Ecology of Populations and Communities BIO 45 by Owen Borville September 27, 2025
A population contains all of the individuals of a particular species that occur in a particular area and have the potential to interact with one another. Populations fluctuate based on factors like: seasonal and yearly changes in the environment, natural disasters like fires and volcanoes, and competition for resources between species. The statistical study of population dynamics is demography, which uses mathematical tools to investigate how populations respond to changes in their environments. Many of these tools have been used to study human populations in addition to all living populations of animals and plants. Life tables were developed to study life expectancy of an individual in a population is a topic of study in demographics.
The study of a population is initiated by determining how many individuals of a particular species exist, and how closely associated they are with each other. Within a particular habitat, a population can be characterized by its population size (N), the total number of individuals, and its population density, the number of individuals with a specific area or volume. Larger populations tend to be more stable than smaller ones because of genetic variability and adaption potential and higher density populations are also more stable than less dense ones. Smaller organisms tend to be more densely distributed than larger ones.
Population research methods include determining a population size and this can be done by simply counting, but often this method is not feasible, particularly if the population is large. Scientists commonly count a population by sampling a representative portion of each habitat and using this sample to estimate the total population. A quadrat can be used for plants or slow-moving organisms. The quadrat is a way of marking off square areas within a habitat with sticks and string or with wood, plastic or metal squares on the ground. After setting the quadrats, researchers then count the number of individuals within the boundaries. Multiple quadrat samples are performed throughout the habitat at several random locations to estimate the population size and density within the entire habitat. The number and size of quadrat samples depends on the type of organisms being studied and their density along with other factors. Larger animals would require a larger quadrat.
For population study of mobile organisms, such as mammals, birds, or fish, scientists use a technique called mark and recapture. This method involves marking a sample of captured animals in some way (such as tags, bands, paint, or other body markings), and then releasing the animals back into the environment to allow them to mix with the rest of the population. Later, researchers collect a new sample, including some individuals that were marked before and recaptured and some that are unmarked.
Using the ratio of marked and unmarked individuals, scientists determine how many individuals are in the sample. Then, calculations are used to estimate the total population size.
N=(number marked first catch x total number of second catch)/(number marked second catch)
There are some limitations to the mark and recapture method. Some animals from the first catch may learn to avoid capture in the second round, giving inflated population estimates. Some animals may prefer or choose to be re-trapped, resulting in an underestimate. Some animals may also be harmed while being marked, reducing their survival. Other methods of population research have been done, including electronic tracking of tagged animals and using data from commercial fishing and trapping operations.
Other types of population measurements:
Species dispersion patterns (or distribution patterns) show the spatial relationship between members of a population within a habitat at a particular point in time. In other words, they show whether members of the species live close together or far apart, and what patterns are evident when they are spaced apart.
Individuals in a population can be equally spaced apart, dispersed randomly with no predictable pattern, or clustered in groups. These are known as uniform, random, and clumped dispersion patterns, respectively. Uniform dispersion is observed in plants that secrete substances inhibiting the growth of nearby individuals (such as the release of toxic chemicals, called allelopathy) and in animals that maintain a defined territory like the penguin. A random dispersion occurs with dandelion and other plants that have wind-dispersed seeds that germinate whenever they happen to fall in a favorable environment. A clumped dispersion may be seen in plants that drop their seeds straight to the ground, such as oak trees, or in animals that live in groups, schools of fish, or herds). Clumped dispersions may also be a function of habitat heterogeneity. Therefore, the dispersion of the individuals within a population provides more information about how they interact with each other than does a simple density measurement. Dispersion also affects the ability to find a mate.
Demography is the study of the dynamics of a population and the statistical study of population changes over time, including birth rates, death rates, and life expectancies. Each of these measures may be affected by the population characteristics already described. Large population sizes can result in a higher birth rate, however it can also result in a higher death rate because of competition, disease, and waste. Higher population density or clumped dispersion can result in higher birthrates because of more encounters. More females in a population can lead to increased birth rates in addition to a larger population of reproductive age individuals (age structure).
In addition, the demographic characteristics of a population can influence how the population grows or declines over time. If birth and death rates are equal, the population remains the same. However, the population size will increase if birth rates exceed death rates. The population will decrease if birth rates are less than death rates. Life expectancy is another important factor as the length of time individuals remain in the population impacts local resources, reproduction, and the overall health of the population. These demographic characteristics are often displayed in a life table.
Life tables provide important information about the life history of an organism. Life tables divide the population into age groups and often sexes, and show how long a member of that group is likely to live. They are modeled after actuarial tables used by the insurance industry for estimating human life expectancy. Life tables may include the probability of individuals dying before their next birthday (mortality rate), the percentage of surviving individuals dying at a particular age interval, and their life expectancy at each interval.
Mortality rate = (number of individuals dying)/(number of individuals surviving) x 1000
Survivorship curves are another tool used by population ecologists. The survivorship curve is a graph of the number of individuals surviving each age interval plotted versus time (commonly with data compiled from a life table). These curves allow us to compare the life histories of different populations. In Type I survivorship, death occurs in the older years, as in humans and mammals. There are few births, but the births are heavily nurtured. Type II survivorship has an equal death rate at all ages, as in birds. Type III survivorship occurs in trees where very few survive the early years, but those that do are more likely to survive to the older years. Many offspring are born, but little care is given to them and enough are born to preserve the species.
A species' life history describes the series of events over its lifetime, such as how resources are allocated for growth, maintenance, and reproduction. Life history traits affect the life table of an organism and a species life history is genetically determined and shaped by the environment and natural selection.
An energy budget is part of all species where the individual must balance energy intake with their use of energy for metabolism, reproduction, parental care, and energy storage (hibernation).
Fecundity is the potential reproductive capacity of an individual within a population, or how many offspring could ideally be produced if an individual has as many offspring as possible, repeating the reproductive cycle as soon as possible after the birth of the offspring. In animals, fecundity is inversely related to the amount of parental care given to an individual offspring. Species that produce many offspring usually provide little care for the offspring and most of their energy is used to produce many tiny offspring and for non-reproductive activities (as in marine invertebrates). Most of these offspring receive little or no parental care and many do not survive, but enough survive to preserve the species.
Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities, even at the expense of their own health, as in humans and mammals. These offspring need much nurturing in order to survive.
Plants with low fecundity produce few energy-rich seeds and each has a good chance to germinate into a new organism. Plants with high fecundity usually have many small, energy-poor seeds that have relatively poor chance of surviving.
The timing of reproduction in a life history also affects species survival. Organisms that reproduce at an early age have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that start reproducing later in life often have greater fecundity or are better able to provide parental care, but they risk that they will not survive to reproductive age. Examples are seen in the fishes.
Semelparity occurs when a species reproduces only once during its lifetime and then dies. These species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive.
Iteroparity describes species that reproduce repeatedly during their lives. Some animals are able to mate only once per year, but survive multiple mating seasons.
Exponential growth in biosciences is when populations of living things have unlimited natural resources and they grow very rapidly while population growth decreases when resources become depleted in a logistic growth. Bacteria growth is a good example of exponential growth, as bacteria reproduce by prokaryotic fission.
Population growth rate is an important concept of exponential growth acceleration, and is the number of organisms added in each reproductive generation and increasing at a greater and greater rate.
In exponential growth, when the population size, N, is plotted on a graph over time, a J-shaped growth curve is produced.
The bacteria example is not representative of the real world where resources are limited and some bacteria will die during the experiment and not reproduce, lowering the growth rate. Therefore when calculating the growth rate of a population, the death rate D, the number of organisms that die during a particular interval is subtracted from the birth rate B, the number of organisms that are born during that interval:
ΔN (change in number) / ΔT (change in time) = B (birth rate) - D (death rate)
The birth rate is usually expressed on a per capita basis for each individual. The instantaneous growth rate is the population at a particular point in time and uses calculus differential notation to replace the change in number and time with an instant-specific measurement of number and time.
dN / dT = bN - dN = (b-d) N
(the d in the first term is the derivative and is different from the death rate, also called d)
The difference between birth and death rates is further simplified by substituting the term "r" for intrinsic rate of increase signifying the maximum per capita growth rate under ideal conditions for the relationship between birth and death rates:
dN / dT = r N
The value "r" can be positive meaning the population is increasing in size or negative meaning the population is decreasing in size or it can be zero where the population's size is unchanging, also known as zero population growth.
Different species have inherent differences in their intrinsic rate of increase (or potential for reproduction) even under ideal conditions. Some species can reproduce more rapidly than other species. The maximal growth rate for a species is its biotic potential or r(max):
dN / dT = r (max) N
When resources are unlimited, populations have exponential growth, resulting in a J-shaped curve. When resources are limited, populations exhibit logistic growth. In logistic growth, population expansion decreases as resources become scarce, and it levels off when the carrying capacity of the environment is reached, resulting in an S-shaped curve.
The logistic growth model was developed to model the reality of limited resources. The carrying capacity, or K, is the population size which represents the maximum population size that a particular environment can support.
The logistic growth equation: dN/dT = r (max) dN/dT = r (max)N(K - N)/K
A graph of this equation yields an S-shaped curve, which is a more realistic model of population growth than exponential growth. There are three different sections to an S-shaped curve. (1) Initially, growth is exponential because there are few individuals and ample resources available. (2) Then, as resources begin to become more limited, the growth rate decreases. (3) Finally, growth levels off at the carrying capacity of the environment, with little change in population over time.
The logistic model assumes that every individual in a population has equal access to resources and equal chance for survival. Some individual have better ability to be successful in their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition. This intraspecific competition intensifies as population size increases and the accumulation of waste products.
Examples of logistic growth include yeast, a microscopic fungus used to make bread and alcoholic beverages. Other examples include sheep and harbor seals, where initial growth slows and levels off as carrying capacity is reached, the maximum population size the environment can sustain.
Population dynamics and regulation: populations are usually not isolated, but rather are engaged in interspecific competition where they share the environment with other species competing for the same resources and help determine how a particular population will grow.
Populations are regulated by density-dependent factors in which the density of the population at a given time affects growth rate and mortality. Populations are also regulated by density-independent factors which influence mortality in a population regardless of population density. Both of these factors help conservationists manage populations and prevent extinction or overpopulation.
Most density-dependent factors are biological (biotic) in nature, and include predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites. Denser populations usually have greater mortality rates. During intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population's rate of growth. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source. An example of density-dependent regulation is the giant intestinal roundworm, a parasite of humans and other mammals where denser populations exhibited lower fecundity and contained fewer eggs.
Many factors including physical and chemical (abiotic), influence the mortality of a population regardless of its density, including weather, natural disasters, and pollution. Population regulation is very complicated and density-dependent and independent factors can interact. A dense population that is reduced in a density-independent manner by some environmental factors will be able to recover differently than a sparce population.
K-selected versus r-selected species are an important topic in population ecology dealing with limited resources and competition. This topic relates to the species' reproductive strategies, habitat, and behavior, especially in the way that they obtain resources and care for their young. It includes length of life and survivorship factors as well. Population biologists have grouped species into the two large categories: K-selected and r-selected, although the categories are really two ends of a continuum.
K-selected species are species selected by stable, predictable environments. Populations of K-selected species tend to exist close to the their carrying capacity where intraspecific competition is high. These species have few, large offspring, a long gestation period, and often give long-term care to their offspring. While larger in size when born, the offspring are relatively helpless and immature at birth. At adulthood, they must develop skills to compete for natural resources. In plants, K-selected species parental care deals with how long it takes to develop offspring or how long the offspring remain on the plant are determining factors in the time to the next reproductive event. Examples of K-selected species are primates, humans, elephants, whales, and plants like oak trees. Oak trees grow very slowly and take many years to produce their first seeds, or acorns. Many acorns are produced, but the germination rate is low. The seeds are large and energy-rich. Therefore, K-selected species mature late, have greater longevity, increased parental care, increased competition, fewer offspring, and larger offspring.
In contrast, r-selected species have a large number of small offspring in unpredictable or changing environments. Long-term parental care is not given and the offspring are relatively mature and self-sufficient at birth. Examples of r-selected species are marine-invertebrates like jellyfish and plants like dandelions, which have small seeds that are dispersed long distances. Many seeds are produced simultaneously to ensure finding a successful environment. These low energy seeds need a hospitable environment to survive. Therefore r-selected species mature early, have lower longevity, decreased parental care, decreased competition, more offspring, and smaller offspring. Bacteria, algae, insects, mice, rabbits, rodents, frogs, plants, and most fish are other r-selected species. New demographic-based models have been developed which incorporate many ecological concepts included in r- and K-selection theory as well as population age structure and mortality factors.
Population dynamics is associated with human population growth. Long term exponential growth has the potential risks of famine, disease, and large-scale death. Depletion of the ozone layer, erosion from acid rain, and damage from global climate change are caused by human activities. Human's carrying capacity has increased and the result is unknown but there is concern. The human population is currently experiencing exponential growth even though reproduction is less than its biotic potential. A consequence of exponential human population growth is a reduction in time that it takes to add a particular number of humans to the Earth. The ability to increase carrying capacity of human population indefinitely be limited without new technological advances, even with increasing population.
Humans are unique in their ability to alter their environment with the purpose of increasing carrying capacity. Human intelligence, society, and communication is part of this ability of humans to create better living conditions, food resources, and technology. Other factors in human population growth are migration and public health as humans have moved across the earth to find better living conditions and have improved healthcare to prevent many major diseases of the past, particularly infectious diseases.
Age structure is an important factor in population dynamics as it is the proportion of a population at different age ranges. Age structure allows better prediction of population growth, plus the ability to associate this growth with the level of economic development in the region. Countries with rapid growth have a pyramidal shape in their age structure diagrams showing a large number of younger individuals and narrowing to a point at the top because of less-than ideal conditions for the elderly. In countries with zero population growth, the age structure is more conical with a larger number of middle-aged and older individuals in the population, as this age structure is more rounded at the top, showing stability. Age structure diagrams range from rapid growth with a sharp point at the top, slow growth with a less pointed top, to stable growth with a rounded top.
Population control efforts in several nations has led to policies such as one-child limits. However, more recently this policy has been phased out due to a dip in population. Another result of human population growth is the effect on the environment, specifically greenhouse gas carbon dioxide.
In community ecology, species interaction involves the dynamic predator-prey cycle where animals hunt other animals for food supply. When prey numbers increase, food supply for predators increases and both populations increase. Eventually the prey population declines from being hunted and the food supply for the predator decreases, which in turn reduces the number of predators. As the predator population decreases, the prey population is allowed to replenish and regrow to previous levels, which in turn allows the predator to access more food supply. The cycle continues.
Defense mechanisms in animals help prevent predation of prey, including mechanical, chemical, physical, or behavioral. Mechanical defenses include thorns on plants or the hard shells of turtles. Chemical defenses in animals and plants release toxic substances to discourage the predator. Some animals like insects can curl up or modify their size or shape for protection. Camouflage is used in prey to hide from predators, as in the chameleon. The walking stick insect uses mimicry to confuse predators. Some animals can play dead to avoid predators or other animals travel in schools or large flocks or herds to avoid being attacked by a predator. Another defense mechanism is aposematic coloration, where an individual uses bright coloration as a warning against predators that they are not safe to eat, after remembering previous experiences with prey of these colors. In Batesian mimicry, a harmless animal has the same coloration as a harmful one with stronger physical or chemical defense mechanisms. In Mullerian mimicry, multiple species share the same warning coloration, but all of them actually have defenses. In Emsleyan/Mertensian mimicry, a deadly prey mimics a less dangerous one to trick predators.
The competitive exclusion principle states that two species cannot occupy the same niche in a habitat and different species cannot coexist in a community if they are competing for all the same resources. When these two species exist in the same habitat, one of them becomes extinct eventually. However, separately each species thrives in different habitats. This exclusion can be avoided if a population switches to a different resource, a different area of the habitat, or feeds during a different time of the day (called resource partitioning). Two organisms occupy different microniches when they coexist by minimizing direct competition.
Symbiotic relationships, or symbioses, are close interactions between individuals of different species over a period of time which impact the abundance and distribution of the associating populations.
Commensalism or a commensal relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits or is harmed. An example is birds nesting in trees, where the bird benefits, but the tree neither benefits or is harmed.
Mutualism is a second type of symbiotic relationship where two species benefit from their interaction with each other. Some scientists believe that these are the only true examples of symbiosis. An example is lichen fungus and photosynthetic algae or bacteria. The algae live inside the cells of the lichen providing energy for the lichen while the lichen provides the algae with a place to live and protection from outside.
Parasitism is when a parasite or an organism that lives in or on another living organism and derives its nutrients from it. In this relationship, the parasite benefits, but the host is harmed by being weakened by the parasite taking its resources. The parasite, however is unlikely to kill the host because the organism would not be able to complete its reproductive cycle by spreading to another host. The parasite is spread to another host when the host eats or consumes a parasite-infected animal, such as a tapeworm inside the intestine of a mammal or animal eaten by humans. A parasite can be spread to humans and other animals by insects like mosquitoes (malaria). The parasite infects the mosquito and is spread to humans by the mosquito.
Foundation species are seen as the base or bedrock of a community by having the greatest influence on its overall structure. These are usually primary producers that bring most of the energy into a community. Examples of foundation species are kelp or brown algae. Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them, such as the photosynthetic corals of the coral reef which contain symbiont organisms within their body tissues that perform photosynthesis (also mutualism). The reef also protects many other species from waves and ocean currents.
Biodiversity is a community's biological complexity and it is measured by the number of different species (richness) in a particular area and their relative abundance (evenness). Species richness is the term that is used to describe the number of species living in a habitat or biome. Species richness varies across the Earth. One determining factor of species richness is latitude, with the greatest species richness occurring in ecosystems near the equator, with warm temperatures, high rainfall, and low seasonality. The lowest species richness occurs near the poles of the Earth, which are very cold, dry, and less suitable for life. Climate predictability or productivity are also important factors of species richness. Island biogeography attempts to explain the relatively high species richness found in certain isolated island chains, including the Galapagos Islands. Relative species abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation species often have the highest abundance of species.
Keystone species are those whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community's structure. An example is the intertidal sea star of North America. Research has shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorous, a necessary inorganic nutrient, to the rest of the community. The absence of this fish would significantly impact the community.
Invasive species are nonnative organisms that, when introduced to an area out of their native range, threaten the ecosystem balance of that habitat. An example is Asian carp fishes introduced to the United States.
Community dynamics are the changes in community structure and composition over time. Sometimes these changes are induced by environmental disturbances, such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state.
Succession describes the sequential appearance and disappearance of species in a community over time. In primary succession, newly exposed or newly formed land is colonized by living things. In secondary succession, part of an ecosystem is disturbed and remnants of the previous community remain.
Primary succession occurs when new land is formed or exposed, such as following the eruption of volcanoes. As lava flows, new land is continually being formed. Weathering and other natural forces help establish pioneer species, or certain hearty plants and lichens with few soil requirements. These species help to further break down the mineral rich lava into soil where other, less hardy species will grow and eventually replace the pioneer species. In addition, as these early species grow and die, they add an ever-growing layer of decomposing organic material and contribute to soil formation. Over time the area will reach an equilibrium state, with a set of organisms quite different from the pioneer species.
Secondary succession occurs, for example, when oak and hickory forests are cleared by wildfire, which burns most vegetation and kills those animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash so that new life can grow. After the fire, the trees are no longer dominant and the first plants to grow back are usually annual plants, grasses, and pioneer species. Then shrubs and small pine, oak, and hickory trees. These organisms are called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species composition is no longer changing and resembles the community before the fire. This equilibrium state is referred to as the climax community, which remains stable until the next fire.
Behavior in biology is the change in activity of an organism in response to a stimulus. Behavioral biology is the study of biological bases for such changes. Many scientists believe that behaviors evolved as a result of pressures of natural selection. However, other scientists believe that living things were designed to have certain abilities and behaviors, or that animals develop behaviors based on their designed ability and from their environment. Ethology is the scientific study of animal behavior. Psychologists study the human mind and behavior, including comparative psychology. Neurobiology is the biology of the nervous system.
Innate behaviors versus learned behaviors is a focal point of behavioral biology. Innate behaviors have a strong genetic component and are largely independent of environmental influences. Learned behaviors result from environmental conditioning. Innate behavior, or instinct, is important because there is no risk of an incorrect behavior being learned because it is built into the system. On the other hand, learned behaviors, although riskier, are flexible, dynamic, and can be altered according to changes in the environment.
Innate or instinctual behaviors rely on response to stimuli. The simplest example of this is a reflex action, an involuntary and rapid response to stimulus, such as pulling your hand quickly from a hot surface, or the knee-jerk reaction when hit below the knee-cap.
Kinesis is an innate behavior of undirected movement in response to a stimulus. Orthokinesis is the increased or decreased speed of movement of an organism in response to a stimulus. An example is woodlice in response to extreme temperature. This movement, although random, increases the probability that the insect spends time in the unfavorable environment. Another example is klinokinesis, an increase in turning behaviors exhibited by bacteria in association with orthokinesis, helps the organisms randomly find a more hospitable environment.
Taxis is the directed movement towards or away from a stimulus. This movement can be in response to light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis) and can be directed toward (positive) or away (negative) from the source of the stimulus. An example of positive chemotaxis is exhibited by the unicellular protozoan Tetrahymena thermophilia. This organism swims using its cilia, at times moving in a straight line, and at other times making turns. The attracting chemotactic agent alters the frequency of turning as the organism moves directly toward the source, following the increasing concentration gradient.
Fixed action patterns are a series of movements elicited by a stimulus such that even when the stimulus is removed, the pattern goes on to completion. Key characteristics of fixed action patterns are innate and hardwired in the nervous system, stereotypic and predictable, triggered by a sign stimulus, carried to completion, unchangeable. An example is yawning in humans. Fixed action patterns help animals survive and reproduce.
Migration is the long-range seasonal movement of animals in response to resource availability and is common in many animals. Animals migrate to find food, mates, breeding grounds, and better weather conditions. Migration is common in birds, salmon and other fish, emperor penguins, and wildebeests. In addition, the monarch butterfly, arctic tern, humpback whale, gray whale, baleen whale, reindeer, bats, sea turtles, zebra, godwit bird, sternidae bird, Canadian goose, zooplankton, dragonfly, hummingbirds, sharks, pronghorn antelope, seals, flamingos, elephant seals, bison, zebra, cranes, desert locust and many more. Many of these migrate long distances per year, even thousands of kilometers or more. Migration is an innate behavior, but not all animals migrate. In addition, those animals that do migrate may not migrate always and sometimes choose not to migrate, depending on the availability of food and resources.
Foraging is the act of searching for and exploiting food resources. Optimal foraging behaviors are feeding behaviors that maximize energy gain and minimize energy expenditure.
Not all animals live in groups, but all animals must mate except those who reproduce asexually. Mating involves signaling to a potential mate. Many animals display mating rituals, or energy-intensive behaviors or displays associated with mating. Other behaviors of animals in groups are labeled as selfish, where only one animal benefits, or altruistic behavior where one animals actions benefit another animal. Cooperated behavior occurs when both animals benefit. All of these behaviors involve communication between animals in a group.
Animals communicate with each other using stimuli known as signals. Visual signals include a particular color on a males body that attracts females and makes other males aggressive, as in the three-spined stickleback fish. Other signals are chemical (pheromones), aural (sound), visual (courtship and aggressive displays), or tactile (touch). These types of communication may be instinctual or learned or a combination of both. However, these types of communication are not seen as the same as language in humans.
A pheromone is a secreted chemical signal used to obtain a response from another individual of the same species. The purpose of pheromones is to elicit a specific behavior from the receiving individual. Pheromones are especially common among social insects and are used by many species for mating, to sound alarms, to mark food trails, and for other, more complex behaviors. Humans are believed to respond to certain pheromones called axillary steroids.
Songs are an example of an aural signal that needs to be heard by the recipient. The songs of birds identify the species and and are used to attract mates. Whales also use songs at very low frequency that travel long distances underwater. Dolphins also communicate with each other using a variety of vocalizations. Male crickets use unique noises with a special organ in their body to attract mates and communicate with others in their species.
Courtship displays are a series of ritualized visual behaviors (signals) designed to attract and convince a member of the opposite sex to mate. These are common in the animal kingdom. Courtships can be either successful or unsuccessful if the display is accepted or not.
Aggressive displays are also common in the animal kingdom, most commonly seen in dogs and other domestic animals but also many wild animals, and are seen as the willingness of an animal to fight but are also seed as a deterrent to other species to avoid fighting.
Distraction displays are seen in birds and some fish to attract a predator away from the nest. This is an example of altruistic behavior because it benefits the young more than the individual performing the display.
Touching is a common form of communication in animals, particularly among primates. Grooming, embracing, greeting, and touching various parts of the body to communicate have been observed in primates and other animals.
Altruistic behaviors are behaviors that lower the fitness of the individual animal but increase the fitness of another individual. These behaviors have been observed throughout the animal kingdom. Examples include worker bees and their queen bee, meerkats guarding their colony from intruders, wolves bring meat from a hunt for pack members. Some animal mothers take care of infants unrelated to them. Some animals have been observed protecting animals of other species. Scientists have long discussed why altruistic behaviors exist and the answer is uncertain, but there are many theories. Altruistic behavior is difficult to explain because animals have been observed helping unrelated animals. The question of whether altruistic is instinctual or voluntary conscience is still being debated.
Finding mates in animals requires significant energy in locating, attracting and mating with a partner. Intersexual selection occurs when individuals of one sex choose mates of the other sex, while intrasexual selection is the competition for mates between species members of the same sex.
Three general matins systems all involve innate as opposed to learned behaviors, are seen in animal populations: monogamous, polygynous, and polyandrous.
In monogamous systems, one male and one female are paired for at least one breeding season. In many animals, however, these relationships last much longer, even a lifetime. Monogamous systems have been observed in wolves, beavers, birds species, and some primates. Reasons for monogamous systems include protection of the partner and offspring.
Polygynous mating systems refers to one male mating with multiple females and the reasoning is believed to be to increase male reproductive success. A variety of animals exhibit polygynous mating, including elephant seals, gorillas, spotted hyenas, red deer, lions, many lizards, and some bird species.
Polyandrous mating systems occur when one female mates with many males. This system is very rare among animals compared to monogamous and polygynous systems. Reasoning for this system is believed to be for increased offspring survival and genetic fitness. Male parental care, access to more resources, and fertility assurance are several reasons for polyandrous mating in animals. Some shorebirds, insects, the seahorse, and pipefish exhibit this system of mating.
Simple learned behaviors in animals include habituation, where an animal stops responding to a stimulus after a period of repeated exposure. An example is when wild animals in national parks lose fear of human visitors and/or human footsteps. Habituation is not the same as taming, which is a later step in the relationship process between humans and animals.
Imprinting is a type of innate learning that occurs at a particular age or a life stage that is rapid and independent of the species involved. This learning allows the infant to develop a bond with its mother or human caregiver. This learning can also recognize a particular habitat or food source and is vital for survival. This learning influences social interactions, predator avoidance, and feeding.
Conditioned behaviors are types of associative learning, where a stimulus becomes associated with a consequence, and the behavioral response is modified by its consequences.
In classical conditioning, a response called the conditioned response is associated with a stimulus that it had previously not been associated with, the conditioned stimulus. The response to the original, unconditioned stimulus is called the unconditioned response. A famous example of this is Ivan Pavlov's experiments with dogs. Dogs are conditioned to hear a bell ring every time they are fed. Then the bell is rung and no food is given, but the dog still salivates expecting to see food. Some conditioning experiments required multiple exposures to the stimulus, but in some cases, the conditioning was learned after only one exposure.
Operant conditioning is when the conditioned behavior is gradually modified by its consequences as the animal responds to the stimulus. An example is the Skinner box experiment, where B.F. Skinner placed rats in a box with a lever that would dispense food when depressed. The rats eventually learned that the lever would dispense food when depressed. This type of learning is the basis of most animal training.
Cognitive learning is the manipulation of information using the mind, the most prominent method of human learning. Wolfgang Kohler is known for cognitive learning research on chimpanzees. He demonstrated that these animals were capable of abstract thought by showing that they could learn how to solve a puzzle. The chimpanzees were able to learn to stack up randomly placed boxes in order to reach a banana that was hanging too high from the floor. H.C. Blodgett performed maze experiments with rats prompted to find their way through a maze to find food.
Sociobiology is the study of animal and human behavior can be explained almost entirely in terms of genetics and natural selection. This field of study was developed by researcher E.O. Wilson in the 1970s. However, other scientists believe that environmental effects on behavior are not emphasized in this field of study, but should be. This topic is focused on the classic nature versus nurture debate of the role of genetics versus the role of the environment in determining the organism's characteristics.
A population contains all of the individuals of a particular species that occur in a particular area and have the potential to interact with one another. Populations fluctuate based on factors like: seasonal and yearly changes in the environment, natural disasters like fires and volcanoes, and competition for resources between species. The statistical study of population dynamics is demography, which uses mathematical tools to investigate how populations respond to changes in their environments. Many of these tools have been used to study human populations in addition to all living populations of animals and plants. Life tables were developed to study life expectancy of an individual in a population is a topic of study in demographics.
The study of a population is initiated by determining how many individuals of a particular species exist, and how closely associated they are with each other. Within a particular habitat, a population can be characterized by its population size (N), the total number of individuals, and its population density, the number of individuals with a specific area or volume. Larger populations tend to be more stable than smaller ones because of genetic variability and adaption potential and higher density populations are also more stable than less dense ones. Smaller organisms tend to be more densely distributed than larger ones.
Population research methods include determining a population size and this can be done by simply counting, but often this method is not feasible, particularly if the population is large. Scientists commonly count a population by sampling a representative portion of each habitat and using this sample to estimate the total population. A quadrat can be used for plants or slow-moving organisms. The quadrat is a way of marking off square areas within a habitat with sticks and string or with wood, plastic or metal squares on the ground. After setting the quadrats, researchers then count the number of individuals within the boundaries. Multiple quadrat samples are performed throughout the habitat at several random locations to estimate the population size and density within the entire habitat. The number and size of quadrat samples depends on the type of organisms being studied and their density along with other factors. Larger animals would require a larger quadrat.
For population study of mobile organisms, such as mammals, birds, or fish, scientists use a technique called mark and recapture. This method involves marking a sample of captured animals in some way (such as tags, bands, paint, or other body markings), and then releasing the animals back into the environment to allow them to mix with the rest of the population. Later, researchers collect a new sample, including some individuals that were marked before and recaptured and some that are unmarked.
Using the ratio of marked and unmarked individuals, scientists determine how many individuals are in the sample. Then, calculations are used to estimate the total population size.
N=(number marked first catch x total number of second catch)/(number marked second catch)
There are some limitations to the mark and recapture method. Some animals from the first catch may learn to avoid capture in the second round, giving inflated population estimates. Some animals may prefer or choose to be re-trapped, resulting in an underestimate. Some animals may also be harmed while being marked, reducing their survival. Other methods of population research have been done, including electronic tracking of tagged animals and using data from commercial fishing and trapping operations.
Other types of population measurements:
Species dispersion patterns (or distribution patterns) show the spatial relationship between members of a population within a habitat at a particular point in time. In other words, they show whether members of the species live close together or far apart, and what patterns are evident when they are spaced apart.
Individuals in a population can be equally spaced apart, dispersed randomly with no predictable pattern, or clustered in groups. These are known as uniform, random, and clumped dispersion patterns, respectively. Uniform dispersion is observed in plants that secrete substances inhibiting the growth of nearby individuals (such as the release of toxic chemicals, called allelopathy) and in animals that maintain a defined territory like the penguin. A random dispersion occurs with dandelion and other plants that have wind-dispersed seeds that germinate whenever they happen to fall in a favorable environment. A clumped dispersion may be seen in plants that drop their seeds straight to the ground, such as oak trees, or in animals that live in groups, schools of fish, or herds). Clumped dispersions may also be a function of habitat heterogeneity. Therefore, the dispersion of the individuals within a population provides more information about how they interact with each other than does a simple density measurement. Dispersion also affects the ability to find a mate.
Demography is the study of the dynamics of a population and the statistical study of population changes over time, including birth rates, death rates, and life expectancies. Each of these measures may be affected by the population characteristics already described. Large population sizes can result in a higher birth rate, however it can also result in a higher death rate because of competition, disease, and waste. Higher population density or clumped dispersion can result in higher birthrates because of more encounters. More females in a population can lead to increased birth rates in addition to a larger population of reproductive age individuals (age structure).
In addition, the demographic characteristics of a population can influence how the population grows or declines over time. If birth and death rates are equal, the population remains the same. However, the population size will increase if birth rates exceed death rates. The population will decrease if birth rates are less than death rates. Life expectancy is another important factor as the length of time individuals remain in the population impacts local resources, reproduction, and the overall health of the population. These demographic characteristics are often displayed in a life table.
Life tables provide important information about the life history of an organism. Life tables divide the population into age groups and often sexes, and show how long a member of that group is likely to live. They are modeled after actuarial tables used by the insurance industry for estimating human life expectancy. Life tables may include the probability of individuals dying before their next birthday (mortality rate), the percentage of surviving individuals dying at a particular age interval, and their life expectancy at each interval.
Mortality rate = (number of individuals dying)/(number of individuals surviving) x 1000
Survivorship curves are another tool used by population ecologists. The survivorship curve is a graph of the number of individuals surviving each age interval plotted versus time (commonly with data compiled from a life table). These curves allow us to compare the life histories of different populations. In Type I survivorship, death occurs in the older years, as in humans and mammals. There are few births, but the births are heavily nurtured. Type II survivorship has an equal death rate at all ages, as in birds. Type III survivorship occurs in trees where very few survive the early years, but those that do are more likely to survive to the older years. Many offspring are born, but little care is given to them and enough are born to preserve the species.
A species' life history describes the series of events over its lifetime, such as how resources are allocated for growth, maintenance, and reproduction. Life history traits affect the life table of an organism and a species life history is genetically determined and shaped by the environment and natural selection.
An energy budget is part of all species where the individual must balance energy intake with their use of energy for metabolism, reproduction, parental care, and energy storage (hibernation).
Fecundity is the potential reproductive capacity of an individual within a population, or how many offspring could ideally be produced if an individual has as many offspring as possible, repeating the reproductive cycle as soon as possible after the birth of the offspring. In animals, fecundity is inversely related to the amount of parental care given to an individual offspring. Species that produce many offspring usually provide little care for the offspring and most of their energy is used to produce many tiny offspring and for non-reproductive activities (as in marine invertebrates). Most of these offspring receive little or no parental care and many do not survive, but enough survive to preserve the species.
Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities, even at the expense of their own health, as in humans and mammals. These offspring need much nurturing in order to survive.
Plants with low fecundity produce few energy-rich seeds and each has a good chance to germinate into a new organism. Plants with high fecundity usually have many small, energy-poor seeds that have relatively poor chance of surviving.
The timing of reproduction in a life history also affects species survival. Organisms that reproduce at an early age have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that start reproducing later in life often have greater fecundity or are better able to provide parental care, but they risk that they will not survive to reproductive age. Examples are seen in the fishes.
Semelparity occurs when a species reproduces only once during its lifetime and then dies. These species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive.
Iteroparity describes species that reproduce repeatedly during their lives. Some animals are able to mate only once per year, but survive multiple mating seasons.
Exponential growth in biosciences is when populations of living things have unlimited natural resources and they grow very rapidly while population growth decreases when resources become depleted in a logistic growth. Bacteria growth is a good example of exponential growth, as bacteria reproduce by prokaryotic fission.
Population growth rate is an important concept of exponential growth acceleration, and is the number of organisms added in each reproductive generation and increasing at a greater and greater rate.
In exponential growth, when the population size, N, is plotted on a graph over time, a J-shaped growth curve is produced.
The bacteria example is not representative of the real world where resources are limited and some bacteria will die during the experiment and not reproduce, lowering the growth rate. Therefore when calculating the growth rate of a population, the death rate D, the number of organisms that die during a particular interval is subtracted from the birth rate B, the number of organisms that are born during that interval:
ΔN (change in number) / ΔT (change in time) = B (birth rate) - D (death rate)
The birth rate is usually expressed on a per capita basis for each individual. The instantaneous growth rate is the population at a particular point in time and uses calculus differential notation to replace the change in number and time with an instant-specific measurement of number and time.
dN / dT = bN - dN = (b-d) N
(the d in the first term is the derivative and is different from the death rate, also called d)
The difference between birth and death rates is further simplified by substituting the term "r" for intrinsic rate of increase signifying the maximum per capita growth rate under ideal conditions for the relationship between birth and death rates:
dN / dT = r N
The value "r" can be positive meaning the population is increasing in size or negative meaning the population is decreasing in size or it can be zero where the population's size is unchanging, also known as zero population growth.
Different species have inherent differences in their intrinsic rate of increase (or potential for reproduction) even under ideal conditions. Some species can reproduce more rapidly than other species. The maximal growth rate for a species is its biotic potential or r(max):
dN / dT = r (max) N
When resources are unlimited, populations have exponential growth, resulting in a J-shaped curve. When resources are limited, populations exhibit logistic growth. In logistic growth, population expansion decreases as resources become scarce, and it levels off when the carrying capacity of the environment is reached, resulting in an S-shaped curve.
The logistic growth model was developed to model the reality of limited resources. The carrying capacity, or K, is the population size which represents the maximum population size that a particular environment can support.
The logistic growth equation: dN/dT = r (max) dN/dT = r (max)N(K - N)/K
A graph of this equation yields an S-shaped curve, which is a more realistic model of population growth than exponential growth. There are three different sections to an S-shaped curve. (1) Initially, growth is exponential because there are few individuals and ample resources available. (2) Then, as resources begin to become more limited, the growth rate decreases. (3) Finally, growth levels off at the carrying capacity of the environment, with little change in population over time.
The logistic model assumes that every individual in a population has equal access to resources and equal chance for survival. Some individual have better ability to be successful in their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition. This intraspecific competition intensifies as population size increases and the accumulation of waste products.
Examples of logistic growth include yeast, a microscopic fungus used to make bread and alcoholic beverages. Other examples include sheep and harbor seals, where initial growth slows and levels off as carrying capacity is reached, the maximum population size the environment can sustain.
Population dynamics and regulation: populations are usually not isolated, but rather are engaged in interspecific competition where they share the environment with other species competing for the same resources and help determine how a particular population will grow.
Populations are regulated by density-dependent factors in which the density of the population at a given time affects growth rate and mortality. Populations are also regulated by density-independent factors which influence mortality in a population regardless of population density. Both of these factors help conservationists manage populations and prevent extinction or overpopulation.
Most density-dependent factors are biological (biotic) in nature, and include predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites. Denser populations usually have greater mortality rates. During intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population's rate of growth. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source. An example of density-dependent regulation is the giant intestinal roundworm, a parasite of humans and other mammals where denser populations exhibited lower fecundity and contained fewer eggs.
Many factors including physical and chemical (abiotic), influence the mortality of a population regardless of its density, including weather, natural disasters, and pollution. Population regulation is very complicated and density-dependent and independent factors can interact. A dense population that is reduced in a density-independent manner by some environmental factors will be able to recover differently than a sparce population.
K-selected versus r-selected species are an important topic in population ecology dealing with limited resources and competition. This topic relates to the species' reproductive strategies, habitat, and behavior, especially in the way that they obtain resources and care for their young. It includes length of life and survivorship factors as well. Population biologists have grouped species into the two large categories: K-selected and r-selected, although the categories are really two ends of a continuum.
K-selected species are species selected by stable, predictable environments. Populations of K-selected species tend to exist close to the their carrying capacity where intraspecific competition is high. These species have few, large offspring, a long gestation period, and often give long-term care to their offspring. While larger in size when born, the offspring are relatively helpless and immature at birth. At adulthood, they must develop skills to compete for natural resources. In plants, K-selected species parental care deals with how long it takes to develop offspring or how long the offspring remain on the plant are determining factors in the time to the next reproductive event. Examples of K-selected species are primates, humans, elephants, whales, and plants like oak trees. Oak trees grow very slowly and take many years to produce their first seeds, or acorns. Many acorns are produced, but the germination rate is low. The seeds are large and energy-rich. Therefore, K-selected species mature late, have greater longevity, increased parental care, increased competition, fewer offspring, and larger offspring.
In contrast, r-selected species have a large number of small offspring in unpredictable or changing environments. Long-term parental care is not given and the offspring are relatively mature and self-sufficient at birth. Examples of r-selected species are marine-invertebrates like jellyfish and plants like dandelions, which have small seeds that are dispersed long distances. Many seeds are produced simultaneously to ensure finding a successful environment. These low energy seeds need a hospitable environment to survive. Therefore r-selected species mature early, have lower longevity, decreased parental care, decreased competition, more offspring, and smaller offspring. Bacteria, algae, insects, mice, rabbits, rodents, frogs, plants, and most fish are other r-selected species. New demographic-based models have been developed which incorporate many ecological concepts included in r- and K-selection theory as well as population age structure and mortality factors.
Population dynamics is associated with human population growth. Long term exponential growth has the potential risks of famine, disease, and large-scale death. Depletion of the ozone layer, erosion from acid rain, and damage from global climate change are caused by human activities. Human's carrying capacity has increased and the result is unknown but there is concern. The human population is currently experiencing exponential growth even though reproduction is less than its biotic potential. A consequence of exponential human population growth is a reduction in time that it takes to add a particular number of humans to the Earth. The ability to increase carrying capacity of human population indefinitely be limited without new technological advances, even with increasing population.
Humans are unique in their ability to alter their environment with the purpose of increasing carrying capacity. Human intelligence, society, and communication is part of this ability of humans to create better living conditions, food resources, and technology. Other factors in human population growth are migration and public health as humans have moved across the earth to find better living conditions and have improved healthcare to prevent many major diseases of the past, particularly infectious diseases.
Age structure is an important factor in population dynamics as it is the proportion of a population at different age ranges. Age structure allows better prediction of population growth, plus the ability to associate this growth with the level of economic development in the region. Countries with rapid growth have a pyramidal shape in their age structure diagrams showing a large number of younger individuals and narrowing to a point at the top because of less-than ideal conditions for the elderly. In countries with zero population growth, the age structure is more conical with a larger number of middle-aged and older individuals in the population, as this age structure is more rounded at the top, showing stability. Age structure diagrams range from rapid growth with a sharp point at the top, slow growth with a less pointed top, to stable growth with a rounded top.
Population control efforts in several nations has led to policies such as one-child limits. However, more recently this policy has been phased out due to a dip in population. Another result of human population growth is the effect on the environment, specifically greenhouse gas carbon dioxide.
In community ecology, species interaction involves the dynamic predator-prey cycle where animals hunt other animals for food supply. When prey numbers increase, food supply for predators increases and both populations increase. Eventually the prey population declines from being hunted and the food supply for the predator decreases, which in turn reduces the number of predators. As the predator population decreases, the prey population is allowed to replenish and regrow to previous levels, which in turn allows the predator to access more food supply. The cycle continues.
Defense mechanisms in animals help prevent predation of prey, including mechanical, chemical, physical, or behavioral. Mechanical defenses include thorns on plants or the hard shells of turtles. Chemical defenses in animals and plants release toxic substances to discourage the predator. Some animals like insects can curl up or modify their size or shape for protection. Camouflage is used in prey to hide from predators, as in the chameleon. The walking stick insect uses mimicry to confuse predators. Some animals can play dead to avoid predators or other animals travel in schools or large flocks or herds to avoid being attacked by a predator. Another defense mechanism is aposematic coloration, where an individual uses bright coloration as a warning against predators that they are not safe to eat, after remembering previous experiences with prey of these colors. In Batesian mimicry, a harmless animal has the same coloration as a harmful one with stronger physical or chemical defense mechanisms. In Mullerian mimicry, multiple species share the same warning coloration, but all of them actually have defenses. In Emsleyan/Mertensian mimicry, a deadly prey mimics a less dangerous one to trick predators.
The competitive exclusion principle states that two species cannot occupy the same niche in a habitat and different species cannot coexist in a community if they are competing for all the same resources. When these two species exist in the same habitat, one of them becomes extinct eventually. However, separately each species thrives in different habitats. This exclusion can be avoided if a population switches to a different resource, a different area of the habitat, or feeds during a different time of the day (called resource partitioning). Two organisms occupy different microniches when they coexist by minimizing direct competition.
Symbiotic relationships, or symbioses, are close interactions between individuals of different species over a period of time which impact the abundance and distribution of the associating populations.
Commensalism or a commensal relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits or is harmed. An example is birds nesting in trees, where the bird benefits, but the tree neither benefits or is harmed.
Mutualism is a second type of symbiotic relationship where two species benefit from their interaction with each other. Some scientists believe that these are the only true examples of symbiosis. An example is lichen fungus and photosynthetic algae or bacteria. The algae live inside the cells of the lichen providing energy for the lichen while the lichen provides the algae with a place to live and protection from outside.
Parasitism is when a parasite or an organism that lives in or on another living organism and derives its nutrients from it. In this relationship, the parasite benefits, but the host is harmed by being weakened by the parasite taking its resources. The parasite, however is unlikely to kill the host because the organism would not be able to complete its reproductive cycle by spreading to another host. The parasite is spread to another host when the host eats or consumes a parasite-infected animal, such as a tapeworm inside the intestine of a mammal or animal eaten by humans. A parasite can be spread to humans and other animals by insects like mosquitoes (malaria). The parasite infects the mosquito and is spread to humans by the mosquito.
Foundation species are seen as the base or bedrock of a community by having the greatest influence on its overall structure. These are usually primary producers that bring most of the energy into a community. Examples of foundation species are kelp or brown algae. Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them, such as the photosynthetic corals of the coral reef which contain symbiont organisms within their body tissues that perform photosynthesis (also mutualism). The reef also protects many other species from waves and ocean currents.
Biodiversity is a community's biological complexity and it is measured by the number of different species (richness) in a particular area and their relative abundance (evenness). Species richness is the term that is used to describe the number of species living in a habitat or biome. Species richness varies across the Earth. One determining factor of species richness is latitude, with the greatest species richness occurring in ecosystems near the equator, with warm temperatures, high rainfall, and low seasonality. The lowest species richness occurs near the poles of the Earth, which are very cold, dry, and less suitable for life. Climate predictability or productivity are also important factors of species richness. Island biogeography attempts to explain the relatively high species richness found in certain isolated island chains, including the Galapagos Islands. Relative species abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation species often have the highest abundance of species.
Keystone species are those whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community's structure. An example is the intertidal sea star of North America. Research has shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorous, a necessary inorganic nutrient, to the rest of the community. The absence of this fish would significantly impact the community.
Invasive species are nonnative organisms that, when introduced to an area out of their native range, threaten the ecosystem balance of that habitat. An example is Asian carp fishes introduced to the United States.
Community dynamics are the changes in community structure and composition over time. Sometimes these changes are induced by environmental disturbances, such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state.
Succession describes the sequential appearance and disappearance of species in a community over time. In primary succession, newly exposed or newly formed land is colonized by living things. In secondary succession, part of an ecosystem is disturbed and remnants of the previous community remain.
Primary succession occurs when new land is formed or exposed, such as following the eruption of volcanoes. As lava flows, new land is continually being formed. Weathering and other natural forces help establish pioneer species, or certain hearty plants and lichens with few soil requirements. These species help to further break down the mineral rich lava into soil where other, less hardy species will grow and eventually replace the pioneer species. In addition, as these early species grow and die, they add an ever-growing layer of decomposing organic material and contribute to soil formation. Over time the area will reach an equilibrium state, with a set of organisms quite different from the pioneer species.
Secondary succession occurs, for example, when oak and hickory forests are cleared by wildfire, which burns most vegetation and kills those animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash so that new life can grow. After the fire, the trees are no longer dominant and the first plants to grow back are usually annual plants, grasses, and pioneer species. Then shrubs and small pine, oak, and hickory trees. These organisms are called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species composition is no longer changing and resembles the community before the fire. This equilibrium state is referred to as the climax community, which remains stable until the next fire.
Behavior in biology is the change in activity of an organism in response to a stimulus. Behavioral biology is the study of biological bases for such changes. Many scientists believe that behaviors evolved as a result of pressures of natural selection. However, other scientists believe that living things were designed to have certain abilities and behaviors, or that animals develop behaviors based on their designed ability and from their environment. Ethology is the scientific study of animal behavior. Psychologists study the human mind and behavior, including comparative psychology. Neurobiology is the biology of the nervous system.
Innate behaviors versus learned behaviors is a focal point of behavioral biology. Innate behaviors have a strong genetic component and are largely independent of environmental influences. Learned behaviors result from environmental conditioning. Innate behavior, or instinct, is important because there is no risk of an incorrect behavior being learned because it is built into the system. On the other hand, learned behaviors, although riskier, are flexible, dynamic, and can be altered according to changes in the environment.
Innate or instinctual behaviors rely on response to stimuli. The simplest example of this is a reflex action, an involuntary and rapid response to stimulus, such as pulling your hand quickly from a hot surface, or the knee-jerk reaction when hit below the knee-cap.
Kinesis is an innate behavior of undirected movement in response to a stimulus. Orthokinesis is the increased or decreased speed of movement of an organism in response to a stimulus. An example is woodlice in response to extreme temperature. This movement, although random, increases the probability that the insect spends time in the unfavorable environment. Another example is klinokinesis, an increase in turning behaviors exhibited by bacteria in association with orthokinesis, helps the organisms randomly find a more hospitable environment.
Taxis is the directed movement towards or away from a stimulus. This movement can be in response to light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis) and can be directed toward (positive) or away (negative) from the source of the stimulus. An example of positive chemotaxis is exhibited by the unicellular protozoan Tetrahymena thermophilia. This organism swims using its cilia, at times moving in a straight line, and at other times making turns. The attracting chemotactic agent alters the frequency of turning as the organism moves directly toward the source, following the increasing concentration gradient.
Fixed action patterns are a series of movements elicited by a stimulus such that even when the stimulus is removed, the pattern goes on to completion. Key characteristics of fixed action patterns are innate and hardwired in the nervous system, stereotypic and predictable, triggered by a sign stimulus, carried to completion, unchangeable. An example is yawning in humans. Fixed action patterns help animals survive and reproduce.
Migration is the long-range seasonal movement of animals in response to resource availability and is common in many animals. Animals migrate to find food, mates, breeding grounds, and better weather conditions. Migration is common in birds, salmon and other fish, emperor penguins, and wildebeests. In addition, the monarch butterfly, arctic tern, humpback whale, gray whale, baleen whale, reindeer, bats, sea turtles, zebra, godwit bird, sternidae bird, Canadian goose, zooplankton, dragonfly, hummingbirds, sharks, pronghorn antelope, seals, flamingos, elephant seals, bison, zebra, cranes, desert locust and many more. Many of these migrate long distances per year, even thousands of kilometers or more. Migration is an innate behavior, but not all animals migrate. In addition, those animals that do migrate may not migrate always and sometimes choose not to migrate, depending on the availability of food and resources.
Foraging is the act of searching for and exploiting food resources. Optimal foraging behaviors are feeding behaviors that maximize energy gain and minimize energy expenditure.
Not all animals live in groups, but all animals must mate except those who reproduce asexually. Mating involves signaling to a potential mate. Many animals display mating rituals, or energy-intensive behaviors or displays associated with mating. Other behaviors of animals in groups are labeled as selfish, where only one animal benefits, or altruistic behavior where one animals actions benefit another animal. Cooperated behavior occurs when both animals benefit. All of these behaviors involve communication between animals in a group.
Animals communicate with each other using stimuli known as signals. Visual signals include a particular color on a males body that attracts females and makes other males aggressive, as in the three-spined stickleback fish. Other signals are chemical (pheromones), aural (sound), visual (courtship and aggressive displays), or tactile (touch). These types of communication may be instinctual or learned or a combination of both. However, these types of communication are not seen as the same as language in humans.
A pheromone is a secreted chemical signal used to obtain a response from another individual of the same species. The purpose of pheromones is to elicit a specific behavior from the receiving individual. Pheromones are especially common among social insects and are used by many species for mating, to sound alarms, to mark food trails, and for other, more complex behaviors. Humans are believed to respond to certain pheromones called axillary steroids.
Songs are an example of an aural signal that needs to be heard by the recipient. The songs of birds identify the species and and are used to attract mates. Whales also use songs at very low frequency that travel long distances underwater. Dolphins also communicate with each other using a variety of vocalizations. Male crickets use unique noises with a special organ in their body to attract mates and communicate with others in their species.
Courtship displays are a series of ritualized visual behaviors (signals) designed to attract and convince a member of the opposite sex to mate. These are common in the animal kingdom. Courtships can be either successful or unsuccessful if the display is accepted or not.
Aggressive displays are also common in the animal kingdom, most commonly seen in dogs and other domestic animals but also many wild animals, and are seen as the willingness of an animal to fight but are also seed as a deterrent to other species to avoid fighting.
Distraction displays are seen in birds and some fish to attract a predator away from the nest. This is an example of altruistic behavior because it benefits the young more than the individual performing the display.
Touching is a common form of communication in animals, particularly among primates. Grooming, embracing, greeting, and touching various parts of the body to communicate have been observed in primates and other animals.
Altruistic behaviors are behaviors that lower the fitness of the individual animal but increase the fitness of another individual. These behaviors have been observed throughout the animal kingdom. Examples include worker bees and their queen bee, meerkats guarding their colony from intruders, wolves bring meat from a hunt for pack members. Some animal mothers take care of infants unrelated to them. Some animals have been observed protecting animals of other species. Scientists have long discussed why altruistic behaviors exist and the answer is uncertain, but there are many theories. Altruistic behavior is difficult to explain because animals have been observed helping unrelated animals. The question of whether altruistic is instinctual or voluntary conscience is still being debated.
Finding mates in animals requires significant energy in locating, attracting and mating with a partner. Intersexual selection occurs when individuals of one sex choose mates of the other sex, while intrasexual selection is the competition for mates between species members of the same sex.
Three general matins systems all involve innate as opposed to learned behaviors, are seen in animal populations: monogamous, polygynous, and polyandrous.
In monogamous systems, one male and one female are paired for at least one breeding season. In many animals, however, these relationships last much longer, even a lifetime. Monogamous systems have been observed in wolves, beavers, birds species, and some primates. Reasons for monogamous systems include protection of the partner and offspring.
Polygynous mating systems refers to one male mating with multiple females and the reasoning is believed to be to increase male reproductive success. A variety of animals exhibit polygynous mating, including elephant seals, gorillas, spotted hyenas, red deer, lions, many lizards, and some bird species.
Polyandrous mating systems occur when one female mates with many males. This system is very rare among animals compared to monogamous and polygynous systems. Reasoning for this system is believed to be for increased offspring survival and genetic fitness. Male parental care, access to more resources, and fertility assurance are several reasons for polyandrous mating in animals. Some shorebirds, insects, the seahorse, and pipefish exhibit this system of mating.
Simple learned behaviors in animals include habituation, where an animal stops responding to a stimulus after a period of repeated exposure. An example is when wild animals in national parks lose fear of human visitors and/or human footsteps. Habituation is not the same as taming, which is a later step in the relationship process between humans and animals.
Imprinting is a type of innate learning that occurs at a particular age or a life stage that is rapid and independent of the species involved. This learning allows the infant to develop a bond with its mother or human caregiver. This learning can also recognize a particular habitat or food source and is vital for survival. This learning influences social interactions, predator avoidance, and feeding.
Conditioned behaviors are types of associative learning, where a stimulus becomes associated with a consequence, and the behavioral response is modified by its consequences.
In classical conditioning, a response called the conditioned response is associated with a stimulus that it had previously not been associated with, the conditioned stimulus. The response to the original, unconditioned stimulus is called the unconditioned response. A famous example of this is Ivan Pavlov's experiments with dogs. Dogs are conditioned to hear a bell ring every time they are fed. Then the bell is rung and no food is given, but the dog still salivates expecting to see food. Some conditioning experiments required multiple exposures to the stimulus, but in some cases, the conditioning was learned after only one exposure.
Operant conditioning is when the conditioned behavior is gradually modified by its consequences as the animal responds to the stimulus. An example is the Skinner box experiment, where B.F. Skinner placed rats in a box with a lever that would dispense food when depressed. The rats eventually learned that the lever would dispense food when depressed. This type of learning is the basis of most animal training.
Cognitive learning is the manipulation of information using the mind, the most prominent method of human learning. Wolfgang Kohler is known for cognitive learning research on chimpanzees. He demonstrated that these animals were capable of abstract thought by showing that they could learn how to solve a puzzle. The chimpanzees were able to learn to stack up randomly placed boxes in order to reach a banana that was hanging too high from the floor. H.C. Blodgett performed maze experiments with rats prompted to find their way through a maze to find food.
Sociobiology is the study of animal and human behavior can be explained almost entirely in terms of genetics and natural selection. This field of study was developed by researcher E.O. Wilson in the 1970s. However, other scientists believe that environmental effects on behavior are not emphasized in this field of study, but should be. This topic is focused on the classic nature versus nurture debate of the role of genetics versus the role of the environment in determining the organism's characteristics.