Introduction to ecology - Ecology is defined as the study of the interactions between organisms and their environment. In 1866, German biologist and evolutionist Ernst Haeckel (1834-1919) used the word "oecology" to denote the study of organisms and their interactions with the world around them. The modern spelling of the word ecology was first used in 1893. In 1927 the British biologist Charles Elton (1900-1991) published the key textbook Animal Ecology. This brought together much of the work that had been done in this field and defined the concept of niche. In the UK, his animal studies earned him the title of "the father of ecology." Ecologists usually recognize two main components of an organism's environment:

1.Abiotic environment - the set of physical factors (e.g. temperature, amount of sunlight, amount of water, pH, etc.) an organism must contend with.

2.Biotic environment - the other interacting organisms; these interactions may be either positive (e.g. prey items, mutualists) or negative (e.g., predators, competitors); the biotic environment for an individual usually is composed both of organisms of the same species as the individual (intraspecific interactions) and organisms of other species (interspecific interactions).

For both the abiotic and biotic environment, the environment shapes the organism in a variety of ways including behaviorally, developmentally and evolutionarily. Ecologists may concentrate on particular aspects of the effect of the environment on organisms and, therefore, there are behavioral ecologists and evolutionary ecologists for example.

Keep in mind that the "interactions" among organisms and their environment occur at the level of the individual: it is individual organisms that interact (e.g., they eat, they are eaten, they release oxygen, they mate) with their environment. Therefore, underlying all of ecology (and all of biology) is the evolutionary process, particularly natural selection. The traits and mechanisms that individual organisms have that allow them to interact with their environment in specific ways are a result of natural selection.

An important distinction we need to make here is the difference between the social/political/philosophical movement of environmentalism and the scientific discipline of ecology. Ecology is the study of natural processes of the planet just as much as any other area within biology. In fact, ecologists have, until relatively recently, been guilty of just the opposite: that is, NOT including humans as a natural part of the planet. Therefore, ecologists have historically used terms like "natural" to mean free of human activity and "unnatural" to mean influenced by humans. Ecologists are certainly concerned about the deterioration of the "environment" in the sense usually used by the popular media and this concern has given rise to the new field of conservation biology. However, you need to remember that ecology is a branch of biology and as such is a scientific area of inquiry aimed at understanding the interactions between living organisms and their environment.


The hierarchical nature of ecology - Ecologists tend to work toward the upper end of the "molecules to biosphere" biological hierarchy. In general, ecology deals with organisms, populations, communities and ecosystems. Hence, there are population ecologists, community ecologists, ecosystem ecologists, etc. As in most categorical systems, not everyone fits neatly into these discrete bins so that scientists may work across different levels simultaneously or at different times.

In Biology 11, we will examine the main levels of ecology which are:

1. Population ecology 2.Community ecology 3.Ecosystem ecology

These three levels are obviously related to each other (e.g., populations are the "building blocks" for ecological community) and hopefully you will see these connections. We will start with ecosystems which provides a framework for understanding the population and community processes we will examine later.

Ecosystem ecology - An ecosystem is defined as all the organisms of a given area combined with the abiotic conditions of that area. Often, an ecosystem is defined as an ecological community combined with the abiotic conditions of that area (a set of interacting populations is an ecological community). Ecosystem ecology is built around two fundamental ideas:

1.Energy flows through ecosystems and this flow occurs in only on direction

2.The matter that living organisms use cycles through ecosystems

A.The flow of energy through ecosystems - The ultimate source of energy for all (actually most) living organisms on the planet is the sun. How does this source of raw energy become usable to living organisms? The answer is through photosynthesis. Recall that photosynthesis is believed to have arisen more than 3 billion years ago and had an enormous impact on life on Earth due to the release and subsequent accumulation of oxygen in the atmosphere. Organisms that are able to photosynthesize -- they are able to directly convert light energy into chemical bond energy -- are termed autotrophic because they can produce their own energy molecules. (We generally think of plants as being photosynthetic, but there are many protists that are photosynthetic as well.) Written simply, photosynthesis is merely the production of energy-rich molecules like glucose (C6H12O6) from the necessary raw materials:

6CO2 + 6H2O + energy ==> C6H12O6 + 6O2

Read page 423 - 433 and complete review questions 1, 2, 3, 4, 5, 6 and 7

These energy-rich molecules can be stored until energy is needed by the photosynthetic organism; the above reaction is then run in the opposite direction to release the energy stored in the chemical bonds of the glucose molecule. This is the process of cellular respiration:

C6H12O6 + 6O2 ==> 6CO2 + 6H2O + energy

Read page 435 - 440 and complete review questions 10, 11 and 14

What about organisms that can't photosynthesize? They still need energy-rich molecules to run the machinery of their bodies; where do they get the needed energy from? They get it from organisms that can photosynthesize. Such organisms that must ingest other organisms or products of other organisms are called heterotrophs. So you should detect a pattern here: the photosynthesizers capture sunlight and convert it into a biologically-usable form; these organisms are then fed on by other organisms; this second group of organisms may be eaten by another group, etc. Because energy is needed to "run" biological systems, and ultimately, the only source of this energy is the sun, organisms like plants that can capture solar energy are essential to all other life on the planet. Photosynthetic organisms are therefore called primary producers because they are the ultimate source of ready-made, biologically-usable energy. Organisms that rely upon the primary producers, directly or indirectly, are called consumers.

We can then imagine a "chain of eating" where an herbivore eats a plant, a carnivore eats the herbivore, another carnivore eats the first carnivore, etc. These relationships are referred to as food chains by ecologists and describe how energy is passed from one organism to the next. The real world is usually more complex in that organisms may eat a variety of different types of foods (e.g., a heterotroph may eat both plants and animals) which produces more intricate food webs instead of chains. (a simplified food web is shown in Figure 16.8 Page 406 of your text)

Complete Review Questions 1, 2, 3, 5, 6, 7, 8

The basic idea of a food chain can be used to examine the flow of energy through an ecosystem by classifying the various organisms as to what trophic level they generally feed on. That is, are they autotrophic? Do they feed directly on plants? Do they usually feed on herbivorous animals? And so on. When we examine the amount of energy present in each of these trophic levels, we see that each level contains less total energy than the level that serves as the food for that level. So if we could stack the total amount of energy in each level one atop the other, we would get a pyramid shape known as a trophic or energy pyramid (Figure 16.15 Page 413): the base of the pyramid would be the primary producers and the top would be the top carnivores. This characteristic shape is a result of two very basic physical laws: the first and second laws of thermodynamics.

First law of thermodynamics: energy cannot be created or destroyed but it can be converted from one form into another.

Second law of thermodynamics: conversion of energy from one form into another always results in a reduction in the total amount of usable energy.

The First law clearly states that life on our planet is dependent on a continuous input of energy. On Earth, that input comes from the Sun. The Second law explains the shape of energy pyramids: as energy at one level is converted into a different form on the next highest level (e.g., grass is converted into wildebeest), that conversion process means some usable energy is lost. Hence, as the energy originally captured by the primary producers is passed up each successive level, each level has less total energy than the one below it. The rule of thumb for efficiency of energy transfer from one level to the next is about 10%, meaning only about 10% of the energy in one level can be effectively transferred to the next highest level. Knowledge of this inefficiency also should help you predict that there may be a limited number of trophic levels possible in any ecosystem: if only 10% is transferred from one level to the next,to support any type of organism. This is why there are only generally only a few (i.e., up to perhaps five) trophic levels in any ecosystem.

It should not be surprising that different types of ecosystems and different parts of the Earth can differ greatly in terms of how much biologically-available energy there is. In other words, areas and ecosystems will differ in terms of how much primary productivity is available to higher trophic levels. Ecologists distinguish between gross primary productivity (GPP; a measure of the total amount of solar energy captured) and net primary productivity (NPP; a measure of the amount of solar energy actually available to consumer trophic levels). In shorthand notation:

GPP - energy used in respiration by the primary producers = NPP

Hence, a comparison of the NPP of different ecosystems gives a measure of how productive they are which can have consequences for the characteristics of the various consumer levels.

Complete Review Questions 9, 10, 11, 12, 13

Read Monocultures on Page 414: According to Edward Wilson what are the five most important plant species to humans? What are monocultures?

Read Pesticides on Page 416: What are pesticides designed to do? DDT was developed with what specific use in mind? What were the positive and negative effects of Dieldrin spraying on the island north of Borneo circa 1955?

Read Biological Amplification on Page 417: Describe the process of biological amplification. Why has the fact that DDT was banned in Canada and the USA not eliminated the the problem for Canadian peregrine falcons?

B.The cycling of matter through communities - Unlike energy, the amount of matter available to living organisms on Earth is essentially fixed: the Sun constantly provides more energy to the Earth, but there is very little matter that is added to or lost from the planet. Therefore, the other major interaction of ecological communities with the physical environment is through biogeochemical cycles. A number of different types of matter cycle through ecosystems; the carbon cycle, the nitrogen cycle, the hydrologic (water) cycle and the phosphorous cycle are the most important and best known. We will examine the first three of these.



All cycles have the same basic structure: 1) a reservoir which represents a source that is directly unavailable to biological organisms, 2) an exchange pool which is the source from which organisms do draw from and 3) the biotic component which are the living organisms:

1.Hydrologic cycle

2.Carbon cycle

3.Nitrogen cycle

Population ecology

Introduction - The ecological unit known as the population is one that is familiar; we are used to thinking about organisms as being grouped into identifiable populations (e.g., the population of Minneapolis, of Minnesota, of the United States, etc.). Population ecologists are generally interested in examining the factors that influence the size of populations through time. Natural populations are often quite dynamic in that their size may change quite dramatically through time. Just as individual organisms can have different attributes (e.g., growth rates, body size,etc.), populations do too. Some of the questions that population ecologists ask include:

What determines the size of a population in a given area?

How do the characteristics of a population change as the population changes (e.g., how does the growth rate of a population change as population size changes?)

How do population characteristics vary among different populations of a species that are experiencing different environments (e.g., how do populations across the entire range of a species differ?)

How do population-level characteristics differ across different species (e.g, why do bacterial populations grow faster than elephant populations?)

Our definition of a population is a simple one: a group of organisms of a single species inhabiting the same geographic area. With this definition in mind, we can start to examine some of the different properties of populations. These population-level characteristics will also have great importance for our next topic, community ecology, since communities are composed of interacting populations.

I.Population dispersion - It should be no surprise that the individuals of a population can be distributed across the area occupied by that population in different ways. Ecologists identify three basic patterns of dispersion: 1) clumped, 2) uniform and 3) random. The dispersion pattern exhibited by a population can be determined by the environment (e.g., clumped resources) or due to characteristics of the species itself (e.g., territoriality).

Read DISTRIBUTIONS OF POPULATIONS and CHAOS THEORY AND BIOLOGICAL SYSTEMS... page 504 - 506: Explain in your own words chaos theory.

II.Population growth - Two of the major (and inter-related) features of populations that are of interest to ecologists are their size and their growth rate. Population size is important because some types of organisms are naturally characterized by large populations while other species are not. Why is this? What effect can this have on community ecology? Similarly, some types of organisms have the capacity for rapid population growth while others do not. This as well can have an important influence on the structure of ecological communities. Plus, both the expected size of populations as well as how fast they can grow can have enormous implications for managing introduced (exotic) species and for preserving endangered populations.

Exponential population growth - If we have a certain number of individuals of a species, how large will the population be at some time in the future?

[Note: We'll often be referring to time in a relative sense where time t + 1 will be one time unit in the future and time t is the current time. The time units can be any relevant units (e.g., minutes, hours, days, years) depending on the organism.]

What factors determine how fast this population grows? This is very much akin to asking how fast you can expect your money to grow in a savings account: you deposit some money in an account and you want to know how much you will have at some later time. What will determine how much money you'll have later? The answer is

1.The amount of money you initially deposit (your principal)

2.The interest rate (the rate at which your money will grow).

Notice that the interest rate is the amount that each dollar will contribute; hence, we could refer to the interest rate as a "per dollar’ rate.

In the simplest situation, suppose your money is in a savings account that has a 3% annual interest rate and you deposit $1,000 at the beginning of the year. How much money will be added to your principal during the first year?: 0.03 x $1,000 = $30. How much will be added during the second year?: 0.03 x $1,030 = $30.90. Therefore, after two years, you should have $1,000 + $30 + $30.90 = $1,060.90 in your account. In both cases, we have determined the rate (number of dollars added per unit time) at which your money is expected to grow each year:

Year one: dollars added/time = $30/year

Year two: dollars added/time = $30.90/year

We can imagine that natural populations grow in essentially the same way: we have some starting number of individuals (the "principal’) and each individual will on average contribute some number of offspring to the population (the "interest rate’). (Note: There is, of course, sometimes great variability among individuals in a population in terms of how many offspring they produce. Assume that we will be using a population "interest rate" that is an average value across all population members.) The starting number of individuals is usually referred to as the starting population size (Nt) and the average contribution made by each population member is called the per capita rate of increase (r) where r is merely the difference between the per capita birth rate (b) and the per capita death rate (d) (i.e., r = b - d). Therefore, we can predict the rate at which individuals are added to the population:

Rate of population increase from time t to time t+1 = Ntr or, N/t = rN

Instead of examining population growth over a set time period, we can put the growth equation in a more general form (by use of differential calculus) and express it as instantaneous population growth (i.e., at any instant in time, how fast is the population growing?): instantaneous growth rate of the population: dN/dt = rN

Therefore, the rate at which individuals are added to the population is a function only of the current population size (N) and how much each of those existing individuals contributes to the population (r). As population size increases, therefore, the rate at which the population is expected to grow should also increase. This type of growth that is limited only by existing population size and per capita rate of increase is known as exponential population growth and is characterized by a J-shaped population growth curve when population size is plotted versus time. Exponential growth means that the population is growing at an ever increasing rate. Notice that:

Population growth is positive (the population is growing) when r > 0 (i.e., per capita birth rate is > per capita death rate)

Population growth is negative when r < 0

There is no change in population size when r = 0

Logistic population growth - Do populations grow according to the equation dN/dt = rN? In other words, do they grow exponentially? The answer is that all populations have the capacity for exponential population growth, but most do not exhibit exponential growth for very long. Why? Recall that this is the observation that Darwin made that led him to propose his mechanism for evolution, natural selection. The idea is, basically, that there will inevitably be some resource that limits individual reproductive output (setting up reproductive competition among individuals leading to natural selection) which ultimately means that population growth will ultimately be limited by some resource. This upper limit on population growth is encompassed within the idea of carrying capacity (K): a given environment (i.e., set of resources) will only be able to maintain a certain population level of a certain species. (Note: the carrying capacity for a species in a particular environment is not the maximum population size possible; it is the population size that can be maintained long-term.) Therefore, a population is expected to "level off’ at the carrying capacity of its environment instead of continuing to grow forever.

Carrying capacity can be easily integrated into the basic exponential growth equation by adding the term [(K - N)/K] to produce the logistic population growth equation:

dN/dt = rN[(K-N)/K]

When population size (N) is small relative to the carrying capacity (K), then the term [(K - N)/K] is approximately K/K or 1 and the population is approximating exponential growth. At some point, however, N will become large enough that the term [(K - N)/K] will start to slow population growth until N = K and [(K - N)/K] now equals 0. At this point, the population growth rate is 0 and the population size is at the carrying capacity of the environment. Logistic population growth is characterized by an S-shaped population growth curve (also called a sigmoidal curve).



Constraints on population growth - The logistic population growth equation has built into it a factor -- [(K - N)/K] -- that causes population growth to slow as carrying capacity is approached. This means that as the population size for a given area -- i.e., the density of the population -- increases, the rate of growth is slowed more and more. Anything that operates in such a way that its effects are felt more intensely as density increases is known as a density-dependent factor. The classic density dependent factor is competition: the effects felt by a population approaching carrying capacity is intraspecific competition for limiting resources. There are also density-independent factors that affect population growth. The difference between the two depends on whether their effects are dependent on the density of the population: with density-dependent factors a larger proportion of the population is affected as population density increases; with density-independent factors, the same proportion of the population is affected regardless of population density. Storms, fires and other physical catastrophes can often act in density-independent ways while biotic interactions (e.g., competition, predation, parasitism, disease outbreaks) are generally density-dependent forces.

r- versus K-selected species - Obviously, not all populations grow in the same way, particularly at the same rates and much of these differences can be tied to the types of environments in which the populations reside. For example, there are some environmental conditions that are very ephemeral -- they do not last long -- and when conditions are right for breeding, an individual must act quickly to take advantage of these conditions or the opportunity will be lost. Other organisms live in much more stable and predictable environments where certain conditions may occur on a known schedule. Natural selection may consequently select for different types of traits in organisms living in these two general types of environments.

Species in unstable environments where conditions favorable for reproduction are infrequent may exhibit traits that allow them to persist until good times roll around and where they can produce large numbers of offspring during these rare favorable times. Such environments are characterized by a high relative importance of density- independent factors and a low relative importance of density-dependent factors and a phenotype that maximizes reproductive output is often favored. Consequently, these types of organisms are often referred to as r-selected species because genetic representation can be enhanced by maximizing the number of offspring produced.

In contrast, species in stable, predictable environments may have enhanced reproductive success by possessing traits that allow them a competitive advantage for resources. Populations of such species are often at or near the carrying capacity of their environment where density-dependent factors are important and these species are generally classified as K-selected species. For these organisms, producing fewer offspring but investing more heavily in each to give them a competitive advantage is the general evolutionary strategy that is favored. On the following page is a list of characteristics associated with these two idealized types of species.






Maturation time






Death rate

Often high

Usually low

Number of offspring per reproductive episode



Number of reproductive episodes per lifetime

Usually one

Often several

Time of first reproduction

Early in life

Late in life

Size of offspring or eggs



Parental care


Often extensive

Another feature that is often associated with the characteristics in the table above is the pattern of survivorship exhibited by individuals of a population. Survivorship is the proportion of individuals in a population that survive to each age. Ecologists have classified organisms according to three basic survivorship patterns when plotting the number of survivors against each age class:

Type I: high juvenile survivorship and high mortality at old age classes (e.g., humans)

Type II: constant rate of mortality across all age classes (e.g., some species of birds)

Type III: high juvenile mortality but then high probability of long life afterwards (e.g., oysters, many insects)

Keep in mind that both the r-selected vs. K-selected dichotomy and the three types of survivorship curves are generalizations and are used only to classify organisms on a broad level.

Complications to population growth - We have been treating populations as if all individuals within a population are essentially the same. Obviously, individuals within populations can differ considerably among themselves in terms of age, sex, reproductive status, health and so on. All of these factors (and many more) can have an impact on the growth characteristics of a population. For example, a population that is initially composed mostly of males may be expected to grow much more slowly than one that has a greater proportion of females. Hence, the sex ratio of a population can have a direct effect on its growth characteristics.

Another factor that can have an influence on the growth of a population is the age structure of the population where age structure refers to the proportion of the population that is in each age class, where age class is an interval of time appropriate for the species of interest. It should be no surprise that a population that is composed of mostly older, perhaps post-reproductive, individuals can be expected to grow more slowly than a population that contains mostly younger individuals that are just starting to reproduce.

Community ecology

Introduction - Ecological communities are composed of interacting populations: your text's definition is "an association of different species living together in a defined habitat with some degree of interdependence." Although somewhat verbose, this definition does capture the essential concept of interaction among the populations. Specifically, community ecologists are interested in the interspecific interactions that occur among the constituent species of a community. So a biological community is different from an ecosystem because the interactions of interest now are strictly between the individual and its biotic environment.

Types of interspecific interactions - It may seem that there could be a nearly infinite number of types of interspecific interactions possible. In some sense this is true: not only can the degree and type of interaction vary depending on the species involved, but the interactions can also change from one community to the next. However, ecologists are able to classify all types of interspecific interactions into a relatively small number of categories. This classification is based on whether a specific interaction between members of separate species is beneficial, harmful or has no effect on the interacting individuals. If we label beneficial interactions (positive interactions) with a "+" sign, harmful interactions (negative interactions) with a "-" sign and interactions with no effect with a "0" (zero), then we can classify each type of interaction into a 3 x 3 matrix:

Therefore, there are only five (+,+; +,-; +,0; -,-; -,0) types of interspecific interactions that can occur within a community and the names of each are provided in the table above. Please keep in mind that the "+", "-" and "0" signs ultimately refer to the effects on the reproductive success -- fitness -- of the individual and therefore these interactions are both the products of natural selection as well as forces for adaptive change within populations.





Predation - Probably the easiest interaction to imagine occurring between two species is where one species uses the other as a resource, often as food. Such is the case with predation where one interactant (the predator) benefits by consuming the other individual (the prey) who obviously is harmed by the interaction. Examples of predation are very well-known to all of us (watch the Discovery channel for more than an hour and you're likely to see some film of predation "in action") and are clearly directly involved with the flow of energy through ecosystems (recall food webs). Secondary consumers are necessarily predators, securing the food resources they need by consuming other consumers.

Two questions that have long interested ecologists are 1) can predators control the size of prey populations? and 2) can prey populations, in turn, control predator population size? Each of these question generates two questions: 1) can predators or prey have an effect on the population of the other? and 2) can predators or prey actually control the population size (i.e., within particular limits) of the other? The answer to whether prey and predators can have an effect on the population size of the other is a definite yes. Many studies have shown that removal of predators or prey can directly affect the population size of the other (e.g., experimental removal/addition of predatory mites on prey mite populations). The effect of prey abundance on predators can be examined by looking at how prey density influences the population size of the predator but also the influence on the foraging (feeding) behavior of the predator. Not surprisingly, predator populations can respond to changes in prey density (e.g., population size). The increase in size of a predator population to growth of a prey population is known as the numerical response of a predator. Individual predators may also change how and on what they forage as the density of prey populations changes. The density-specific use of particular types of prey by a predator is known as the functional response of a predator. The actual effects on predator and prey populations by the action of the other is usually very complex in nature: each population is likely to be interacting with many other species (parasites, other predators/prey, competitors) and changes in each of these may have complex effects on all others (e.g., hare and lynx cycles are not simply due to direct responses by each population to changes in the other).

Because of the presumed strong selective force predators and prey represent for each other, there are myriad examples of how predation has selected for traits in both predators and prey as a result of the interaction. Some predators have become very efficient at capturing prey (e.g., running speed and anatomical structure of cheetahs; "lures" of anglerfish and turtles; sit and wait behavior of some predators) and prey have evolved traits to escape predation. Of the latter category, physical defense, behavioral responses, cryptic coloration, and aposematic coloration (with associated defenses) are the best known classes of adaptation. The last category also has allowed for the evolution of two types of mimicry: Müllerian mimicry where two to several dangerous or unpalatable species resemble each other, and Batesian mimicry where harmless or palatable species mimic a dangerous or unpalatable species.



Parasitism - Parasitism generally is different from predation in that parasites often do not quickly kill (if at all) the organisms (the host) they use; parasites are also usually smaller than the host organisms. Herbivory can be considered either predation (if the herbivore kills the plant quickly) or parasitism if only parts of the plant are eaten and the plant remains alive. Parasites may live within the host (endoparasites) or live on the outside of the hosts (ectoparasites). Because parasites and hosts generally live in close contact with each other, parasitism is classified as a form of symbiosis, a close interaction between two or more species. (Mutualism and commensalism -- see below -- are also considered symbiotic relationships.) Due to this close relationship, the degree to which both host and parasite can be a selective force on the other is magnified. Consequently, coevolution -- the evolutionary response of two or more closely interacting species -- has been studied in many host-parasite relationships.

Competition - Competition is the negative effect of two or more individuals using the same limited resource. Note that competition is a "-,-" interaction in that both interactants are harmed by the interaction. We have already seen that competition can occur between individuals of the same species (intraspecific competition) and is the explanation for the leveling off of populations at carrying capacity. Keep in mind that "competition" doesn't necessarily mean that organisms are fighting with each other for access to the resources, although that can happen, and is referred to as interference competition. More commonly (and usually more importantly), it is merely the use of a common limited resource (exploitative competition) that negatively affects both species.

When discussing interspecific competition, we are considering the effects of two (or more) species using some of the same resources. To aid in understanding these interactions, it is useful to introduce the concept of the niche. The niche of the organism (also referred to as the ecological niche) is the sum total of all resources used and characteristics of the environment tolerated by the organism. In shorthand, if the habitat is the organism address, the niche is the organism's occupation or role in the community. Ecologists generally contrast the fundamental niche of an organism (the entire range of ecological conditions under which the organism could be found) with its realized niche (the ecological conditions the organism is actually found under in nature). Because the niche of an organism is in large measure determined by the resources used by that organism, it is a useful concept for examining the effects of interspecific competition. Basically, we can estimate how much the niches of two species overlap to get a measure of how strong the competitive effect is between species.

There is a general belief in community ecology that two species cannot occupy the same niche in the same community and still coexist; one species will eventually exclude the other from the community. This is the basis of the competitive exclusion principle that was first proposed by G.F. Gause based on his work with competing species of Paramecium. Competition among species is believed to lead to both resource partitioning (selection favors individuals in one or more competing species that can use slightly different resources; e.g., warblers) and character displacement (selection favors individuals in one or more of the competing species that have different traits that affect how they use the resources; e.g., Galapagos finches).

Interestingly, the effects of interspecific competition can be influenced by both the physical and biotic environment. For example, in a classic experiment, T. Park demonstrated that the outcome of competition between two species of flour beetles (Tribolium spp.) can be reversed by changing the temperature and humidity characteristics of the environment. Hence, competitive effects are not merely always the same; they depend on the other characteristics of the environment. Competition also does not have to be equal in terms of its effects on each competing species: competition may be symmetrical (equal effect on both species), but is more often asymmetrical in that it has a greater effect on one of the competing species. It is generally held that the reason exotic (introduced) organisms are often able to reach extraordinary population sizes is because they are freed from the effects of natural competitors (as well as predators and parasites).

Mutualism - Opposite to competition, mutualism is a "+,+" interaction such that both interacting individuals are benefitted by the presence of the other. Mutualists generally live in close contact with each other and mutualism is therefore a symbiotic relationship. The effects and evolution of mutualistic interactions are fairly straightforward to understand: individuals of both species are benefitted so that individuals that do partake of interaction will have higher fitness than individuals that do not. Hence, the "mutualistic" trait should spread in both populations. Classic examples include ant-acacia interactions, plant-pollinator relationships, mycorrhizal fungi associated with plant roots, and nitrogen fixing bacteria (Rhizobium) in root nodules of many legumes. It is believed that mutualistic interactions may also evolve from parasite-host relationships: there may be selection on the parasite population to decrease its negative effects if keeping the host alive and healthy bestows fitness benefits to the parasite. The (apparent) mutualistic interaction between some fungi and some algae -- together known as lichen -- may have started this way. In fact, it is not clear whether lichen actually represent a mutualistic interaction in all cases.

Commensalism - Commensalism, a "+,0" interaction, exists where an individual of one species benefits from the presence or activity of another species, but individuals of the second species are not affected. Although commensalism may theoretically be common, actually demonstrating no effect of one species on another is very difficult in nature (e.g., clownfish anemone interaction; remora-shark interaction). Possible examples of commensalism include the growth of epiphytic plants on trees; ant-following birds; cattle egrets associated with large herbivores.

Amensalism - Interactions where one individual is harmed but the other is not affected may be common in nature and generally are not the result of coevolution between the two species. It should not be difficult to imagine that one species may grow or behave in a particular way or produce certain waste products that harm other species but who in turn have no effect on the other species. For example, trees growing next to small understory plants may not be affected by the presence of the small plants but the understory plants may be harmed due to competition for soil resources and the effects of shading by the trees.

Because amensalistic interactions are generally not the products of selective forces promoting the interaction (also potentially true of commensalism), they are often of substantially lesser interest to ecologists (although the interactions may have a significant impact on particular individuals and populations). Generally, predation, parasitism, competition and mutualism are the interspecific interactions of primary concern to community ecologists.

The complexity of interspecific interactions in real communities - Communities (except perhaps in experimental, laboratory conditions) do not consist of only two interacting species. Instead, there are a multitude of different interactions and any one species may have many interactions of all sorts with many other species. This often makes it very difficult to determine what is really important in determining not only the size and growth rates of populations but also the overall structure of ecological communities. Examination of research preformed in the intertidal community of the Pacific northwest should demonstrate the potential complexity of these interactions.

The intertidal is the area of ocean beaches that daily experiences fluctuations in water level due to the action of the tides. A particular community resides here and the intertidal area studied by R. Paine included a species of mussel (Mytilus), a species of barnacle (Balanus) and a starfish (Pisaster) that fed on the mussels. When Pisaster is excluded from an area of the intertidal, the mussel competitively excluded the barnacle by taking over all available rock surface. However, when Pisaster was present, the mussel and the barnacle were able to coexist. Hence, by depressing (through predation) the population of the mussel (the better competitor for space) below a level that it could exclude the barnacle, Pisaster allowed coexistence of two competing species. This is an example of how particular interspecific interactions can influence other interspecific interactions and affect the structure of an entire community. Because of the overall importance of Pisaster in the intertidal community, Pisaster is termed a keystone species (here, a keystone predator) in this community.

Community change - Ecological communities are not static entities but rather change through time. This should make sense because the building blocks -- populations -- certainly are expected to change through time as the resources they rely on change. As populations change, interspecific interactions may change in terms of their effect on the structure of a community.

The process by which ecological communities change is referred to as succession: the gradual sequential changes in composition of an ecological community. Frederick E. Clements, an american ecologist pioneered the use of the quadrat to study and identify the different species that make up a community. By clearing a measured area of all its vegetation, he showed that in each geographical zone, plants succeed each other in a particular sequence, developing toward a "climax" vegetation specific to that community. Succession has historically been viewed as a developmental process that communities go through much like the development that individuals go through from infancy to maturity. In fact, it has been proposed that communities develop like a "superorganism" in that groups of species interact closely much like components (e.g., organs) of individual organisms. However, more recent studies seem to indicate that such tight, cohesive interaction among species as a community develops does not really occur. Instead, the individualistic model of community development seems to be supported where species are added or lost from communities based on the individual requirements of the species and as the community changes.

We can identify two major patterns of succession:

A.Primary succession - Community development in an area devoid of life (e.g., colonization of sand dunes, lava flows, bare rock exposed by glaciers)

B.Secondary succession - Community development in an area where an existing community is in place but is disturbed by some process

Still inherent in our concept of succession is the idea that there is some predictability in the process. This should also make some sense. Think about primary succession: as the first organisms to invade alter the existing conditions, this may allow other species to invade, etc. Traditionally, the successional stages (or seres) are arranged from the pioneer community to the climax community. The climax community was originally thought to be a stable endpoint of the successional process, and to some extent this does appear to be true for many communities, although there can be a great deal of natural change that occurs within mature communities. (see Figure 20.29 in Nelson)

There are also predictable, general patterns that can be observed during community development. One of the major changes is the relative prevalence of r-selected species in early stages of succession and the relative dominance of K-selected species in later stages. Below are some other major differences that are often observed in comparing early versus late successional stages for a given area.

Early successional stage

Late successional stage







Energy use

Less efficient

More efficient

Nutrient cycling


More efficient

Species diversity



Stability of populations



Stability of community composition




Types of communities - Not surprisingly, different geographic areas support different types of communities. For example, the tropical rain forests of the world are located not far from the equator. Tropical rain forests have particular characteristics that distinguish them from other types of communities. These distinctive features of communities allow ecologists to categorize communities into different types. The largest, most inclusive community classification is the biome: a major ecological community classification (based on dominant vegetation and adaptations to the particular environment). The "environment" here primarily refers to the abiotic environment in general and temperature and precipitation specifically.

You should know the names of all 10 types of biomes, their general distribution and their general temperature and precipitation characteristics as indicated by the figure below.














Behavioral ecology

Costs of Grouping: INCREASED feeding competition

INCREASED risk of parasitic infection

Benefits of Grouping:


A. Dilution effect - reduces individual risk of being ATTACKED

B. Selfish herd - reduces own risk of being SELECTED

C. Confusion effect - reduces chance that attack will SUCCEED

D. Corporate vigilance - reduces chance attack will SUCCEED; enables individuals to devote more time to feeding


A. Improves SUCCESS rate against large or difficult prey

B. Improves ACCESS to prey through group territoriality