Ecology

Ecology is the study of how organisms relate to their natural environments. The two principal concerns of ecologists are the distribution and abundance of organisms. Why are animals, plants, and other organisms found where they are, and why are some common and others rare? These questions are rooted in the theory of evolution. In fact, it is difficult (and not often worthwhile) to separate modern ecological matters from the concerns of evolutionary biologists. Ecology can be divided according to several levels of organization—the individual organism, the population, the community, and the ecosystem.

Environment and Natural Selection

An ecologist views organisms as consequences of past natural selection brought about by their environments. That is, each organism represents an array of adaptations that can provide insight into the environmental pressures that resulted in its present form. Adaptations of organisms are also revealed by other features, such as the range of temperature an organism can tolerate, the amount of moisture it requires, or the variety of food it can exploit. Food and space for living are considered resources. Factors such as temperature, light, and moisture are conditions which determine the rate of resource utilization. When ecologists have discovered the full range of resources and conditions necessary for an organism’s existence, they have discovered its niche.

Many species, such as many insects and plants, have a large reproductive output. This compensates for high mortality imposed by natural selection. Other species, such as large mammals and birds, have fewer offspring. Many of these animals care for their young, thus increasing the chances that their offspring will survive to reproduce. These are two different strategies for success, based upon the principle that organisms have a finite energy budget. Energy acquired from food (animals) or sunlight (plants) must be partitioned among growth, maintenance, and reproduction. The greater the energy allocated to the care of offspring, for example, the fewer the offspring that can be produced.

The concept of an energy budget is a key to understanding evolutionary strategies of organisms, as well as the energetics of ecosystems. The amount of energy fixed and stored by an organism is called net production; this is the energy used for growth and reproduction. Net production is the difference between gross production (the amount of energy assimilated) and respiration (metabolic maintenance cost). The greater the respiration, the less energy will be left over for growth and reproduction. Endothermic animals, which physiologically regulate their body heat (mammals and birds), have a very high respiration rate relative to ectotherms (reptiles, amphibians, fish, and invertebrates), which cannot. Among endotherms, smaller animals have higher respiration rates than larger ones because the ratio of body surface area (the area over which heat is exchanged with the environment) to volume (the size of the “furnace”) decreases with increasing body size.

Demography and Population Regulation

Although single organisms can be studied regarding adaptations, in nature, most organisms exist in populations rather than as individuals. Some organisms reproduce asexually (that is, by forming clones) so that a single individual may spawn an entire population of genetically identical individuals. Populations of sexually reproducing organisms, however, have the property of genetic variability since not all individuals are identical. That is, members of a population have slightly different niches and will, therefore, not all be equally capable of living in a given environment. This is the property upon which Charles Darwin’s theory of natural selection depends: Because not all individuals are identical, some will have greater fitness than others. Those with superior fitness will reproduce in greater numbers and, therefore, will contribute more genes to successive generations. In nature, many species consist of populations occupying more than a single habitat. This constitutes a buffer against extinction: If one habitat is destroyed, the species will not become extinct because it exists in other habitats.

Two dynamic features of populations are growth and regulation. Growth is simply the difference between birth and death rates, which can be positive (growing), negative (declining), or zero (in equilibrium). Every species has a genetic capacity for exponential (continuously accelerating) increase, which will express itself to varying degrees depending on environmental conditions: A population in its ideal environment will express this capacity more nearly than one in a less favorable environment. The rate of growth of a population is affected by its age structure—the proportion of individuals of different ages. For example, a population which is growing rapidly will have a higher proportion of juvenile individuals than one which is growing more slowly.

Populations may be regulated (so that they have equal birth and death rates) by many factors, all of which are sensitive to changes in population size. A population may be regulated by competition among its members for the resource that is in shortest supply (limiting). The largest population that can be sustained by the available resources is called the carrying capacity of the environment. A population of rodents, for example, might be limited by its food supply such that as the population grows and food runs out, the reproductive rate declines. Thus, the effect of food on population growth depends upon the population size relative to the limiting resource. Similarly, parasites that cause disease spread faster in large, dense populations than in smaller, more diffuse ones. Predators can also regulate populations of their prey by responding to changes in prey availability. Climate and catastrophic events such as storms may severely affect populations, but their effect is not dependent upon density and is thus not considered regulatory.

Interactions Between Species

Competition occurs between, as well as within, species. Two species are said to be in competition with each other if and only if they share a resource that is in short supply. If, however, they merely share a resource that is plentiful, then they are not really competing for it. Competition is thought to be a major force in determining how many species can coexist in natural communities. There are several alternative hypotheses, however, which involve such factors as evolutionary time, productivity (the energy base for a community), heterogeneity of the habitat, and physical harshness of the environment.

Predator-prey interactions are those in which the predator benefits from killing and consuming its prey. These differ from most parasite-host interactions in that parasites usually do not kill their hosts (a form of suicide for creatures that live inside other creatures). Similarly, most plant-eating animals (herbivores) do not kill the plants on which they feed. Many ecologists classify herbivores as parasites for this reason. There are exceptions, such as birds and rodents that eat seeds, and these can be classified as legitimate predator-prey interactions. Predators can influence the number of species in a community by affecting competition among their prey: If populations of competing species are lowered by predators so that they are below their carrying capacities, then there may be enough resources to support colonization by new species.

In many cases, the interaction between two species is mutually beneficial. Mutualism is often thought to arise as a result of closely linked evolutionary histories (coevolution) of different species. Termites harbor protozoans in their guts that produce an enzyme which can break down cellulose in wood. The protozoans thus are provided with a habitat, and termites can derive nourishment from wood. Some acacia trees in the tropics have hollow thorns which provide a habitat for ants. In return, the ants defend the trees from other insects which would otherwise damage or defoliate them.

Communities of organisms are composed of many populations that may interact with one another in a variety of ways: predation, competition, mutualism, parasitism, and so on. The composition of communities change over time through the process of succession. In terrestrial communities, bare rock may be weathered and broken down by bacteria and other organisms until it becomes soil. Plants can then invade and colonize this newly formed soil, which in turn provides food and habitat for animals. The developing community goes through a series of stages, the nature of which depends on local climatic conditions, until it reaches a kind of equilibrium. Often, this equilibrium stage, called climax, is a mature forest. Aquatic succession essentially is a process of becoming a terrestrial community. The basin of a lake, for example, will gradually be filled with silt from terrestrial runoff and accumulated dead organic material from populations of organisms within the lake itself.

Ecosystems

Ecosystems consist of several trophic levels, or levels at which energy is acquired: primary producers, consumers, and decomposers. Primary producers are green plants that capture solar energy and transform it, through the process of photosynthesis, into chemical energy. Organisms that eat plants (herbivores) or animals (carnivores) to obtain their energy are collectively called consumers. Decomposers are those consumers, such as bacteria and fungi, that obtain energy by breaking down dead bodies of plants and animals. These trophic levels are linked together into a structure called a food web, in which energy is transferred from primary producers to consumers and decomposers until finally all is lost as heat. Each transfer of energy entails a loss (as heat) of at least 90 percent, which means that the total amount of energy available to carnivores in an ecosystem is substantially less than that available to herbivores.

As with individual organisms, ecosystems and their trophic levels have energy budgets. The net production of one trophic level is available to the next-higher trophic level as biomass (mass of biological material). Plants have higher net productivity (rates of production) than animals because their metabolic maintenance cost is lower relative to gross productivity; herbivores often have higher net productivity than predators for the same reason. For the community as a whole, net productivity is highest during early successional stages since biomass is being added more rapidly than later on when the community is closer to climax equilibrium.

In contrast to the unidirectional flow of energy, materials are conserved and recycled from dead organisms by decomposers to support productivity at higher trophic levels. Carbon, water, and mineral nutrients required for plant growth are cycled through various organisms within an ecosystem. Materials and energy are also exchanged among ecosystems: There is no such thing in nature as a “closed” ecosystem that is entirely self-contained.

Descriptive, Experimental, and Mathematical Ecology

The science of ecology is necessarily more broadly based than most biological disciplines. Consequently, there is more than one approach to it. Ecological studies fall into three categories—descriptive, experimental, and mathematical.

Descriptive ecology is concerned with describing natural history, usually in qualitative terms. The study of adaptations, for example, is descriptive in that one can measure the present “value” of an adaptive feature, but one can only conjecture as to the history of natural selection that was responsible for it. On the other hand, some patterns are discernible in nature for which hypotheses can be constructed and tested by statistical inference. For example, the spatial distribution (dispersion) of birds on an island may be random, indicating no biological interaction among them. If the birds are more evenly spaced (uniform dispersion) than predicted, assuming randomness, however, then it might be inferred that the birds are competing for space; they are exhibiting territorial exclusion of one another. Such “natural experiments,” as they are called, depend heavily upon the careful design of statistical tests.

Experimental ecology is no different from any other experimental discipline; hypotheses are constructed from observations of nature, controlled experiments are designed to test them, and conclusions are drawn from the results of the experiments. The basic laboratory for an ecologist is the field. Experiments in the field are difficult because it is difficult to isolate and manipulate variable factors one at a time, which is a requisite for any good experiment in science. A common experiment that is performed to test for resource limitation in an organism is enhancement of that resource. If food, for example, is thought to be in short supply (implying competition), one section of the habitat is provided more food than is already present; another section is left alone as a control. If survivorship, growth, or reproductive output is higher in the enhanced portion of the habitat than in the control area, the researcher may infer that the organisms therein were food-limited. Alternatively, an ecologist might have decreased the density of organisms in one portion of the habitat, which might seem equivalent to increasing the food supply for the remaining organisms, except that it represents a change in population density as well. Therefore, this second design will not allow the researcher to differentiate between the possibly separate effects of food level and simple population density on organisms in the habitat.

Mathematical ecology relies heavily upon computers to generate models of nature. A model is simply a formalized, quantitative set of hypotheses constructed from sets of assumptions of how things happen in nature. A model of population growth might contain assumptions about the age structure of a population, its genetic capacity for increase, and the average rate of resource utilization by its members. By changing these assumptions, scientists can cause the model population to behave in different ways over time. The utility of such modeling is limited to the accuracy of the assumptions employed.

Modern ecology is concerned with integrating these approaches, all of which have the common goal of predicting the way nature will behave in the future, based upon how it behaves in the present. Description of natural history leads to hypotheses that can be tested experimentally, which in turn may allow the construction of realistic mathematical (quantitative) models of how nature works.

All-Encompassing Nature

People have historically viewed nature as an adversary. The “conquest of nature” traditionally meant human encroachment on natural ecosystems, usually without benefit of predictive knowledge. Environmental problems like pollution, species extinction, and overpopulation may be viewed as experiments performed on a grand scale without appropriate controls. The problem with such experiments is that the outcomes might be irreversible. A major lesson of ecology is that humans are not separate from nature, but are constrained by the same principles as other organisms on Earth. One object of ecology, then, is to learn these principles and apply them to Earth’s ecosystem.

Populations that are not regulated by predators, disease, or food limitation grow exponentially. The human population, on a global scale, grows this way. All the wars and famines in history have scarcely made a dent in this growth pattern. Humankind has yet to identify its carrying capacity on a global scale, although regional famines provided insight into what happens when local carrying capacity is exceeded. The human carrying capacity needs to be defined in realistic ecological terms, and constraints like energy, food, and space must be incorporated into the calculations. For example, knowledge of energy flow—there is more energy at the bottom of a food web (producers) than at successively higher trophic levels (consumers)—means that more people could be supported as herbivores than as carnivores. This area of ecological study was more prevalent than ever in early 2023, as Earth's population reached 8 billion. Some scientists warned the carrying capacity of the planet may not be able to support more than a few generations at this number.

The study of disease transmission, epidemiology, relies heavily on ecological principles. Population density, rates of migration among epidemic centers, physiological tolerance of the host, and rates of evolution of disease-causing parasites are all the subjects of ecological study.

An obvious application of ecological principles is conservation. Before habitats for endangered species can be set aside, for example, their ecological requirements, such as migratory routes, breeding, and feeding habits, must be known. This also applies to the introduction (intentional or accidental) of exotic species into habitats. History is filled with examples of introduced species that caused the extinction of native species. Application of ecological knowledge in a timely fashion, therefore, might prevent species from becoming endangered in the first place.

One of the greatest challenges of the twenty-first century is the global loss of habitats, particularly in the tropics, which contain most of Earth’s species of plants and animals. Species in the tropics have narrow niches, which means that they are more restricted in range and less tolerant of change than are many temperate species. Therefore, destruction of tropical habitats, such as rainforests, leads to rapid species extinction. These species are the potential sources of many pharmaceutically valuable drugs, and they represent a genetic record of millions of years of evolutionary history. Tropical rainforests are also prime sources of oxygen and act as a buffer against carbon dioxide accumulation in the atmosphere. Ecological knowledge of global carbon cycles permits the prediction that destruction of rainforests will have a profound impact on the quality of the air.

Principal Terms

Adaptation: A genetic (intrinsic) feature of an organism which, through natural selection, enhances its fitness in a given environment

Community: All the populations that exist in a given habitat

Ecosystem: The biological community in a given habitat, combined with all the physical properties of the environment in that habitat

Environment: All the forces and things external to an organism that directly affect it

Fitness: The contribution of an organism to future generations; the perpetuation of its genes through reproduction

Habitat: The place where an organism lives; for example, a pond or forest

Natural Selection: A change in the genetic makeup of a population as a result of different survival and reproduction rates (fitness) among its members

Niche: The role of an organism in its environment; the sum of all factors that define its existence (temperature, energy requirements, and so on)

Population: All the individuals in a habitat which are of the same species

Resource: A requirement for life, such as space for living, food (for animals), or light (for plants), not including conditions such as temperature or salinity

Bibliography

Begon, Michael, John L. Harper, and Colin R. Townsend. Ecology: Individuals, Populations, and Communities. 4th ed. Blackwell, 2005.

Carson, Rachel. Silent Spring. 1962. Mariner Books, 2022.

Elton, Charles. Animal Ecology. Franklin Classics, 2021.

Hutchinson, G. Evelyn. The Ecological Theater and the Evolutionary Play. Yale UP, 1969.

Krebs, Charles J. The Message of Ecology. Harper, 1987.

Molles, Manuel C., Jr. Ecology: Concepts and Applications. 9th ed. McGraw, 2022.

Morin, Peter Jay. Community Ecology. 2nd ed. Oxford University Press, 2013.

Pianka, Eric R. Evolutionary Ecology. 6th ed. Benjamin/Cummings, 2000.

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"What is Ecology?" The Ecological Society of America, www.esa.org/about/what-does-ecology-have-to-do-with-me. Accessed 24 Sept. 2024.