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Ecology is a field of biology that studies "the distribution and abundance of organisms and the interactions that determine distribution and abundance."[1] These interactions include both biotic (occurring between living individuals) and abiotic interactions (occurring between and individual and its environment). There are several subdisciplines of ecology that focus on different levels of organization and different types of interactions, such behavioral, disease, sensory, and functional ecology.

Ecology is often confused with environmentalism. While familiarity of the term "ecology" emerged with the growing environmental concerns of the 1960s and '70s, the two have separate meanings and goals. Ecology seeks to understand natural phenomena in the environment whereas environmentalism is a social and political movement.[2] Even though they are separate, the two are closely related, as environmentalism relies on data and evidence from ecologists to support their goals and ecologists apply their findings to conservation efforts and management strategies.

Organizational Complexity

The discipline of ecology is split up into several subfields, ranging in order of organizational complexity. The smallest field of ecology, is organismal ecology, focusing on the the adaptations and physiology of an individual. Population ecology is focused on the interactions that individuals of the same species have, affecting their population dynamics. Community ecology investigates how populations of different species interact and affect community composition. Ecosystem ecology incorporates the abiotic factors that influence individuals, populations, and communities. The largest scale of ecology studies the entire biosphere, specifically the processes that occur in it, how changes in climate and organismal abundance affect it, and reconstruct previous climates and ecosystems on Earth.

Though evolutionary biology is a separate field of biology, the two are tightly linked, each not fully explaining phenomena without the other. G. E. Hutchinson combined the two topics through the metaphor of an "evolutionary play in an ecological theater,"[3] and as T. G. Dobzhansky states, "nothing in evolution makes sense except in the light of evolution."[4] While evolution is still heavilly debated and studied, it is important to take it into account when studying each of the levels of organization and to make sense of adaptations, distributions, and life histories.

Population Ecology

Population ecology is concerned with the interactions between members of a single species. A population is defined as a group of potentially interacting individuals of the same species that occupy the same area and can interbreed, for sexually reproducing species.[5] The geographic area that defines a population can be distinct, such as an isolated lake or mountain peak, or it can be subjective, based upon the goal of the scientist studying the population.[6]

Interactions that occur in populations are said to be interspecific. The most prominent of these interactions is competition for resources, such as water, food, shelter, and mates. Competition can be strong, where individuals control large territories to secure resources, or it can be less strong, such as in eusocial species like humans and wolves.


Population dynamics is an important aspect of population ecology that studies how populations change in size and density. Two important growth models are the exponential and logistic growth curves. The exponential growth equation is as follows: Nt = N0ert, where N is the population size at time t, e is Euler's number, and r is the intrinsic growth rate. This model predicts a population continually growing faster and faster without bound. Exponential growth is rare in nature, but examples include bacteria and humans.

Most populations in nature do not grow exponentially, they reach a certain density threshold that limits growth, called the carrying capacity. Thomas Malthus described how at some point in time an expanding population must exceed supply of prerequisite natural resources, i.e., population increases exponentially resulting in increasing competition for means of subsistence, food, shelter, etc. This concept has been termed the "Struggle for Existence". Resources may be plentiful at small population sizes, so initially a population will appear to grow exponentially, however, as resources become less available due to competition, the population growth rate slows down. This is described the by the logistic growth model as follows: dN/dt = rN[(K - N) / K], where K is the carrying capacity. Once a population has reached its carrying capacity, it will often remain at that population size, with stochastic fluctuations. Most populations follow this model.


Modern approaches to population ecology have incorporated the concept of metapopulations. Inspired by the work of E. O. Wilson and R. H. Macarthur's work on island biogeography, metapopulations are important for discussing smaller populations that are connected to other nearby populations through colonization, emigration, and extinction. Certain populations can act as sources for the metapopulations whereas others act as sinks and cannot sustain their own populations. Metapopulations have important conservation concerns, as increased habitat fragmentation increases the liklihood of local extinction of a species. Prioritizing which "islands" in a metapopulation should be protected can help to preserve and maintain healthy populations of charismatic species, such as butterflies and birds.

Community Ecology

Community ecology incorporates all the populations of species that occur in the same area and interact.[7] The main research questions of community ecologists focus on interactions, niches, strucutre, composition, and assembly.

Similar to populations, communities may be difficult to define the boundaries for. Two models for community definition are proposed. F.E. Clements proposed that communities are distinct from eachother, with a set of species unique to each, while H. A. Gleason proposed that communities are not discrete, but continuous, with species prevalence melding communities together through an ecotone.[8][9] Even though both explanations are in opposition to each other, both are seen in nature. Discrete communities can occur where forests abruptly turn to agricultural land, whereas continuous communities can occur where a grassland gradually turns to a mature forest. The distinction between which model to use depends on the characterisitcs of the ecotone, ranging from abrupt (Clementonian) to gradual (Gleasonian).[10]

Ecological Niche
Interspecific Interactions
Positive Interactions
Negative Interactions
Community Structure and Composition
Community Assembly


  1. Begon, M., Harper, J. L., & Townsend, C. R. (2006). Introduction: Ecology and its Domain. In Ecology: From Individuals to Ecosystems (4th ed., pp. xi-xii). introduction, Blackwell.
  2. Institute of Food and Agricultural Sciences. (n.d.) Confusing Ecology with Environmentalism. Fort Lauderdale. University of Florida Extension.
  3. Hutchinson, G. E. (1973). The Ecological Theater and the evolutionary play. Yale Univ. Press.
  4. Dobzhansky, T. (1973). Nothing in biology makes sense except in the light of Evolution. The American Biology Teacher, 35(3), 125–129.
  5. Encyclopædia Britannica, inc. (n.d.). Population ecology. Encyclopædia Britannica. Retrieved March 9, 2022, from
  6. Begon, M., & Harper, J. L. (2006). Life, Death and Life Histories. In C. R. Townsend (Ed.), Ecology: From Individuals to Ecosystems (4th ed., pp. 89–131). chapter, Blackwell.
  7. Krohne, D. T. (2018). The Structure of Communities. In Ecology: Evolution, application, integration (pp. 276–297). essay, Oxford University Press.
  8. Dunbar. (1970, January 1). Clements & Gleason - two contrasting views of vegetation associations. UNM Grad Ecology. Retrieved March 11, 2022, from
  10. Bowersox, M. A., & Brown, D. G. (2001). Measuring the abruptness of patchy ecotones. Plant Ecology, 156(1), 89–103.