ECOLOGY: The Study of Ecosystems

Ecology (from the Greek oikos meaning "house" or "dwelling", and logos meaning "discourse") is the study of the interactions of organisms with each other and their environment.

The hierarchy. Define each of the following.

Ecology is a science, not a sociopolitical movement (e.g., environmentalism, conservation, etc.).

The Ecologist engages in the hypothetico-deductive method to pose questions and devise testable hypotheses about ecosystems. Often, this involves the generation of complex mathematical models to simulate ecosystems. These models represent idealized systems to which real systems can be compared for their predictive value. Sometimes, when a very large scale project is logistically impossible to perform, a computer model is used to predict expected results.

An ecosystem consists of

Levels of Ecological Study

Organismal Ecology

An organism's ability to survive extremes of its environment reflect its evolutionary history.
Homeostasis is the process of an organism maintaining a stable internal environment (with respect to temperature, salt balance, etc.).

  • regulators use metabolic means to maintain homeostasis in response to environmental changes.

  • conformers are less able to metabolically maintain homeostasis. Their internal environment is governed primarily by the external environment.

    Another example: osmoregulation - regulation of internal salt/water balance

  • anadromous (fish that migrate from salt to freshwater habitats annually) and catadromous (fish that migrate from freshwater to marine habitats annually) maintain constant salt balance in their tissues via their excretory systems (kidneys), even when their environments vary. They are regulators

  • Echinoderms, have no excretory system, so are strictly limited to marine environments. Their tissues have the same salinity as sea water. They are conformers

    Circadian Rhythm: Individual Organisms have a 24-hour Cycle

    The cycle of biological processes that recur on a 24-hour cycle is known as Circadian Rhythm.
    Most living things have Circadian Rhythms, and humans are no exception.

    The physiological features of any species determine where it can live. Species with similar ecological tolerances will live in the same ecosystem, and their interactions are the focus of community ecology

    Community Ecology: The Study of Symbiosis

    Evolution by natural selection is driven by ecological interactions between organisms and between organisms and their environment. Today we'll explore the former.

    Biological coevolution is the evolutionary change of one species triggered by interaction with another species.

    Example: Wolves hunt caribou, chasing them down to capture them. The slower caribou are more likely to be preyed upon, leaving the faster individuals to reproduce. The resulting faster offspring will be even more difficult for the wolves to catch, and only the fastest wolves (or perhaps the wolves who are genetically capable of devising strategies to hunt very fast prey) will get enough food to survive. An evolutionary "Cold War" ensues.

    Example: Flowers compete for pollinators. The plants that produce flowers (with colors, shapes and scents specific to particular pollinators) that attract the most effective pollinators will set the most seed and have the most offspring.

    The various species in an ecosystem have evolved over the millennia in response to pressures from both the environment and from other species with which they live and interact.

    In Greek, sym means "together" and bios means "life". Symbiosis means living together. This term refers to members of two different species (i.e., two populations) that have coevolved to have some type of ecological interaction that affects both populations.

    Some Terminology for Interacting Species:

    Here are some of the theoretical types of interactions that can evolve over many generations.

    "+" means that the population benefits from the interaction

    "-" means that the population is harmed by the interaction

    "0" means that the population is not affected by the interaction

    type of interaction

    pop'n A

    pop'n B

    nature of effect

    obligate mutualism



    obligatory; both populations benefit










    NOT obligatory; both pop'ns benefit








    populations inhibit one another








    populations do not affect one another








    predator (A) kills & consumes prey (B)








    parasite (A) exploits the host (B), but

    does not kill it outright












    parasitoid (A) lives within, and eventually kills host (B) at point of metamorphosis from one life cycle stage to another.








    commensal (A) benefits; host (B) not









    A unaffected; B inhibited





    Examples of Symbiotic Interactions

    Obligate Mutualism

    The termite and its intestinal flagellate symbionts exemplify Obligate Mutualism: Neither organism can survive without the other.


    The Clown Fish and its Sea Anemone partner both benefit from the relationship: Nemo gets a safe home that protects him from predators, and he fiercely protects his sea anemone from predators. He also feeds the anemone. How cute is that?


    The Green Anole (Anolis carolinensis) is native to the southern United States. In the 1960's, The Brown Anole (Anolis sagrei) was introduced from Cuba. The two species vie for habitat and food resources, and it appears that the exotic Brown Anole has displaced the native Green Anole in some physical spaces, such as lower shrubbery and grass. The Green Anole generally lives higher up in the trees and foliage than the Brown Anole does. This result of competition is known as resource partitioning.


    In a case of true neutralism, two populations interact, but neither would have any effect on the evolutionary fitness of the other. Because all organisms in an ecosystem are interconnected in some way, true neutralism is not likely to occur, and would be very difficult to prove. The term is often used to describe interactions in which the effects of two populations on each other are simply negligible. Say....a Bactrian Camel and a Longtailed Tadpole Shrimp, both living in the Gobi desert.
    (Okay, it's a stretch, but you get the idea.)


    This is possibly the most familiar type of symbiosis. The predator species (in the illustration below, the Lion (Panthera leo)) kills and consumes the prey species (in this case, a Cape Buffalo (Syncerus caffer). You can no doubt think of dozens of other examples of predation.

    Predation has driven the evolution of some truly amazing phenomena, such as crypsis (camouflaging coloration), aposematism (warning coloration), mimicry, and other ways animals avoid being eaten.

    Some Interesting Coevolutionary Results of Predation: Camouflage and Warning Coloration
    Know the difference!


    A parasite is an organism that takes up residence in or on a host organism and feeds on the host's body without killing it outright.
    An organism that is host to an adult parasite is known as the definitive host. An organism that is host to a juvenile parasite is known as an intermediate host. The definitive host is usually a predator of the intermediate host, and the life cycle is completed when the definitive host eats the intermediate host, freeing the larval forms to take up residence, as shown below in the tapeworm life cycle.

    There is just no end to the creativity of parasite evolution.


    A parasitoid acts as a parasite of its host until some critical point of its life cycle--such as metamorphosis from juvenile to adult or onset of reproduction--at which point it kills the host. If you recall the movie "Alien" then you've seen Hollywood's best representation of a parasitoid.
    But there's nothing a writer can invent that nature hasn't already done better. Check out the
    Brain-stealing Fungus and the Gordian Worm.


    In this case, one species benefits from the presence of another, which is not affected by the presence of the first species. An example is the Cattle Egret. As large grazers move through the grass, they stir up insects. Cattle Egrets follow them and get a banquet. The large grazers are neither helped nor harmed by the presence of the birds.


    Amensalism occurs when species A impedes the success of species B, but is neither positively nor negatively affected by the presence of the species B. This is commonly the effect when one species produces a chemical compound (as part of its normal metabolic reactions) that is harmful to the other species.
    Allelopathy, in which some plants produce chemical compounds that inhibit the growth of nearby would-be competitors, is one type of amensalistic interaction. For example, the Black Walnut Tree (Juglans nigra) produces compounds in its roots that inhibit the growth of other trees and shrubs.

    Another famous (and useful) example is the production of penicillin by Penicillium notatum. This antibiotic compound inhibits the growth of many species of bacteria (in this picture, it's Staphylococcus aureus) by interfering with the normal formation of peptidoglycan in the cell wall.

    Note that one could assert that these relationships are actually competition. However, the populations affected by the toxic compounds of the amensal "partner" really have no chance. It would be more accurate to hypothesize that these amensalistic relationships are the end result of natural selection that occurred because of past competition between Species A and Species B.