Animal Distribution and Ecology

Ecology can be defined as  the scientific study of interactions that determine the distribution & abundance of organisms.  Since this is a course in animal ecology, we will focus on animals, which we will define fairly generally as organisms that can move around during some stage of their life and that must feed on other organisms or their products.

Let's explain the terms in the definition of ecology.

Distribution refers to where organisms are found.  We can study distribution on different scales:

• where found geographically
• where found in terms of habitat
• how distributed spatially within habitat

Abundance refers to how many organisms occur.  We can ask different questions about abundance:

• does a species occur in many habitats?  If so, it will appear abundant on a large scale -- we will encounter it in many places.
• are there large numbers of individuals of a species in a habitat where it occurs?  If so, a species may be rare or abundant on a large scale, but in certain localities it will be abundant.
• we can also look at abundance in terms of numbers of species, rather than in terms of individuals of a single species.  We can ask whether an area has many different species or only a few species.

Interactions refer to the relationships between an organism or species and aspects of its environment.  The environment refers to the surroundings of an organism or species, and is generally considered to consist of two categories of factors:

• biotic factors refer to other organisms that interact with an organism or species, or the organic products of those organisms.  Examples of biotic factors include:
• the species that produce the food eaten by an organism
• species that feed on and harm the organism, including:
• predators: species that kill and eat their prey and have no long term interaction with them
• parasites: species that live on or in their host over a long period of time and harm, but are unlikely to directly kill, the host
• parasitoids: species whose eggs are laid on the host (typically on the larval stages of insect hosts) and which then develop in or on the host, harming it as parasites do, but that eventually grow large and kill the host
• brood parasites: species (typically birds) that lay eggs in the nests of their host species.  The hosts care for these young and their own young are usually harmed or killed
• iterspecific competitors of the organism -- other species that use the same resources and deplete supplies of those resources so that they negatively impact an organism
• mutualists -- species whose presence is helpful or essential to the organism, and who are helped by the organism
• members of same species, through:
• intraspecific competition: use of same resources, so members of same species affect each other negatively
• behavioral interactions
• abiotic factors refer to non living aspects of the environment that affect an organism, such as oxygen, water, pH, salinity,...

Note that biotic and abiotic factors interact.  For example, plants (biotic factors) in an environment tend to increase the amount of oxygen (an abiotic factor.) The above explanations of distribution, abundance, and interactions should indicate that we can study ecology on a various different levels.  The main levels studied by ecologists are:
 

• individuals.  We can consider how individuals are affected by the environment; this can determine whether they can survive (which will affect their distribution) and how well they reproduce (which will affect their abundance.)  We will spend some time early in this course looking at how the physiology of individual organisms relates to their survival and reproduction in the environments where they occur.
• populations.  A population is a group of organisms of the same species within a defined area.  We can look at the factors that determine how large a population grows,  that regulate it at a certain size, or that cause population size to fluctuate.  A large part of this course will be spent studying populations
• communities.  A community usually refers to all the organisms within an area.  We can also talk about a community of some type of organism, such as the community of rodents in a field in West Tennessee.  We will look at factors affecting the numbers of species in communities later in this course.
• ecosystems.  An ecosystem refers to all the organisms within an area and the abiotic factors that affect it.  We will not consider ecosystems very much in this course; ecosystem interactions are strongly dependent on plants so they often fall outside the area of animal ecology, although you should all realize that animals are part of an entire ecosystem and interact with the entire ecosystem.

The final aspect of the definition of ecology that we started with is that ecology is a scientific study.  Scientific study means using the scientific method, which is discussed below.  It is an important part of the definition of ecology because it indicates that to study ecology we must be doing the things associated with science -- testing hypothesis with objectively obtained, repeatable data.  It is important to consider this with regard to ecology because we get a lot of information about organisms and their environments in ways that are NOT scientific.  For example, I like birds, and I like to go out birding to see how many different species I can observe in some area or time.  When I'm out birding, I am NOT being an ecologist -- I'm getting information about the natural world but it is biased, not repeatable, not objective.  In contrast, I also study birds, and when I am getting nest success information on a population of meadowlarks I try my best to do it in a way that IS repeatable and objective; while doing that, I am being an ecologist.

The Scientific Method involves the following steps:

1. ask questions about the natural world
2. develop possible explanations, answers to these questions, that can be tested by doing experiments or taking observations.  An hypothesis is a plausible, testable explanation for some phenomenon observed in the natural world.  It must be consistent with what is already known about the world, and it must be possible to take data in a repeatable, objective manner to test the hypothesis.
3. make predictions: observations or experimental results that we would expect to observe if the hypothesis we are testing is true.
4. take data -- through experimentation and/or observation determine whether we see the predictions that are predicted based on our hypothesis.  These data must be taken in a repeatable way.  Ideally, we take data such that we look at just one factor at a time; this often involves having a control group which we do not manipulate and an experimental group which we manipulate in just one way.
5. evaluate our hypothesis based on the data:
6. if our prediction is NOT met, then our hypothesis must be false
7. if our prediction IS met, then our hypothesis MIGHT be true -- we say it is "supported."  This is a weaker conclusion than we would get if our prediction was not met.  This is because it is always true that there might be some other hypothesis that makes the same prediction as the hypothesis we are testing.  The result of this is that we never know for sure that an hypothesis is true.

While we never know for sure that any hypothesis is true, we can conduct experiments that allow us to say it is very very likely that a hypothesis is true.  To do this, we use a method called strong inference, which involves:

• considering several different hypotheses that might explain the phenomenon we're studying
• develop predictions of the hypotheses that are mutually exclusive -- that is, a prediction of one hypothesis is something that is NOT predicted by the other hypotheses

If we can do this for every hypothesis we, or anyone, can think of, and only support one hypothesis, then we're pretty sure it's true.  It remains possible however that someone smarter will come along and think of another hypothesis that explains all the results just as well as the one we thought was true. Testing hypotheses in ecology can be fairly difficult.  A less formal definition of ecology (don't learn this one for the tests!) is "science under the worst possible conditions."  The main reason for this is that when we are looking at organisms in their environments, they are affected by many factors, and it is hard or impossible to change one without changing others.  It is thus very hard to do a real controlled study in ecology.  Ecologists take different approaches to this problem:

• laboratory studies.  Populations of many species have been grown and studied in laboratory conditions.  The advantage to this approach is that by bringing organisms into the lab, ecologists can reduce the number of factors affecting them and change factors one at a time.  The disadvantage is that the factors that affect a population in the lab may be different from those that really affect it in nature -- by creating a laboratory situation we risk creating a situation so different from nature that what we determine in the lab does not apply in nature.
• field observations.  By observing organisms in different environments, or at different times of the year, we may be able to determine the factors that are really affecting them.  Careful observation of natural communities is the basis for developing hypotheses in ecology.  However, often many different factors vary together so it is difficult to know which really causes any patterns we observe.
• field experiments.  We can sometimes change factors in a field setting to see how they affect the populations we are studying.  These ideally allow us to test our hypotheses in a natural setting so that it is less likely that the factors we study have no real importance to the species than it would be for a laboratory study.  It is hard or impossible, however, to change just one factor at a time in a natural setting.(http://www.utm.edu/departments/biology/rirwin/441_442/441introlec.htm)