<<Go back

Community Ecology

Key Unifying Themes:

In the last chapter, population ecology was mostly about vital statistics, and the evolutionary significance of them. Now, in community ecology, we're dealing with interactions between biotic and abiotic factors, something, I think, is the real meat of ecology. So here we go.


Communities are one level higher than populations organizationally. Instead of lots of individuals of the same species, communities are groupings of multiple species.


Communities are partly characterized by their diversity. There are two parts to diversity:

species richness, and relative abundance

Species richness is really straightforward the understand - it is basically the number of species that exist in a community.

Relative abundance really means the proportions of the species in a community. For example, even though an environment might be technically species rich (it has a lot of species), it might break down to this:

90.0% frogs
01.0% lions
01.0% tigers
01.0% bears
02.0% wombats

Which isn't really that diverse.

So diversity is made up of both species richness and relative abundance.

Why are some communities made up of the organisms they way they are? Well, some ecologists tried to answer that, and the first theory was the individualistic hypothesis - which says it's because of chance that all the organisms worked out to form a fully functional community. This isn't really accurate, so we have this interactivity hypothesis, which has a more accurate view:

The communities we see are as is because of the mandatory and complex interactions between biotic factors, which form a fully functional community unit.

Interspecific Interactions

In population ecology, we talked about intraspecific interactions, which are interactions that take place within species. Now in community ecology, with multiple species running around the house, we talk about interspecific interactions, which are interactions between individuals of different species.

Interspecific Interactions and Evolution

The interactions in communities that take place between species drive a lot of the evolution that occurs. This is because when there's competition between two species (such as they fight over resources or food), they have to continually adapt to each other, which is coevolution.

Effects of Interspecific Interaction Between Two Species

There are many types of interaction between species. One obvious type is predation: which is basically a predator eating prey. This is symbolized by a +/-, where the + represents the predator benefiting, while the - represents the prey losing (bigtime).

Other examples of interactions:

+/-: Parasitism - a parasite gets nutrients from the host body
+/+: Symboisis - two organisms feed off of each other - they are using each other to both of their advantage.
+/0: commensalism - one organism derives nourishment from another, without severely harming it
+/+: mutualism - same as symboisis

Defense against Predation


Cryptic Coloration versus Aposematic Coloration
They are opposites, really. Cryptic coloration is kind of blending within one's environment, while aposematic coloration is warning potential predators away with bright, dangerous-looking colors.

Mimicry In nature, mimicry is often incredibly useful. I mean, what's cooler than mimicking a snake when you're just this tiny harmless caterpillar?
Batesian - imagine a catepillar mimicking a snake. Take a look at figure 53.8 at 1113. Self explanatory.
Mullerian - Two, unpalatable (meaning you can't put it on a plate, or you can't really eat it) animals mimick each other, so it looks like there's more than just one of each.

Other forms of mimicry invovle pretending to be food in order to lure hapless victims.

Competitive Exclusion Principle

Competitive Exclusion Principle, in very simple terms, is this: There's only room for one at the top. Anyways, it basially states that no two species with the same resource requirements can live in the same area.