Community Processes: More on Competition

Theory

How it works

Competition Theory

We would like to understand why some places have more species than others. As an example, there are only about

10 species of lizards in Missouri, but New Mexico has about 40.

The tropics have many more species of plants and animals then the temperate zone.

Competition Theory

How might competition for resources enhance species diversity?

Compare a species rich system with a species poor system. How do they differ and why?

What might contribute to these differences?

Competition Theory

Possible explanations: Greater breadth of the resource

availability curve. Greater stability of the resource

availability curve. Predation – effectively reduces

resource utilization curves. Greater specialization – results from

competition.

Do these Galapagos

finches show some level of specialization

?

Competition Theory

Something to keep in mind for later: notice that the morphology of the birds corresponds with their use of the food resource.

Much of our competition theory is dependent on the close relationship between habitat/resource use and morphology. Is this reasonable?

Competition Theory

Most of what we know is based on Lotka-Volterra models. The models are actually quite simple, they are an extension of the logistic growth model we already considered.

They are differential equations, and have no explicit solution.

How do we analyse these models?

Here we

have a stable

equilib-rium.

The equilib-

rium here is

unstable.

In both this

case, and that on the next slide,

there is no stable equilib-rium.

Competition Theory

What are the conditions for stability?

Under what conditions can we get coexistence of both species?

K-compatibility. -compatibility (intraspecific

competition must be greater than interspecific competition).

Competition Theory

Pretty early on, there was an interesting paper by Francisco Ayala titled: Invalidation of the competitive exclusion principle.

Ayala used fruit flies in a competition experiment, and found the results did not mimic those of Gause. What went wrong?

It turns out that the isoclines are not always linear. Systems are sometimes more complex

than the Lotka-Volterra models suggest.

Competition Theory

What kind of evidence do we need to test the veracity of the competition hypothesis?

Here it is important to remember how science is supposed to work … conjecture and refutation.

A little history

Much of the early work on competition theory didn’t start until a famous address by G. Evelyn Hutchinson. A paper based on that address was titled “Homage to Santa Rosalia: why are there so many kinds of species?”

The history

Hutchinson found this interesting pattern: a size ratio of about 1.4.

Jim Brown worked on desert rodents in the Great Basin, and found a similar ratio.

Jared Diamond, Peter Grant, Eric Pianka, and a host of others found similar ratios, although the ratios kept changing.

The history

For some time, competition theory was dogma. Competition explained everything, and everything was explained in the context of competition.

What’s wrong with this picture?

The history

Let’s take a look at what MacArthur did.

He surveyed birds feeding in pine trees. His famous paper includes the following figure:

The history

The figure shows very nice separation of species, just what you might expect in a competitively structured community.

On the other hand, look at the amount of sampling time and the number of observations. It turns out to be a pretty small sample size.

The history

It turns out to be the same with the work of Jim Brown on desert rodents. In almost all of these cases, the results were accepted because the fit our expectations, not because they were right. In other words, it was not a process of conjecture and refutation, but a process of conjecture and confirmation.

Lets go back to the beginning: what do we know about competition?

Competition Theory

Our problem is this: We know that morphology predicts

ecology. It is relatively easy to study morphological

differences, and much more difficult to study differences in habitat / resource use.

When we look at the morphological patterns, they show nice separation. Why?

Competition Theory

One explanation is that competition has selected for differences in the species. In other words, the species have become more specialized, and thus reduced competitive overlap.

Competition Theory

Are all features of an organisms biology changed easily?

Consider the compression hypothesis.

Competition Theory

However, morphology lags behind ecology. That is, the selection pressures experienced by organisms today, will be exhibited as the morphological attributes of the future.

What we see when we look at morphology is the ‘ghost of competition past.’

Competition Theory

Before we can argue that competition structures communities, we must remove ‘phylogenetic effects.’

The morphology of an organism is a consequence (we suspect) of competitive effects as well as its phylogenetic history.

Can you see how ignoring phylogeny might color our perception of interspecific competition?