Download Community Processes: More on Competition Theory

Community Processes: More
on Competition Theory
How it works
Competition Theory

We would like to understand why some
places have more species than others.

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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
equilibrium.
The
equilibrium here
is
unstable.
In both
this case,
and that
on the
next slide,
there is
no stable
equilibrium.
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 LotkaVolterra 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:

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
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?