Biology Reference
In-Depth Information
displays often involve assessment of an opponent's fighting potential, so individuals
vary their fighting tactics from contest to contest. The fixed strategies in the hawk-dove
game ('sealed bids') are, therefore, also rather simplistic. Nevertheless, the game
illustrates a valuable way of thinking about how evolution will proceed whenever there
is competition. The key question is: what will be the evolutionarily
stable
outcome?
In this chapter, we will now use 'ESS thinking' to examine how individuals compete for
scarce resources, focusing on the two problems: where to search and how to behave.
We will start by discussing the simplest form of competition,
exploitation
, which simply
means 'using up resources', and then go on to describe another form of competition,
resource defence
, in which individuals keep others away from resources through
dominance or territoriality. Next, we will show how competition for either food or mates
often leads to individuals in a population showing variability in competitive behaviour,
with mixtures of producers and scroungers or fighters and sneaks. Finally, we will link
this variation to the concept of animal personalities.
Competition by exploitation:
the ideal free distribution
The ideal free model
Let us start with a simple model. Imagine there are two
habitats, a rich one containing a lot of resources and a poor
one containing few, and that there is no territoriality or
fighting, so each individual is free to exploit the habitat in
which it can achieve the higher pay-off, measured as rate of
consumption of resource. With no competitors, an individual
would simply go to the better of the two habitats and this is
what we assume the first arrivals will do. But what about the
later arrivals? As more competitors occupy the rich habitat,
the resource will be depleted, and so less profitable for further
newcomers. Eventually a point will be reached where the
next arrivals will do better by occupying the poorer quality
habitat where, although the resource is in shorter supply,
there will be less competition (Fig. 5.1). Thereafter, the
two habitats should be filled so that the profitability for an
individual is the same in each one. In other words, competitors
should adjust their distribution in relation to habitat quality
so that each individual enjoys the same rate of acquisition
of resources. This theoretical pattern of distribution of
competitors between resources was termed the 'ideal free'
distribution by Stephen Fretwell (1972) because it assumes
that animals are free to go where they will do best (there is no
exclusion of weaker competitors by stronger ones) and that
the animals are ideal in having complete information about
the availability of resources.
Rich habitat
Poor habitat
a
Number of competitors
Fig. 5.1
The ideal free distribution. There is
no limit to the number of competitors that
can exploit the resource. Every individual is
free to choose where to go. The first arrivals
will go to the rich habitat. Because of
resource depletion, the more competitors the
lower the rewards per individual so at point
a
the poor habitat will be equally attractive.
Thereafter, the two habitats should both be
filled so that the rewards per individual are
the same in each. After Fretwell (1972).
