in density at various sites from environmental factors; there may be some factor that
has been overlooked to account for the remaining variability instead of attributing it
to interspecific competition.Since species prefer to use some habitats over others, we ask how does this choice
change when resources become limiting? There are two hypotheses that we should
consider. We start with the theory of optimal foragingwhich predicts that when
resources are not limiting, species should concentrate their feeding on the best types
of food or the best types of habitats and ignore the others no matter how abundant
they are. When resources are limiting, a species should expand its niche to include
other types of food, habitat, etc. This is the expected response under intraspecific
competition.
When two species are present one might expect both to respond to declining resources
by expanding their niches and so increase the overlap. However, Rosenzweig’s
(1981) theory of habitat selectionintroduces a second hypothesis. This predicts that
when resources are limiting, species should contract their niches as a result of inter-
specific competition. He considered two different situations: we start by assuming
that there are two species, 1 and 2, and two habitats, A and B. In the first case, called
the distinct preferencecase, both species use both habitats but each prefers to use
a different one (i.e. species 1 treats A as the better and B as the poorer habitat, while
species 2 does the reverse). In the second case, the shared preferencecase, again
both species use both habitats, but now both treat the same habitat A as the better
and B as the poorer one.
In either case we should first consider the habitat choice of a species when no com-
petitors are present. Under conditions of abundant resources, such as food, a species
should confine itself to its preferred habitat. As density increases and resources become
limiting through the feeding of other individuals, the species will continue to remain
in the better habitat (A) so long as the food intake rate is greater than what it would
be in the poorer habitat (B). At some point, density in habitat A increases so that
intake rate falls until it equals that in habitat B: at this stage the species should not
confine itself to A but should expand its habitat use in such a way that densities keep
intake rates similar in the two habitats. The intake rate at which the species changes
from one to two habitats is called the marginal value.
Now we consider what happens when there is a competitor present and resources
are limiting. The outcome depends on which of the two preference cases occurs. First,
in the distinct preference case each species will confine itself to its preferred habitat
rather than expand into the other habitat. Therefore, when resources are limiting,
species will specialize, contract their niches and reduce overlap. When resources are
abundant they should use either one or both habitats depending on their intake rates
in the two habitats. Second, in the shared preference case we have to assume that
one of the species (1) is dominant and can exclude by behavior, or other means such
as scent marking, the second species from the preferred habitat A. If species 2, the
subordinate, is to coexist with species 1 then it must be more efficient at using the
less preferred habitat B than species 1. If the dominant is more efficient in both habi-
tats then it will exclude species 2. Therefore, when resources are limiting, one species- the dominant – will not change its habitat choice. In contrast, the subordinate will
change its choice from A to B: the competitive effect is asymmetrical, with the dom-
inant having a large effect on the subordinate while the reverse effect is small.
COMPETITION AND FACILITATION BETWEEN SPECIES 151
9.6.4The theory of
habitat selection