untitled

We should note that we do not have actual measures of the food supply in these

examples, so we cannot be sure that we are seeing competition. In Serengeti,

Tanzania, wildebeest are regulated by lack of food in the dry season (Mduma et al.

1999), so that overlap with this species at this time should result in competition.

However, overlap in both diet and habitat between wildebeest and several other

ungulate species increases or does not change between wet and dry seasons (Hansen

et al. 1985; Sinclair 1985). One interpretation could be that interspecific com-

petition is asymmetrical, with the impact of the rarer ungulates on the numerous

wildebeest being real but very slight, while the reverse does not occur because these

other ungulates are kept at low density by predation (Sinclair et al. 2003).

So far we have discussed the patterns of occupancy and utilization of habitats as if

they were constants for a species, or that they changed only seasonally. However,

longer-term studies are now showing that species densities vary in the same habitat

and they also change over a longer time scale measured in years. Thus populations

may go through periods when there are abundant resources and, although there is

overlap with other species, even at the supposedly difficult time of year there is no

competition (Fig. 9.16). Occasionally there are periods of resource restriction and it

is only at these times that one sees competition and niche separation (Wiens 1977).

Some of the predicted outcomes from interspecific competition include the reduction

of populations, the contraction of niches, and exclusion of species from communi-

ties. However, these predictions are also to be expected when species have non-

overlapping resource requirements but share predators, especially when predators

can increase their numbers fast.

Let us suppose there is a predator that is food limited, and which feeds on two

prey species. The prey are both limited by predation and not by their own food sup-

plies. If species 1 increases in number then this should lead to more predators, which

in turn will depress species 2 numbers. This result is called apparent competition

COMPETITION AND FACILITATION BETWEEN SPECIES 153

Selective constraints

Selective constraints

Individual

generation

Variation in resource-utilization trait

Time

A

B

Fig. 9.16Changes
over time in the mean
(thick line) and variance
(shaded area) of a
selective constraint such
as resource availability.
At times A and B there
are “bottlenecks” when
competition is more
likely. (After Wiens
1977.)

9.7 Competition in variable environments

9.8 Apparent competition

9.8.1Shared
predators