Biology Reference
In-Depth Information
a competitor to a resource. Nicholson (
1954
) distinguished contest and
scramble competition. In contest competition, an animal is either fully
successful or it is unsuccessful in utilizing a resource, i.e., the resource is
completely utilized by one of the competitors. In scramble competition,
the competitors attempt to utilize as much as possible of a resource to a
certain minimum. In other words, competitors share a resource in various
proportions (Barker
1983
). Barker pointed out that both contest and
scramble competition may lead to interference and provided further
discussion of the relationship between contest and scramble, and inter-
ference and exploitation competition.
The foregoing discussion has shown that in cases where competition
occurs, it should be for some resource, whether it be space, food, or some
other resource that is in limited supply. The role of resource limitation is
discussed in the following, and in particular the question of how solid is
the evidence for its importance.
Resource limitation
Supposed competition for resources is widely used to explain ecological
patterns. However, Levin (
1970
) has shown that resource limitation of all
species in a system is not necessary for extinction to occur. Specifically,
even two species that feed on different food resources that are not in
limited supply cannot indefinitely coexist if they are limited by the same
predator. This is because each species increases when the predator
decreases, and vice versa, and since each species must have a different
threshold predator level at which it stabilizes, the species with the higher
threshold will replace the other. Therefore, the only important criterion
for species coexistence is that limiting factors (whether food resource,
predation, etc.) differ and are independent.
The Red Queen hypothesis (Van Valen
1973
) (the same as the Rat
Race hypothesis, Rosenzweig
1973
) assumes that resources are limiting
and that the most important component of a species' environment is the
species with which it interacts. Therefore, changes in any of the inter-
acting species modifies the environment of all the other species with
which it interacts, and even if there are no changes in the abiotic
environment, species will still evolve.
Various models that have been established to fit species abundance
patterns are based on the assumption that species' abundances are deter-
mined by resource allocations (e.g., Tokeshi
1990
,
1999
). One of these
models, the Random Assortment model, was fitted successfully to data for
