Species Competition and Predation - Ecological Systems - page 223

because individual prey are killed. This does not, however, preclude some positive
benefits of predation, or more generally of natural enemies, on species that are
attacked, as these species may well be adapted to their natural enemies, and suffer
some negative effects when natural enemies are removed. For example, grasses
may be adapted to grazers that remove old, less productive biomass as well as
enhancing nutrient recycling [ 8 ]. In the case of competition, harm is mutually
negative for the participant species, though it is often lopsided with some species
being harmed much more than others.
Resource consumption may reduce resource availability to individuals in species
that depend on the resource. In this case, harm occurs when lowered resource
availability leads to reduced fitness of individuals because they suffer directly by
consuming less resource, have to expend more energy or materials to obtain the
resource, take greater risks to obtain it, or have to divert time from other beneficial
activities to do so. Competition is also assumed to occur by direct negative
interactions between individuals seeking the resource. It is not necessary in such
instances for resource consumption to lower resource availability, but the presence
of other individuals decreases the ability of a given individual to consume
resources, or in the process of seeking resources, individuals harm each other in
other ways, for instance through fighting [ 9 , 10 ].
Predation and species-enemy relationships necessarily have strong effects in
ecosystems, as they form the paths of energy and material flows [ 1 ]. The role of
competition between species is less obvious, often indirect, and frequently contro-
versial [ 11 ]. Although it is easy to verify that a predator consumes a prey species, it is
much more difficult to demonstrate that one consumer species harms another con-
sumer species through their resource consumption activities. While numerous rigor-
ous experimental studies have firmly established that competition between species is
frequently a strong force in nature, the effects of interspecific competition on various
community properties have been difficult to establish. An abundance of theoretical
work provides hypotheses, but rigorously testing of them in nature has proved to be
difficult and often controversial [ 12 - 14 ]. Thus, although competition as a strong force
is well established, the effects of that force are not.
Several difficulties arise in the study of competition. First, the consequences of
competition between species (interspecific competition) do not rest with its absolute
strength but with its strength relative to competition within species (intraspecific
competition) [ 15 ]. Competition within species constrains the tendency of one
species to harmanother. Second, competition can be constrained by other interactions,
such as predation, in some cases limiting its effects, but potentially interacting with
competition in complex ways [ 16 , 17 ]. Third, the natural world is extremely variable
in time and space. This variability not only makes clear trends difficult to discern, it
potentially interacts with competition modifying the outcome [ 15 ]. Thus, although
competition can be shown to be present and strong, and is believed to have important
implications for numerous community phenomena, clear tests of predictions have
often been elusive.
The predictions from predation are most often of a different character from those
of competition, yet they need not be. Competition comes from the interactions