Environmental Engineering Reference
In-Depth Information
Figure 2.1. Three simple kinds of species interactions. (a) A direct consumer-resource
interaction (solid arrows) in which species 1 (S 1 ) is the consumer and species 2 (S 2 ) is the
resource. The consumer derives a net nutritional benefit (hence +) by directly feeding
on the resource; and the resource, being the victim, suffers a cost (hence -). (b) A direct
competitive interaction (soild arrows) between two species in which each species phys-
ically preempts the other species' access to resources (hence a mutual -/- interaction).
(c) A system in which two species (S 1 and S 2 ) vie for a common resource (S 3 ) through
direct consumer-resource interactions. As a consequence, the two competitor species
have a mutually indirect (hence dashed line) negative effect on each other mediated by
the abundance of the resource species.
animal, then the cost is the victim's life. If the victim is a plant, then the cost
is loss of some plant tissue such as leaves or stems. (Herbivores rarely kill and
consume an entire plant—leaves, flowers, stems, and roots—in the same way
that carnivores kill and consume their herbivore prey.)
If two species within a trophic level usurp one another's access to the
same resources by holding territories or through direct physical struggles
then each species pays the price for such interactions. Such competitors will
have a direct mutually negative effect on each other's abundance.This is de-
noted by two arrows each with a minus sign (figure 2.1b).
Two species may influence each other's abundance in less direct ways.
Suppose that two species shared a common resource but never interacted
directly with each other for access to that resource. In this case, the con-
sumer-resource interaction reduces the availability of the resources through
consumption. As a consequence, one species reduces the availability of re-
sources for the other species. So, the one species has a negative effect on the
other species. Here, both species again are competitors, but the effect is in-
direct (as denoted by the dotted line) as opposed to direct (denoted by solid
lines).
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