Environmental Engineering Reference
In-Depth Information
plants are linked to herbivores, and herbivores are linked to carnivores. Such
linkages indicate that plants are consumers (predators) of soil nutrients, her-
bivores are consumers (predators) of plants, and carnivores are consumers
(predators) of herbivores. Ecologists give such consumer-resource interac-
tions a special name—trophic interactions. Species engaging in a particular
kind of trophic interaction belong to the same trophic level of the food
chain. So, for example, species engaging in herbivory belong to the herbi-
vore trophic level, species preying on herbivores belong to the carnivore
trophic level, and so on.
In addition, plant species are limited by, and thus must compete for, light
and soil nutrients. Herbivore species may therefore compete for limited
plant resources and carnivores may potentially compete for an even more
limited number of herbivores that comprise their prey. Limiting resources
and the need to compete for them can lead to ecological innovation in the
way species vie for their share of resources.Thus, we can elaborate our ver-
tical conception of an ecological system by envisioning horizontal linkages
within a trophic level as species engage in various strategies to maximize
consumption of particular resources.
Conceptualizing Predation and Competition
Together, the vertical chain comprised of consumer-resource links coupled
with horizontal links between species at the same trophic level create a
highly interconnected web of life—a food web. Individual species within
this web are sandwiched between their predators, their resources, and their
competitors.The easiest way to imagine the implications of such complex-
ity is to begin by drawing food web diagrams that depict the interdepen-
dencies among species created by their linkages and the nature of each
species' net effect on the other species (figure 2.1). Such an approach as-
sumes that we can ignore the diversity of individuals within a species and
understand interactions simply on the basis of a typical or average individ-
ual.This is a good staring point for conveying principles that can be later
elaborated with the added complexity of variety within a species.
In the case of a consumer-resource interaction, the arrow pointing from
the consumer to the resource is denoted by a minus sign and the arrow
pointing from the resource to the consumer is denoted by a plus sign (fig-
ure 2.1a) called a (+/-) link.This implies that the consumer derives a net
nutritional benefit (hence +) by directly feeding on the resource; and the
resource, being the victim suffers a cost (hence -). If the victim is another
