Environmental Engineering Reference
In-Depth Information
Figure 8.2.
Food web diagrams in which the composition of plant (P) herbivore (H)
and carnivore (C) species remains unchanged but the degree of interconnectedness in
lines of dependency varies. The systems range from low diversity (low degree of inter-
connectedness) as depicted in Food Web I to high diversity (high degree of intercon-
nectedness) as depicted in Food Web II. In Food Web I species tend to be specialized in
their use of prey resources and hence have a unique role to play in the ecosystem that
complements the other species. The uniqueness of the role diminishes (i.e., functional
redundancy increases) as the degree of interconnectedness increases.
documented that drought-induced species composition change was posi-
tively related to grassland plant species diversity (measured as species rich-
ness), consistent with expectations of the Diversity-Stability Hypothesis.
Although stability was measured in terms of the degree of fluctuation in
plant species composition, the study provides insight into another element
of diversity known as
resistance
(see Box 8.1). Frank and McNaughton ef-
fectively showed that species-rich systems were more resistant (i.e., changed
less) to the disturbance than systems with lower diversity.
A review of the broad ecological evidence (McCann 2000) revealed
that greater species diversity is related to greater ecosystem stability. The
evidence further indicates, however, that species diversity per se does not
drive this relationship. Rather, ecosystem stability depends upon the nature
of the trophic linkages among species and the relative strength of the inter-
actions between species. Specifically, the most stable systems are those in
which there are a few strongly interacting species and many weakly inter-
