Environmental Engineering Reference
In-Depth Information
acting species. This asymmetry cre-
ates the necessary conditions for the
many weakly interacting species to
counterbalance the destabilizing
effects of a few strongly interacting
species (McCann 2000).Thus, weak-
ly interacting species that individual-
ly might contribute minimally to a
specific ecosystem service may col-
lectively provide an essential func-
tional role in ecosystems.
Ecosystem stability thus depends
upon the nature of the trophic link-
ages among species and the relative strength of the interactions between
species.
Instability brought about by fluctuations in species abundances or bio-
mass also has important financial implications for natural resource-based
economies (Armsworth and Roughgarden 2003). Instability increases the
risk that an exploited resource will not replenish itself because production
of species biomass becomes highly variable.There may even be complete
loss of long-term sustainability if the feeding linkages among the harvested
species and all others in the system are additionally disrupted by habitat frag-
mentation as described in chapter 6. Essentially, the Diversity-Stability
Hypothesis proposes that maintaining diversity is a way to ensure against
fluctuations that cause declining ecosystem function or performance.Thus,
biodiversity may provide insurance against loss of ecosystem function (Yachi
and Loreau 1999).
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
. Frank and McNaughton
effectively showed that species-rich
systems were more resistant (i.e.,
changed less) to the disturbance
than systems with lower diversity.
Rivets and Redundants
The nature of the feeding linkages among the species in figure 8.2 also de-
termines the functional role of species in the ecosystem. This may have
added implications for ecosystem stability, or rather resistance to loss of
species. For example, food web I depicts herbivore species 1 and 2 (i.e., H
1
and H
2
) as specialists on their respective plant resources (P
1
and P
2
); food web
III depicts them both as being generalist and completely overlapping in their
resource use. In essence, the herbivores in food web I have a unique func-
tional role in the ecosystem whereas in food web III they are completely
functionally redundant to one another.The possibility for different func-
