Environmental Engineering Reference
In-Depth Information
tional roles has led ecologists to develop various hypotheses about the link
between species functional diversity and ecosystem function.
When species tend to be specialized, as in food web I, they each have a
specific “job” in the ecosystem.The emerging level of an ecosystem serv-
ice, say rate of organic material decomposition for soil creation, rate of ni-
trogen fertilizer cycling, or rate of plant production, then increases
incrementally with each new specialist species that is part of the ecosystem.
That is, there is functional complementarity among species.This kind of
scenario is the basis for the “Rivet Hypothesis” of species diversity and
ecosystem function (Ehrlich and Ehrlich 1981).The Rivet Hypothesis likens
species in an ecosystem to rivets on an airplane wing.You can lose a few
rivets with limited consequences. If you lose too many, however, the in-
tegrity of the whole wing (ecosystem) is compromised.The wing (ecosys-
tem) then loses its ability to maintain critical function and breaks apart.
Alternatively, when species are so generalized that they overlap completely,
as in food web III, they are able to back-up one another's roles.That is to
say they are functionally redundant.The Functional Redundancy Hypoth-
esis of species diversity and ecosystem function (Walker 1992) envisions
classes of species doing different kinds of jobs. But, within a class, say graz-
ing herbivores, several species may be doing the same job. In this case, loss
of one or a few species from that class does not jeopardize ecosystem func-
tion because other species can back
up the lost species. The burden of
current evidence, derived from a syn-
thesis of fifteen field experiments in-
volving plant communities across the
globe, suggests that species tend to
play a complementary role to each
other (i.e, they tend to be rivets)
rather than have a functionally redun-
dant role (Schmid et al. 2001).
The Rivet Hypothesis likens species
in an ecosystem to rivets on an air-
plane wing. You can lose a few riv-
ets with limited consequences. If
you lose too many, however, the
integrity of the whole wing
(ecosystem) is compromised.
Diversity-Productivity
Relations
Much of life on earth is sustained by the fact that green plants use sunlight
to stimulate a physiological process—photosynthesis—that converts carbon
dioxide and water into sugars and oxygen. The sugars especially are the
building blocks that lead to plant structure (roots, shoots, and leaves) and,
