Environmental Engineering Reference
In-Depth Information
emphasis on the biological integrity of the system than simply ensuring
that all the biotic elements are present.
Functional diversity refers to the characteristics of ecosystems and
includes ecosystem complexity at dif erent levels of organization such as
trophic levels (Cousins, 1991). This approach uses trophic-level analysis
to relate species diversity to functional ecosystem parameters such as food
web structure or the transfer of energy, water and chemicals between dif-
ferent trophic levels. Functional diversity can be interpreted as the number
of species required for a given ecological process.
In managed ecosystems, although every species may have a particular
role, it does not follow that these roles are of equal importance. Ecologists
acknowledge that some species have a greater ecological impact than
one might expect from their abundance or biomass and these have been
referred to as keystone species (Power et al., 1996). Some ecosystem
studies indicate that only a small number of the numerous species found
in ecosystems perform key functions or so-called keystone roles and that
most species perform a perfunctory role (Holling, 1992). For example,
beavers have been shown to have a profound impact on streams, forests
and wetlands through dam construction.
Many species may play keystone roles that involve interdependencies
with other species (Daily et al., 1993). The elimination of any single com-
ponent of an ecosystem could lead to an unanticipated unravelling of com-
munity structure and to local extinctions of some species. Bird dispersers
such as the blackcap (
Sylvia atricapilla
) interact with shrub species to
inl uence l oristic diversity, spatial patterns of vegetation development and
plant dynamics in patchy Mediterranean vegetation in southern France
(Debussche and Isenmann, 1994). Seeds deposited by blackcaps under-
neath pioneer shrubs (which have positive 'nurse' ef ects on other plants)
were more likely to survive than in the open i eld and birds may actually
trigger dynamic successional processes initiated by pioneer woody peren-
nials in Mediterranean grasslands and shrublands.
The 'keystone role' of a species may also depend on whether a number
of other species can assume its functional role within the ecosystem
(Schindler et al., 1989). Functional redundancy is known to occur if
other species can perform similar roles (Hutchinson, 1961: Walker, 1992).
Although there is evidence that the deletion of some species has very little
ef ect on ecosystem functioning, in many diverse tropical forests there are
so many rare species that collectively they may have an important impact
on the ecosystem. Here, top predators in ecosystems are relatively scarce
because they are large in size and at the top of the food chain but may
nevertheless be important in terms of ecosystem structure.
Most research has focused on which species are important here and now.
