Environmental Engineering Reference
In-Depth Information
to allow options for future biological control, it is imperative that a portfo-
lio of predator species (i.e., predator species diversity) from which to choose
is actively maintained on ecological landscapes.
Invasion Resistance
As explained in chapter 7, biotic invasions occur when species are trans-
ported beyond their current range distribution into conditions in which
their descendants thrive.The likelihood that a species can invade depends
in good part on biophysical features of the environment. For example, the
success of invasions may depend upon the availability of bare areas, requi-
site soil fertility, and suitable climate (Kennedy et al. 2002).These biophys-
ical conditions for invasion may be mitigated by the presence of native
species that can regulate the competitive environment the invader faces. So,
it stands to reason that the likelihood of successful invasion may decline as
the number of native species with which the invader must compete in-
creases. In essence, biodiversity should lead to ecosystem stability by resist-
ing species invasions and associated decline in ecosystem function (see Box
8.1; Kennedy et al. 2002).
This idea was tested using a large-scale field experiment at the Cedar
Creek Long-Term Ecological Research site in Minnesota, United States
(Kennedy et al. 2002).The experimental system consisted of 147 different
experimental plots in which resident plant diversity (species richness) was
initially varied from one, two, four, six, eight, twelve, and twenty-four grass-
land species drawn at random from a pool of twenty-four species. Invaders
were allowed to enter the experimental area from the surrounding weedy
fields through natural dispersal processes. The experiment revealed that
species richness had a strong suppressing effect on invader establishment suc-
cess (there were fewer invaders in high diversity plots than in low diversity
plots) and on the ability of invaders to proliferate (invaders had smaller max-
imum size in high diversity plots than in low diversity plots).The study sug-
gests that loss of biodiversity may exacerbate likelihood of invasion; it can
diminish resistance against invasion once other factors that insulate habi-
tats from invasion, such as natural geographical barriers, are compromised
by human transport of nonnative species.
These examples illustrate that biodiversity can have important functions
in ecosystems, the value of which may go well beyond that measured in
terms of material goods. Conserving such functions should not be viewed
as competing with economic activity, but rather as essential and consistent
with a pursuit of human well being (Myers 1996).
