Environmental Engineering Reference
In-Depth Information
and fifty years (Ferraz et al. 2003); in California sage-scrub it can be up to
seventy-five years (Crooks and Soulé 1999).Theory shows that the level of
extinction rises exponentially with the degree of habitat loss. Initially small
amounts of habitat loss should precipitate only low numbers of species ex-
tinctions. Greater amounts of loss can cause a sharp rise in extinction rate.
As explained above, it may be the highly competitive species that are most
susceptible to extinctions. However, highly competitive species tend to be
highly abundant at the onset of habitat loss so it will be some time before
their populations disappear. Species should also disappear sequentially in
rank-order of their competitive ability.These factors together imply that
fragmentation can precipitate a chain of species extinctions that are irre-
versible once the fragmentation process is initiated.Without some careful
thinking, current habitat fragmentation may pass on a considerable legacy
of debt—the “extinction debt”
(Tilman et al. 1994)—the burden of
which will only truly be realized by
our grandchildren and generations
beyond.
We must be careful not to ascribe
absolute truth to the assertions made
by the various theories described
above. After all, the predictions are
only as accurate as the assumptions
on which they are based conform to
nature.We must ask several questions:
Does a trade-off exist between com-
petitive and dispersal ability among
species? Is the likelihood of coloniz-
ing any patch on a landscape equal across all patches? Can these spatial
processes be examined without considering the effects of predators on the
competitors? Theoretical examinations that relax these assumptions sug-
gest that the extinction debt idea is robust (insensitive) to changes in as-
sumptions (Kareiva and Wennergren 1995).The ultimate arbiter, however,
will be data derived from experimentally testing the predictions on large
landscapes. Such experimentation may, however, be logistically challenging
and even impossible on ethical grounds.Thus data to address this issue may
not be easily forthcoming.
Theory shows that the level of
extinction rises exponentially with
the degree of habitat loss. Initially
small amounts of habitat loss
should precipitate only low num-
bers of species extinctions. Greater
amounts of loss can cause a sharp
rise in extinction rate. Without
some careful thinking, current habi-
tat fragmentation may pass on a
considerable legacy of debt—the
“extinction debt.”
