Biology Reference
In-Depth Information
tion of alien species (Cox 1999). For most groups of organisms, extinc-
tions have been concentrated in insular environments, including islands
and freshwater lakes and streams. Evidence from past extinction episodes
suggests that diversity does not rebound quickly in geological time
(Kirchner and Weil 2000). The marine fossil record, for example, shows
that rates of origin of species are correlated with extinction rates about 10
million yr earlier. Recovery of diversity appears to be a complex process
of adaptive radiation that is in some degree self-reinforcing. This implies
that the current extinction episode will require millions of years for
recovery of biodiversity, even with the increased rate of speciation that
may result from introductions of organisms to new geographical areas.
The long-term impact of the massive introduction of species to new
biogeographic regions, given the roles both of speciation and extinction,
may not itself cause an overall reduction of the diversity of species,
regionally or globally (Rosenzweig 2001). In the short run, introductions
are likely to increase local species diversity, at least in continental regions,
since extinctions triggered by the impacts of aliens are often slow. Num-
bers of naturalized plants, especially, have far exceeded the number of
alien-caused plant extinctions (Sax et al. 2002). On oceanic islands,
extinctions tend to occur more quickly, and the evolutionary time
required for speciation to offset extinction may be much longer than in
continental regions.
Over the long term, biotic relaxation is expected to occur where
species diversity has been raised above a level set by environmental con-
ditions that influence the rate of natural invasion, speciation, and extinc-
tion for the geographical region. Biotic relaxation is simply the decline in
diversity due to an excess of extinctions over appearance of new species
by invasion and speciation. Relaxation has been demonstrated in the bio-
tas of oceanic islands formed by rising postglacial sea levels. In the Gulf of
California, for example,Wilcox (1978) showed that islands formed in this
manner held more lizard species than could be maintained and that the
longer the island has been isolated, the fewer species remained.
In small areas of habitat that become isolated, relaxation can occur
very quickly. Barro Colorado Island, Panama, was formed by the flooding
of Gatun Lake in 1913. On this island of tropical forest, 15.6 km
2
in
size, more than 27% of the original breeding bird species were lost by the
early 1980s (Karr 1982). In cases such as this, many of the losses are due
to stochastic fluctuations in environmental conditions or population
demography that result in total mortality or reproductive failure of small
populations.
