Biology Reference
In-Depth Information
from hybridization and genetic swamping (Daehler and Carino 2001), as
have some animals.The ability of native species to make the evolutionary
adjustments necessary to prevent their extirpation or extinction varies
greatly, and is a key concern of conservation biology.
The impact of alien species as agents of extirpation and extinction is
of great concern because most habitats and regions of the world are being
flooded by alien species in a very short period of ecological and evolu-
tionary time. In most North American ecosystems, for example, more
than 10% of species are now aliens, with more appearing every year.This
rate of invasion exceeds by several orders of magnitude the invasion rate
in pre-European time.
The extinction potential of this new era of alien invasions rivals the
wave of extinction accompanying the colonization of the New World,
Australia, New Zealand, and many oceanic islands by humans. More than
half of the large mammals of North America disappeared at the end of the
Pleistocene, coincident with the appearance and spread of human hunters
through the New World.That human overkill is the probable mechanism
of these extinctions is strongly supported by much evidence. Simulation
of the population dynamics of the North American megafauna, species by
species, confirms that human hunting is adequate to account for the
extinction of almost all species (Alroy 2001).
In Australia, where 23 of 24 genera and 85% of species of large mam-
mals disappeared about 46,000 yr ago, human colonization and its eco-
logical impacts are among the most likely causes of extinction (Miller
et al. 1999; Roberts et al. 2001). In New Zealand, recent data and analy-
ses indicate that all 11 species of moas were hunted to extinction by
humans in less than 100 yr, following arrival of Polynesians in the thir-
teenth century
A
.
D
. (Holdaway and Jacomb 2000). Throughout Melane-
sia, Micronesia, and Polynesia, colonization by humans between 1,000 and
30,000 yr ago resulted in extinction of more than 2,000 species of birds
(Steadman 1995).
Evolutionary Responses of Native Species
One of the striking features of many invasions is a massive, early popula-
tion outbreak that makes the alien form conspicuous and often highly
destructive. In some species this outbreak may occur immediately after
initial introduction; in others it occurs only after a delay of years, decades,
or even a century. Following the outbreak, however, the population of the
