Biology Reference
In-Depth Information
Alien Species, Biotic Homogenization,
and Invasional Meltdown
As alien species invade and alter native communities, endemic native
species disappear and the communities of similar climatic zones in differ-
ent world regions become homogenized.The number of successful alien
species is strongly correlated with the number of native species whose
existence is threatened (McKinney and Lockwood 2001). In the short
run, as alien species become naturalized, the richness of species, especially
of plants, tends to increase (Sax et al. 2002). In Hawaii, for example, the
native flora consists of 1,294 species, 71 of which are now extinct. About
1,090 species have become naturalized in Hawaii, however, so the flora
has nearly doubled in number of species. For vertebrate animals on
islands, on the other hand, establishment of alien species and extinction of
native species tend to balance each other very closely. In Hawaii, 64 bird
species have become extinct since humans arrived, whereas 55 species
have become naturalized.
The extent of homogenization in some types of ecosystems is often
not at all obvious. In stream faunas of mountainous regions, for example,
human land use and, increasingly, global climate change, tend to promote
the spread of downstream organisms into higher elevation, upstream areas
(Scott and Helfman 2001). Downstream faunas tend to comprise gener-
alist and alien species, so their spread is often accompanied by extirpation
of species endemic to highland areas.
The disruption of native communities by alien invaders has a distinctly
ominous potential: the process tends to exhibit positive feedback. Sim-
berloff and Von Holle (1999) coined the term “invasional meltdown” for
such a process. Simply stated, invasional meltdown means that successful
invaders facilitate the invasion of still other alien species.This might occur
because aliens disrupt the structure or function of the ecosystem so as to
create conditions favoring invasion by other species, including other
aliens. It might also occur because aliens create conditions or constitute
resources that specifically favor other aliens.
The precise nature of synergistic effects has been documented by two
recent analyses. Simberloff and Von Holle (1999) examined a large num-
ber of articles in journals of ecology and conservation biology to deter-
mine the frequency of positive interactions among introduced species.
Out of 190 interactions that could be categorized, 10 were mutually ben-
eficial (mutualistic), 12 were beneficial to one species and harmless to the
other (commensal), 156 were beneficial to one and harmful to the other
