Biology Reference
In-Depth Information
habitats, and it is useless for all those species not yet transported by humans. One
consequence of our limited predictive abilities is that practical governmental efforts
to assess risk from alien species may focus on the hazards a species poses, rather
than the likelihood of its establishment or spread (e.g., Bomford, 2003).
Interestingly, the extent to which the recipient location has already been invaded
by other species can impinge on establishment success of new arrivals. Earlier inva-
sions may synergistically facilitate the success of later invasions - and thereby
magnify impacts on native ecosystems - in a process referred to as “invasional
meltdown” (Simberloff and Von Holle, 1999). This occurs when earlier invaders
provide resources - in the form of food, nutrients, pollination services, mycorrhizal
associations, seed dispersal, or habitat - critical to the successful survival of later-
arriving aliens. For example, the blind snake,
Ramphotyphlops braminus
, could not
have survived introduction to Hawaii without its alien food sources (ants, termites)
being introduced first. In this instance, the snake is ecologically benign, but many
facilitated introductions are not. Facilitation frequently takes the form of acquisition
of novel mutualisms among species (Simberloff and Von Holle, 1999; Richardson
et al., 2000b), but it may also be effected by alterations of habitats, resource-supply
rates, or disturbance regimes (Simberloff and Von Holle, 1999; Richardson et al.,
2000b; Ricciardi, 2005) or by protection from predators or competitors (O'Dowd
et al., 2003; Grosholz, 2005). These mutualisms may re-unite species that co-
evolved together and were independently transported to the new location, but more
often they involve generalists that can successfully form mutualistic pairings with
a wide array of potential partners (Richardson et al., 2000b). Moreover, an alien
may successfully establish but not become invasive until a facilitator species is later
introduced (cf. Grosholz, 2005). The importance of invasional meltdown is that it
provides a positive-feedback loop that makes recipient habitats more prone to addi-
tional invasions, accelerates the accumulation rate of alien species, and magnifies
impacts. This phenomenon makes invasion and ecological disturbance more likely
to occur over time, raising the concern that the rate of establishment, as well as the
magnitude of impacts, may be increasing. It also makes predicting the impacts of
any particular introduction more difficult.
We may also assess establishment success from a broader, community-level
perspective. In this case, alien species richness (number of naturalized alien spe-
cies) has been correlated with a variety of factors in an attempt to identify whether
particular areas or habitat types are more prone to alien invasion. Regional richness
in alien species has been correlated with human population numbers, land area,
disturbance, and native-species richness, and these may vary in importance across
spatial scales (Lonsdale, 1999; McKinney, 2001; Sax, 2002). With respect to
human population, temporal growth in numbers of naturalized aliens has been cor-
related with increasing human population (Mauchamp, 1997; K.G. Smith, 2006a),
and spatial variation in species richness has been correlated with variation in human
population numbers (McKinney, 2001, 2002; Espinosa-Garcia et al., 2004; Gido
et al., 2004). Many of these correlations are not ecologically surprising. Increasing
land area should generally lead to increased species numbers because larger areas
tend to hold greater habitat diversity, which will itself be correlated with increased
