Invasive Species - Ecological Systems - page 161

colonization and impact are presented. Next, management approaches to assessing,
preventing, and mitigating this problem are considered. The entry ends with a brief
glimpse at some of the emerging issues that will likely be the foci of future research.

Pattern and Process in Biological Invasion

The process of invasion comprises a sequence of events involving the transport,
introduction, establishment, and spread of organisms into a new region. Organisms
in various life stages may be moved by natural dispersal (e.g., passive transport by
wind, water currents, or animals; active transport by the organism's own
movements) or, far more frequently, by human activities (e.g., transportation
systems carrying people or material) across a geographic barrier that previously
defined the limits of the historic range of the species. Most organisms will die soon
after arrival, or reproduce for only a couple of generations; thus, the vast majority of
introduction events fail to produce a sustainable population. If a sufficient number
of healthy individuals arrive in a suitable habitat when conditions are favorable,
then a self-sustaining population will develop and the species is said to be
established. Although populations can sometimes establish from very small num-
bers, higher numbers of introduced individuals and more frequent introduction
events (collectively termed propagule pressure ) contribute to a higher probability
of establishment [ 6 ].
In general, the more species introduced to an area, the more that become
established in that area [ 7 ]. Lonsdale [ 8 ] presented an instructive model to describe
the number of nonnative species in a region, E:

E ¼ I S

where I is the number of species introduced ( colonization pressure [ 7 ]) and S is the
product of the survival rate of each species. S is a function of both the biological
traits of the nonnative species and the environmental conditions of the target
habitat; for example, all other things being equal, a higher survival rate would
result from a closer match between the species' physiological requirements and the
prevailing habitat conditions.
There is a variable time lag between initial introduction and establishment,
followed by an exponential increase in abundance until the population reaches
limits imposed by local abiotic and biotic conditions, at which point population
growth diminishes. The range expansion of the species (increase in area occupied
per unit time) is correlated with its population growth. The lag phase may range
from being negligible (e.g., for a rapidly reproducing species) to extensive - during
which the species may remain inconspicuous for years or decades prior to becoming
abundant and widespread [ 9 , 10 ]. For example, the first outbreak of the European
gypsy moth ( Lymantria dispar ) in North America occurred two decades after it was
initially released. A mussel introduced from the Red Sea remained rare for about
120 years prior to developing dense colonies on the Israeli Mediterranean coast [ 9 ].