Environmental Engineering Reference
In-Depth Information
Recognition of the lag phase phenomenon is critical to management; otherwise, it
may lead to inaccurate assessments of benign invasion risk and low impact, as well
as missed opportunities to control a nonnative species population while it was still
small [
10
]. Non-mutually exclusive factors contributing to lag phases include:
(1) density-dependent (Allee) effects, in which the organism's birth rate is
correlated with its population density [
11
]; (2) adaptation and selection of new
genotypes; (3) a change in the composition of the recipient community (e.g., the
introduction of a pollinator or seed disperser [
12
], or the extinction of a dominant
resident predator) that triggers the explosive growth of a previously subdued
nonnative species; and (4) changing abiotic conditions (e.g., climate change [
13
])
that release the nonnative species from physiological constraints. Furthermore, the
inability to detect an inconspicuous population in its early growth stages is often
responsible for a substantial delay in the discovery of a nonnative species. Substan-
tial lags in detection, caused by inadequacies in monitoring and taxonomic exper-
tise, are a major hindrance to effective management [
14
].
The range expansion of an introduced species tends to fall into a few general
patterns, each of which is characterized by an establishment lag phase, an expansion
phase, and, when a geographic limit to suitable habitat is realized, a saturation
phase [
15
]. In the simplest pattern, the species expands its range linearly through
time; this pattern is the result of random short-distance dispersal outward in all
directions through a homogeneous environment, and is often exhibited by rodents
such as muskrats. The expanding range is modeled as a circle whose radius
increases at a constant rate [
16
]. The probability of invasion at a given site is
inversely proportional to the distance from the edge of the expanding colony and
directly proportional to time.
A second pattern is defined by a slow initial rate of linear spread followed by an
abrupt shift to a higher linear rate. This biphasic pattern, which has been observed in
invasive birds such as the European starling (
Sturnus vulgaris
), occurs when long-
distance migrants generate new satellite colonies not far from the primary colony;
the coalescence of satellites into the expanding primary colony generates a higher
linear rate of expansion. A third pattern occurs when long-distance dispersers create
numerous remote satellite colonies that begin to expand their range independent of
each other; their continuous coalescence generates an exponential expansion phase,
as exhibited by European cheatgrass (
Bromus tectorum
) in North America and tiger
pear cactus (
Opuntia aurantiaca
) in South Africa [
15
,
17
]. In this pattern,
a prolonged lag phase often occurs prior to conspicuous exponential growth.
Genetic adaptation is another mechanism that can produce the enhanced rate of
expansion that characterizes the second and third patterns, but the occurrence of
long-distance migrants is probably the more common cause. Via long-distance
“jumps,” migrants may establish satellite colonies that are remote from the
expanding edge of the primary colony; the overall rate of range expansion is driven
more by the number of these satellite colonies than by their individual size [
16
].
The pattern is more pronounced where human vectors dominate dispersal, such that
there would be multiple introductions of satellite colonies within a region (e.g., the
transport of zebra mussels and aquatic weeds between river basins by recreational
boats, or introductions of a marine invertebrate along a coastline via ballast water