Biology Reference
In-Depth Information
Allee effects are also important to consider individually and are defined as “a
positive relationship between any component of individual fitness and either num-
bers or density of conspecifics” (Stephens et al. 1999). In Washington state, fecun-
dity of the nonnative smooth cordgrass (
Spartina alterniflora
) was greater in
high-density areas than in low-density areas, resulting in a mean spread rate of 31%
compared with 19% (Davis et al. 2004; Taylor et al. 2004). This example allee
effect in the low-density areas is a weak effect, where the spread rate slows but still
occurs, but is still an important factor in considering forecasts of the spread of
smooth cordgrass.
6.2.2 Spatial Considerations
Understanding the relationship of spread rates to spatial scale, both resolution and
extent, is another important aspect of forecasting. An invading species may not
have the same rate of spread at different leading edges of its expansion, and rates
may change through time as they are moderated by other factors like climate. For
example, the spread of hemlock woolly adelgid (
Adelges tsugae
) in the Eastern
United States had different rates through time at different leading edges and at the
same location through time (Evans and Gregoire 2007). Different modeling
approaches and different datasets to predict woolly adelgid spread all yielded vary-
ing rates, but were consistent in predicting spread rates to differ geographically. The
adelgid spread more quickly in the southern US than at the northern edge, probably
because of colder temperatures in the north. Additionally, at a smaller spatial scale,
counties on the leading edge were invaded more slowly if the leading edge arrived
during a colder winter, further evidence that interannual climatic variation may also
affect the rate of spread.
Spread rates may also differ depending on the spatial resolutions. Large scale
(like continental) spread rates result from long distance dispersal events which are
often human mediated, like the spread of zebra mussels (
Dreissena polymorpha
)
appearing in western US waterways unconnected to the heavily infested ones in the
eastern US. However, at the local scale such as a preserve or meadow allee effects
can be important, where spread rates differ as a result of population dynamics.
These rates of spread would be measured in units of meters per year, while the long
distance dispersal events measurements differ by orders of magnitude.
6.2.3
Vectors of Invasion
Including vectors or pathways of spread in predictive models may also be important
in forecasting invasions (Leung et al. 2006). Spread of an invasive species by non-
anthropogenic means may be predictable, though difficult; spread by humans will
be harder to predict as humans may have less predictable pattern in their movement
