Environmental Engineering Reference
In-Depth Information
being explored for predicting rates of spread from simple dispersion or determinis-
tic spatial models to stochastic models (see Hastings
et al
. 2005 for review).
Spread models may have more limitations than potential distribution models
because they often are heavily dependent on complete information on the dis-
tribution and abundance of the target species, and the predictions of the estab-
lishment, growth, reproduction, and migration of metapopulations in complex
environments. Moderately sessile organisms such as plants might provide simple
cases to begin developing estimates of species spread. h ere may be a link between
establishment success and invasion success (i.e. frequency and abundance) for
many species. However, accurate monitoring of the distribution, abundance, and
spread of metapopulations, species, and genotypes remain rare in the ecological
literature (e.g. Harrison 1991).
h e costs associated with invading species may be environmental, economic, or
impacts to human health. Assessing environmental risks includes potential losses
or declining populations (or genotypes) of native species, declines in ecosystem
services (e.g. water quality, fi re prevention), or undesirable eff ects on ecosystems
processes (e.g. increased fi re frequency). h ere are many examples of native spe-
cies declines including the eff ects of Dutch elm disease caused by the fungus
(
Ophiostoma ulmi
) on elm trees (
Ulmas
spp.) or the loss of native populations of
Phragmites
due to invasive non-native genotypes of same species. About 42% of
the species listed on the United States h reatened and Endangered species list, are
listed because of threats from non-native species (Wilcove
et al
. 1998). Direct and
immediate losses of native species might be rare, but are exemplifi ed by the loss of
12 native species of birds on Guam due to the voracious invading brown treesnake
(Fritts and Rodda 1998). Quantifying reductions in populations of native species,
loss of native genetic diversity, and extinctions requires non-market valuations
(Stohlgren and Schnase 2006).
Invasive species can degrade habitat quality for native species, aff ect nutrient
cycling, and promote disturbances such as wildfi re (Mack
et al
. 2002). h ese
impacts may be slow and chronic, such as the salinization of soils invaded by salt
cedar (
Tamarix
spp.), or they may be cataclysmic such as the rapid spread of aquatic
weeds such as hydrilla (
Hydrilla verticillata
) in the southeastern United States, or
the spread of sudden oak death in California (
Phytophthora ramorum
).
Total annual costs have been mentioned, but we often lack site-specifi c costs
and valuations for individual species, partly because we have poor maps of the
distribution and abundance of species, much less damage estimates throughout
their ranges.
Invasive species can directly and indirectly aff ect human health. Direct aff ects
are seen by the 100 human deaths due to West Nile virus in the United States
between 1999 (when it arrived) and 2004 (
http://www.cdc.gov/ncidod/dvbid/
westnile/surv&controlCaseCount04_detailed.htm).
Several human deaths
