Environmental Engineering Reference
In-Depth Information
estuaries, rivers, and lakes from ballast water, with small organisms (larvae, eggs,
pelagic stages, etc.) being stored on board in the ships home port before transpor-
tation and release in a receptor port (Lodge 1993). Still, even rough estimates of
the abundance, viability, and condition of arriving organisms are unknown—for
invasive or less-invasive species. Likewise, small seeds of non-native annual weeds
contaminate native forage and crops. It is very costly to examine and purify every
large bag of seeds. Shipping manifests rarely describe organic hitchhikers in su -
cient detail to accurately assess exposure.
h e quantifi cation of 'propagule pressure' for biological organisms remains elu-
sive in risk assessment. For aquatic organisms (e.g. invasive fi sh, zooplankton),
propagule pressure might be quantifi ed as the number and viability of reproduct-
ive units arriving at a given location (Lodge 1993). However, many invasive species
do not have large, obvious, easily counted propagules, and quantifying propagule
pressure over large areas is problematic. Many pathways and corridors to invasion
are poorly understood. Corridors may include the matrix of roads and riparian
zones which may facilitate the spread of invasive riparian plants such as purple
loosestrife or tamarisk. Railroads also are linear, disturbed, habitats of invasion for
many non-native plants species. Can any country say how many seeds, spores, and
pelagic stages arrive undetected? How many establish in each habitat? As di cult
as this task seems, some estimates are possible for some species (based on trade and
transportation volumes and patterns, surveys, and rudimentary models). We need
more 'practice' estimating exposure.
Surveys in the early stages of most invasions are made di cult by small population
size, patchy distributions, and the cryptic nature of many initially-rare species in
complex landscapes and waterways. Cost and e ciency of information gain are
major considerations because only a small percentage of any area can be aff ordably
surveyed. Completely random survey techniques may be unlikely to detect new
cryptic invaders, especially if costs constrain sampling intensity and complete-
ness. However, probabilistic, iterative sampling (e.g. surveys guided to particularly
vulnerable habitats) may be an important tool in risk assessments (Crosier and
Stohlgren 2004; Stohlgren and Schnase 2006).
h e fi rst step in risk surveys is augmenting initial opportunist or subjective sur-
vey information with more systematic, less-biased, and more comprehensive sur-
veys in an iterative approach (Fig. 2.3). In the initial phase, only a few established
individuals or populations are known to investigators. h ey may add a few other
observations nearby in similar habitats (highly probable strata) with some loca-
tions in less-probable habitats to get a conceptual, fi rst-approximation model of
the species distribution and abundance (i.e. a species presence and absence model
and map). Upon the initial sightings, the proper authorities are alerted for rapid
response, containment, and restoration eff orts. Species a nities to habitat types
are noted, as are information gaps such as un-surveyed habitat types or areas.
