Invasive Species - Ecological Systems - page 171

Maintenance Control
When dealing with nonnative species with strong Allee effects, eradication may
involve culling individuals to bring a population below sustainable levels [ 11 ].
If eradication fails, or is impossible, the next option is maintenance control of the
invader at acceptable population levels, using mechanical, chemical, or biological
control methods. Mechanical control, such as hunting, may be particularly effective
on islands and other geographically restricted areas. Chemical control involves the
application of pesticides to reduce the abundance of a target species, but high
economic costs and human health risks constrain the application of chemicals over
large areas. Moreover, pesticides often impact nontarget species (including native
competitors), sometimes to the benefit of the target itself [ 113 ].
Biological control involves the introduction of a nonnative species (usually
a predator, herbivore, or parasite) to reduce an established nonnative pest to less
harmful densities. This technology is considered to be a more desirable alternative
to pesticide use, despite its potential for unanticipated consequences. Because the
introduced agents can disperse beyond the target area and evolve to exploit new
hosts, nontarget species may be attacked and even driven to extinction [ 17 , 114 ].
The assumption underlying biological control is that nonnative species proliferate
to harmful levels because they have escaped their natural enemies. However,
indirect (e.g., competitive) effects may sometimes be more important than top-
down consumer regulation. Under these situations, the introduction of a biological
control species may have a counterproductive effect [ 115 ]. Difficulties in predicting
such complex community interactions can obviously compromise ecological risk
assessments.
Future Directions
The questions underlying invasion ecology - that is, why some species are more
successful and have greater impact than others, why some systems are more
vulnerable to invasion, and how ecosystem functions and services are affected by
invasion - are clearly of societal importance and will remain relevant in the future,
as invasive species are increasingly viewed as a biosecurity issue [ 87 ]. The extent
and impact of invasions will be further exacerbated by climate change, and
synergies between nonnative species and other human-mediated stressors will
become more frequent. Future research foci will include the consequences
associated with cultivation of novel biofuels and bioenergy crops [ 116 ] and the
expanded use of genetically modified organisms [ 117 ]. Moreover, there may be
increasing interest among conservation biologists to relocate native species deemed
to be threatened by climate change or other stressors, and some plants and animals
could be moved well beyond their historical ranges [ 73 ]. Each of these practices
will have potentially high ecological risks whose assessment will require more