Biology Reference
In-Depth Information
empirical information to develop or modify weed control procedures, and also to
justify invasive plant control. In contrast to agricultural systems, a major focus of
weed management in natural production systems is to assess the risk that new plant
species will become invasive in order to prevent their spread. Byers et al. (2002)
identify four levels of
risk assessment
associated with the biological stages of
exotic species invasion:
●
Arrival (risk associated with entry pathways)
●
Establishment (risk of forming viable, reproductive populations)
●
Spread (risk of expanding the range or extent)
●
Impact (risk of having a measurable effect on existing species or communities)
Thus, management of weeds in natural systems focuses primarily on detection and
eradication of potentially invasive plants that are not yet widespread. The assess-
ment of invasive plants, however, has proven more difficult than simply finding,
counting, and controlling them (Auld et al. 1987; Hobbs and Humphries 1995;
Leung et al. 2002; Pitafi and Roumasset 2005).
17.3
The Question of Whether, Not How
Management of invasive plants is a general strategy that encompasses prevention,
eradication, and control.
Prevention
of species invasion involves procedures that
inhibit or delay establishment of weeds in areas that are not already inhabited by them.
These practices restrict the introduction, propagation, and spread of weeds on a local
or regional level. Quarantines, surveys, and monitoring are the first steps in prevention
of invasive species.
Eradication
is the elimination of a plant species from a field, area,
or entire region. It requires the complete removal of seed and vegetative parts of a spe-
cies. Eradication is usually attempted only in small areas or those with high-value
crops or land use because of the difficulty and high costs associated with the practice.
Eradication of small weed patches, however, is a low-cost tactic for management of
invasive plant species, especially when compared with control costs of species with
much broader distribution (Rejmánek 2000; Radosevich et al. 2007).
Control
practices
reduce or suppress weeds in a defined area but do not necessarily result in the elimina-
tion of any particular species. Similar to control,
containment
is often a goal of man-
agement of invasive plants, where the infestation is held to a defined geographic area
and not allowed to spread. This strategy involves habitat manipulation through plant
community restoration, often at landscape and regional scales.
Finnoff et al. (2005) developed models to examine the economics of invasive
species. They identified allocations of manager time and capital to prevent
versus
control invasive species in order to achieve an acceptable risk (Leung et al. 2002).
Figure 17.3 is an analysis of managers who are risk neutral (RN), mildly risk-
adverse (RA1), moderately risk-adverse (RN2), and highly risk-adverse (RN3) at
four monetary discount rates. These results indicate that managers select activities
that seem least risky, which means less prevention and more control (Finnoff et al.
