Biology Reference
In-Depth Information
agement requires that an assessment of both possible and real damage be made
prior to the introduction of weed control tactics (Norris et al. 2003; Radosevich
et al. 2007). The most common thresholds applied to weeds are those that relate
to damage, economics, and action. A
damage
threshold describes the plant popu-
lation at which negative impact to a crop is detected. It is usually expressed as
plant density or biomass per area. An
economic
threshold is the weed density or
damage level at which control measures should be taken to prevent further eco-
nomic injury from being incurred. The economic threshold is also called the
eco-
nomic injury level
(EIL) (Norris et al. 2003), which implies that the costs of
control should be less than the loss that would have occurred had nothing been
done. An
action
threshold is the weed population level at which some interven-
tion is needed to preclude further damage. A method to construct thresholds is
discussed in Radosevich et al. (2007).
In addition, the traditional views of succession are now being challenged by
scientists who believe that the process is more climate- and disturbance-driven than
driven by competition (Westoby et al. 1989; Briske et al. 2003). These scientists
argue that many equilibrium states probably exist among plant communities during
succession and that transitions among these states occur when an
ecological thresh-
old
is crossed (Kimmins 1997). According to Briske et al. (2003), ecological
thresholds separate multiple equilibrium states and can be distinguished by changes
in community structure and composition. Many exotic invasive plant species are
believed to disrupt native ecosystem function by their presence (Vitousek et al.
1996; Hobbs and Huenneke 1992; Quigley and Arbelbide 1997; Sheley and Petroff
1999; Harrod 2001).
The possible impacts of exotic invasive plant species on ecological thresholds
of natural production systems are only now being considered by land managers.
The introduction of IPM concepts also has been slow to emerge in natural pro-
duction systems, although the threshold most easily recognized by land managers
is the recent directive for EDRR. In this case, control tactics are employed to
eradicate the patch once a new exotic species that is deemed to be harmful is
found in a new area. This
action
implies that any damage by the weed to the plant
community is too much. According to Hobbs and Humphries (1995), this type of
threshold should be employed in a natural production system only when a
resource or area is extremely valuable and the risk to it by presence of invasive
plant is great. It must be recognized, however, that eradication of a few small
patches or isolated plants may be an extremely cost-effective form of weed man-
agement. In late stages of succession, if disturbance to the natural system is
severe or if the presence of exotic plants is ubiquitous, the economic threshold
(EIL) would be a better measure of the cost-effectiveness for restoration than
EDRR. In this case, the damage to a plant community by various levels of inva-
sive plant species and the costs of restoration would be compared with the long-
term gain in ecosystem function from the action. In most well-established natural
plant communities, species diversity, complexity, and coexistence are the rule,
rather than direct competition (Vandermeer 1989). Thus, it is possible in many
cases that the threshold for action is higher than 0. It also seems possible that both
