Biology Reference
In-Depth Information
abiotic feedbacks are now believed sufficient to reverse these patterns of
dominance.
Many research findings report spotted knapweed to exhibit positive feedback
with microbes and mycorrhizae in its introduced habitats, and the plant certainly
functions as an aggressive competitor with native plant species. Further, spotted
knapweed appears to have at least temporarily escaped specialist herbivores and
perhaps some specialist pathogens present in its native country. The species may or
may not exhibit allelopathic characteristics that enhance its invasiveness and com-
petitive characteristics, but this fact may be irrelevant given controls on seed pro-
duction and seedling survivorship. We find no fault with those studies that have
found that single species of herbivores can increase the competitive abilities of
spotted knapweed under a specified set of resource conditions (Callaway et al.
1999; Thelen et al. 2005). However, we do find that the experimental findings of
Pokorny et al. (2005) and Jacobs et al. (2006) are much more consistent with our
observations regarding the relative inability for individual spotted knapweed seed-
lings to become established or persist in intact communities when multiple special-
ist herbivore insects are present. Further, the herbivore studies by Corn et al. (2006)
and Story et al. (2006) match the management findings of Cortilet and Northop
(2006), and monitoring studies of Michels et al. (2007), documenting spotted knap-
weed decline due to biological control insects.
Our studies of herbivore impacts in the presence of strong interspecific competi-
tion and moderate to low soil resources also suggest similar declines (Seastedt et al.
2007 and unpublished results). Collectively, these studies argue that insect herbivory
is effective at controlling densities of spotted knapweed, and that the degree of control
will vary depending on site characteristics. Thus, we believe that (1) multiple herbiv-
ores appear to prevent spotted knapweed from exhibiting compensatory responses to
tissue removal and seed destruction by herbivory, and (2) that the long-term effect of
seed reduction in the presence of competing species is sufficient to reduce the densi-
ties of spotted knapweed. What we do not know, however, and what remains an open
research issue, is the range of habitats in which this response is likely to occur.
We predict that the suite of insects released against spotted knapweed will even-
tually be effective across at least a substantial portion of the introduced range of this
species. This prediction argues that many areas may not require additional manage-
ment activities beyond monitoring plant and insect abundance. However, studies
also indicate that the reduction in densities of spotted knapweed may be a slow
process, and one that might be facilitated by cultural techniques (e.g., adding desir-
able plant seeds; Jacobs et al. 2006). Use of biological control agents does not pre-
clude the use of other proactive control measures for spotted knapweed, but the
evidence for the efficacy of biological control insects published since 2006 reduces
the urgency to proactively manage this plant species with other mechanical or
chemical techniques. Accordingly, the use of both cultural and biological control
tools in natural areas will likely decrease spotted knapweed densities to a level
where the plant persists as neither an ecological nor as an economic concern. We
believe that the moniker, “the wicked weed of the West” (Alper 2004) may now be
transferred to another invasive plant species.
