Local- vs. landscape-scale indirect effects of an invasive weed on native plants
Abstract
Insect-mediated indirect interactions between native plant species recently have been
shown to be important determinants of plant performance in a number of ecological communities.
However, the potential indirect effects of exotic plant invasion on native plant species are not well
understood. We examined whether the presence or proximity of the targeted exotic weed, musk
thistle, influences the magnitude of attack on native thistles by the introduced biological control,
flowerhead weevil. At the local scale, we quantified weevil egg densities on heads of the native
wavyleaf thistle growing at different distances (0 to 100 m) from patches of the exotic thistle.
Densities were significantly higher when the native thistle occurred within, vs. 30 to 50 m or 80 to
100 m from, patches of the exotic thistle, indicating a strong local ‘‘spillover effect.’’ At larger
scales, we measured egg densities on wavyleaf thistle within grassland landscapes (2.4 × 2.4 km²)
with varying infestation densities of the invasive musk thistle. We found that egg densities increased
significantly with increasing invasive thistle densities measured at larger site and landscape scales.
Because flowerhead weevil feeding substantially reduces seed production of wavyleaf thistle, exotic
thistle populations are likely to have indirect negative effects on these native thistles. Our results
provide strong empirical evidence that exotic plants can increase the attack on native plant species
by maintaining populations of a shared insect herbivore. This finding suggests that persistence of
exotic weeds in less-successful biocontrol programs will magnify the nontarget effects of weed
biocontrol insects.
URI
http://hdl.handle.net/10057/3409http://dx.doi.org/10.1614/0890-037X(2004)018[1250:LVLIEO]2.0.CO;2