Invasive grass indirectly alters seasonal patterns in seed predation.

Invasive species threaten ecosystems globally, but their impacts can be cryptic when they occur indirectly. Invader phenology can also differ from that of native species, potentially causing seasonality in invader impacts. Yet, it is unclear if invader phenology can drive seasonal patterns in indirect effects. We used a field experiment to test if an invasive grass (Imperata cylindrica) caused seasonal indirect effects by altering rodent foraging and seed predation patterns through time. Using seeds from native longleaf pine (Pinus palustris), we found seed predation was 25% greater, on average, in invaded than control plots, but this effect varied by season. Seed predation was 24-157% greater in invaded plots during spring and autumn months, but invasion had no effect on seed predation in other months. One of the largest effects occurred in October when longleaf pine seeds are dispersed, suggesting potential effects on tree regeneration. Thus, seasonal patterns in indirect effects from invaders may cause underappreciated impacts on ecological communities.

Niche lability mitigates the impact of invasion but not urbanization.

Native species can coexist with invasive congeners by partitioning niche space; however, impacts from invasive species often occur alongside other disturbances. Native species' responses to the interactions of multiple disturbances remain poorly understood. Here we study the impacts of urbanization and an invasive congener on a native species. Using abundance (catch-per-unit effort) and vertical distribution of native green anoles (Anolis carolinensis) and invasive brown anoles (Anolis sagrei) across a gradient of natural-to-urban forests, we ask if niche shifting (lability) is occurring, and if it can mitigate impacts from one or both disturbances. We use generalized linear models to relate species abundances across the landscape to urbanization, forest structural complexity, and congener abundances (i.e., A. sagrei); and test for an interaction between urbanization and congener abundance. Our data show that A. sagrei presence results in a 17-fold upward shift in vertical niche of A. carolinensis-an 8.3 m shift in median perch height, and models reveal urbanization also drives an increase in A. carolinensis perch height. A. carolinensis and A. sagrei abundances negatively and positively correlate with urbanization, respectively, and neither species' abundance correlate with congener abundance. Despite a positive correlation between A. sagrei abundance and urbanization, our results do not show evidence of this interaction affecting A. carolinensis. Instead, niche lability appears to enable the native species to mitigate the impact of one driver of decline (invasive competition) while our data suggest it declines with the second (urbanization).