Invasive species act as a filter on the native community, leading to shifts in species composition by selectively affecting species with specific functional strategies. While many invasive plants display traits associated with high resource acquisition and fast growth, native plants in variable environments tend to adopt one of two strategies: fast growth during high resource availability to avoid stress (resource acquisitive) or slow growth during resource poor conditions to tolerate stress (resource conservative). Though the majority of research to date has focused on competitive effects among live plants, many invasive species also alter communities through their production of litter, yet how this litter shifts the functional composition of communities remains unknown. To understand these effects, I manipulated the presence of invasive annual grasses and their litter in an annual grassland and followed the demographic rates of six native annual forb species that varied in their resource acquisition strategy.
Results/Conclusions
I found that live grass competition alone marginally decreased per capita growth rates of resource-acquisitive natives and had no effect on resource-conservative natives. The addition of litter, however, led to declines in both types of natives, with substantially stronger declines in resource-acquisitive natives through differential effects on seed set and germination. In this grassland, invaders turn a favorable environment into an unfavorable one through the production of litter, limiting the capacity of both resource-acquisitive and resource-conservative natives to maintain population growth under high resource conditions, but with stronger effects on acquisitive natives. This study demonstrates that invasive plants have the potential to dramatically shift the functional composition of native communities not just through live competition, but also through lagged effects of litter.