Voles mediate functional trait diversity along a resource gradient

Disentangling the effect of multiple ecological processes on plant trait composition is complicated by the fact that both top‐down and bottom‐up processes may affect similar traits. We examined the interacting role of resource variation and vole herbivory on functional trait patterns in an annual California grassland. We manipulated vole herbivory via exclosures at eight grassland sites along a steep resource gradient and measured plant composition and functional traits over 3 years. Plants with resource acquisitive functional traits were favoured at sites with increasing resources. Vole herbivory influenced community‐weighted mean (CWM) leaf nitrogen (N) and seed mass, suggesting these traits may mediate plant susceptibility to vole herbivory. After 3 years, CWM leaf N increased in the absence of the voles, as did CWM seed mass, although this increase in CWM seed mass only occurred at higher resource sites. Vole exclusion at high‐resources sites also increased the functional diversity of leaf N and seed mass by the end of the experiment. Overall, environmental filtering primarily structured the dominant plant trait strategies, but vole herbivory also influenced the functional diversity of traits that influence herbivore susceptibility, particularly at resource‐rich sites. Thus, habitat filtering and herbivory can operate on different dimensions of plant functional composition to influence the species and functional composition of communities. A free Plain Language Summary can be found within the Supporting Information of this article.