Seed dispersal by caching rodents is a context-dependent mutualism in many systems. Plant populations benefit when seeds in shallow caches germinate before the rodent returns for them, often gaining protection from beetles and a favorable microsite in the process. Regular caching in highly unfavorable microsites, conversely, could undermine the dispersal benefit for a population. Plant invasions could disrupt the dispersal benefits of seed caching for native plants by attracting rodents to the protection of a dense canopy which then inhibits the establishment of native seedlings beneath it. To determine whether rodents disproportionately cache seeds under the dense canopy of an invasive grass (Pennisetum ciliare) in southeastern Arizona, we used nontoxic fluorescent powder and ultraviolet light to locate caches made from millet seeds offered to rodents in the field. We estimated the degree of disproportionate use of the grass for caching cover by rodents with a general model for estimating use relative to availability of cover types.
Results/Conclusions
The disproportionate use of plant cover for caching by rodents, dominated by genus Chaetodipus, depended on moonlight, with use of cover roughly proportional to plant cover at the new moon and highly disproportionate at the full moon. This agrees with past studies of rodent foraging and caching behavior being influenced by illumination. Across all moon phases, when rodents cached under plants, they cached disproportionately often under the invasive grass, given its relative cover. Directed dispersal of native seeds to the base of an invasive grass could magnify the competitive effect of the grass on native plants, further reducing their recruitment regardless of whether rodents would otherwise operate as net mutualists or net consumers. By increasing the spatial overlap of plant species, caching rodents increase those plants’ resource niche overlap and undermine their ability to coexist. Although the best-studied cases of mutualism disruption have involved the extinction of animal pollinators, the perturbations in ecological dynamics due to plant invasions could disrupt plant-animal dispersal mutualisms as well.