Caching rodents disproportionately disperse seed beneath invasive grass

Abstract

Seed dispersal by caching rodents is a context-dependent mutualism in many systems. Plants benefit when seed remaining in shallow caches germinates before being eaten, often gaining protection from beetles and a favorable microsite in the process. Caching in highly unfavorable microsites, conversely, could undermine the dispersal benefit for the plant. Plant invasions could disrupt dispersal benefits of seed caching by attracting rodents to the protection of a dense invasive canopy which inhibits the establishment of native seedlings beneath it. To determine whether rodents disproportionately cache seed under the dense canopy of an invasive grass in southeastern Arizona, we used nontoxic fluorescent powder and ultraviolet light to locate caches of seed offered to rodents in the field. We fitted a general habitat-use model, which showed that disproportionate use of plant cover by caching rodents (principally Chaetodipus spp.) increased with moonlight. Across all moon phases, when rodents cached under plants, they cached under the invasive grass disproportionately to its relative cover. A greenhouse experiment showed that proximity to the invasive grass reduced the growth and survival of seedlings of a common native tree (Parkinsonia microphylla) whose seeds are dispersed by caching rodents. Biased dispersal of native seed to the base of an invasive grass could magnify the competitive effect of this grass on native plants, further reducing their recruitment and magnifying the effect of the invasion.