Spatial patterns of acorn dispersal by rodents: do acorn crop size and ungulate presence matter?

Seed dispersal is qualitatively effective when it increases recruitment probability. A poorly studied factor likely affecting recruitment is the spatial distribution of dispersed seeds. Seed-caching animals are thought to disperse seeds in a way that reduces clumping and density to impede cache pilfering. Furthermore, dispersal might differ depending on whether the seed is immediately consumed versus cached for later consumption, and might differ depending on the ecological context. The main objectives of this study were to determine: 1) the spatial pattern of seed dispersal by rodents in a heterogeneous environment; 2) whether the patterns differ among years and among acorn competitor exclosure treatments, and 3) whether rodents create different spatial patterns of dispersal for acorns that are cached versus consumed immediately following dispersal. We studied the degree of spatial aggregation of acorn dispersal by rodents using two different estimators derived from the Ripley K and the Diggle G functions. We also analyzed various metrics of dispersal distances. For both analyses we used observed acorn dispersal patterns in two years differing in crop size and inside versus outside exclosures restricting access to acorn-consuming ungulates. During 2003, a year with a larger crop size, maximum seed dispersal distances were less, and the pattern of dispersed seeds was more clumped, than in 2004, a year with a smaller crop size. Median dispersal distances did not differ between years. In the presence of ungulates, seed dispersal was marginally sparser than in their absence. Cached acorns were dispersed more sparsely than acorns eaten immediately. These results have important implications for the quality of seed dispersal for oak recruitment that are likely relevant to other systems as well. [ABSTRACT FROM AUTHOR]