A Model for Seed Scatterhoarding: Coevolution of Fox Squirrels and Black Walnuts

Abstract

Granivores are likely to store food in numerous, widely scattered, small caches if they are unable to defend concentrated large caches against interspecific competitors. This scatterhoarding of seeds makes it impossible for individuals to defend all their scattered caches against intraspecific competitors as well. Optimal spacing of scattered caches should result from a balance between decreasing loss of caches to naive competitors with decreasing density and increasing cost of storage with decreasing density. A mathematical model predicting optimal density is presented. One prediction of the model is that as the (θ) of habitat suitable for seed burial surrounding a seed source is decreased, the average distance (D) a cache is taken from the source by scatterhoarders should increase. Another prediction is that increasing the number of seeds (N) at a source, either by an increase in the size of a single seed crop or by the presence of 2 or more conspecific seed producers in close proximity to one another, should increase D. One trait of trees which increases single crop sizes and average seed dispersal distance (D) is the pattern of withholding energy from reproduction some years to allow unusually large crops during mast years. Three field tests of the model and its predictions were conducted: (1) Juglans nigra seeds were buried at 3 densities and their survival in time from predation by Sciurus niger was found to increase with decreasing density; (2) average distances that J. nigra saplings occurred from 16 parent walnut trees were found to have a statistically significant correlation with the D calculated from the model and the θ of suitable habitat surrounding each tree; (3) Sciurus niger individuals were observed to scatterhoard Juglans nigra seeds in a pattern that maintained a mean cache density that gives low rates of loss to naive competitors as indicated from field test (1). The results strongly suggest maintenance of optimum cache density by S. niger. The coevolution of trees which have mast years and the animals that scatterhoard their seeds is discussed.