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Harvard Forest Symposium Abstract 2013

  • Title: A Conceptual Model Of Scatter Hoarding With Direct Implications For Seed Dispersal: The Habitat Structure Hypothesis
  • Primary Author: Michael Steele (Wilkes University)
  • Additional Authors: Salvatore Agosta (Center for Environmental Studies and Department of Biology, Virginia Commonwealth University); Thomas Contreras (Washington & Jefferson College); Leila Hadj-Chikh (Wilkes University); Chioma Ngumezi (Brigham's and Women's Hospital, Harvard University); Peter Smallwood (University of Richmond)
  • Abstract:

    Scatter hoarding is a common food hoarding strategy of many granivores that is thought to facilitate seed dispersal. The most widely accepted model of scatter hoarding, the Optimal Density Model, predicts that the spatial distribution of scatter hoards results from a trade-off between the energetic cost of spacing caches and the increased risk of pilferage as cache densities increase. Here, we present evidence that eastern gray squirrels (Sciurus carolinensis) rely on an alternative strategy in which preferred food items are stored in open habitats, beyond crowns, where the probability of predation rates are higher but the risk of cache pilferage is reduced. We tested this hypothesis (hereafter the Habitat Structure Hypothesis) in three field experiments. In the first, we presented squirrels with acorns of two different sizes at the bases of individual oaks, and measured the distances that acorns were dispersed in relation to the canopy of the presentation tree. In the second experiment, we mapped the acorn (Quercus spp.) caches of individually marked free-ranging gray squirrels to determine the shortest distance of each cache from the edge of tree canopy. In the third, we tested the effects of both tree canopy cover (HSH) and cache density (ODM) on pilferage rates by simulating acorn caches at high and low densities in two cover types: under canopy and in the open, 0-30m from canopy edges. Scatter-hoarding decisions of squirrels strongly supported this hypothesis. In the first two experiments, squirrels cached larger, more profitable acorns at significantly greater distances from canopy edges than smaller acorns, which were more often cached under trees. In the third experiment, in which we tested the effects of both tree canopy cover and cache density on pilferage rates, we found no effect of density on pilferage rates, but significantly higher pilferage rates under canopy cover. Our results indicate that more profitable items are placed outside canopy, and that selection of such cache sites are a way of decreasing cache pilferage by conspecifics. We suggest that this behavior may reflect a general strategy of many scatter hoarding species that likely increases the probability of dispersal and establishment of seeds.

  • Research Category: Biodiversity Studies