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

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.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 de? creasing 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 are (0} 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, shouldmcrease 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 6 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 maintenace of optimum cache density by S. niger. The coevolution of trees which have mast years and the animals that scatterhoard their seeds is discussed. nprvnl mnHeP vnuirre]' ut/ilnutdispersal model: squirrel: walnut enough to make whole cones an ineffective source of seeds for mice, and the chipmunks that can open cones 1 Manuscript received 26 September 1977; accepted 15 February 1978. are driven away from larders for the short period be? fore chipmunks enter hibernation (Smith 1968). The system of individual, intrageneric territories effectively defends an individual's source of cones and its larder from its 1 source of competition, other individ...

Scatter hoarding by the Central American agouti: a test of optimal cache spacing theory

“…Our results showed that the density of agouti-made caches declined with increasing cache distance, in agreement with the prediction of Clarkson et al (1986), but not Stapanian & Smith (1978), who predicted that density should not be affected by distance. Several earlier studies have reported a similar negative relationship (Cowie et al 1981;James & Verbeek 1985;Clarkson et al 1986;Jansen et al 2004).…”

“…A key prediction of optimal cache spacing theory (OCST, Stapanian & Smith 1978;Clarkson et al 1986) is that cache spacing increases with food value. Our study is among the first (but see Hopewell et al 2008) to test this prediction in terms of economic value, that is, value-of-exchange, by comparing cache pilferage and cache spacing across a spatial gradient of ambient food availability, which we quantified as the number of seeds over relative agouti abundance (cf.…”

“…We created three 5 Â 5 grids in each plot during 14-20 May with 2.5, 5 and 10 m interspacing, corresponding to cache densities of 0.16, 0.04 and 0.016/m 2 . These densities are similar to those used in pilferage experiments by Stapanian & Smith (1978, 1984. At each position, we cached one palm seed that was thread-tagged by drilling a 1 mm hole to the distal end of the seeds and tying a 100 cm green nylon thread to the seed, attached the free end of the thread to the nearest sapling or tree, and marked that tree at eye level with a numbered tag.…”

“…Several studies have shown that cache robbery is indeed density dependent (e.g. Stapanian & Smith 1978, 1984Waite 1988;Daly et al 1992;Tamura et al 1999;Male & Smulders 2007a; but see Kraus 1983;Jensen 1985;Henry 1986;Van Horik & Burns 2007). Stapanian & Smith (1984) proposed that scatter-hoarding animals must space out their caches to a degree that optimally balances the benefits of reducing cache robbery risk and the costs of carrying food items to caches: 'optimal cache spacing' (Stapanian & Smith 1978;Clarkson et al 1986;Dally et al 2006).…”

Seed predation and defleshing in the agouti-dispersed palmAstrocaryum standleyanum

“…The second hypothesis -the 'theft-prevention hypothesis' -was that defleshing reduces post-dispersal seed predation by lowering the likelihood that seed-eating mammals recover cached seeds. Odour enhances cache detection by rodents and other macrosmatic mammals (Murie 1977, Stapanian & Smith 1978, Vander Wall 1991, Vander Wall et al 2003, and decaying fruit flesh and other soft material has a stronger odour than the seeds themselves. Defleshing might reduce cues that other animals use to find caches, and increase the recovery advantage for the cache owner.…”