Reciprocal pilfering in a seed-caching rodent community: implications for species coexistence

Abstract

Cache pilfering rates have been reported to be unsustainably high in many seed-caching rodent communities, but the dynamics of pilfering is largely unknown at the community level. In this study, we examined rates of seed-cache pilfering in a species-rich community of granivorous rodents in pair-wise trials. We compared the ability of each species to pilfer from conspecifics as well as heterospecifics to determine if pilfering is symmetrical or asymmetrical in the community. During the study, pilfering was more or less symmetrical among three scatter-hoarding species of rodents, averaging 28% (SD = 26%) of caches pilfered in 24 h, while the lone larder-hoarding species was unable to pilfer and experienced cache loss at the rate of 16 ± 14% of caches in 24 h to the other species. Pilfering was reciprocal among the scatter-hoarding species among conspecifics and heterospecifics despite differences in caching behavior (cache depth, size, and location). These finding support the hypothesis of reciprocal pilfering and are consistent with theories of the coexistence of ecologically similar species by lessening the effects of competition among species at the resource level and demonstrate that species with a pilfering disadvantage may need to exhibit different caching behaviors (e.g., larder-hoarding) to prevent competitive exclusion. Many rodents scatter-hoard seeds to survive periods when other food is scarce. Because these caches are usually undefended, individuals may experience significant theft of seeds. We reasoned that individuals that scatter-hoard seeds extensively are likely to have many of their caches pilfered, and that to counteract this loss, they should also be very effective pilferers of other animal’s caches. Conversely, animals that seldom scatter-hoard seeds are likely to be poor pilferers. This suggests that the ability to pilfer is part of the adaptive strategy of scatter-hoarding animals and that the more they scatter-hoard, the more they pilfer. Individuals that are unable to replace lost caches may not survive periods of food scarcity. There should be intense competition for stored seeds within communities of scatter-hoarding rodents, and this competition is manifested not only at the time of seed harvest but also as pilferage of cached seeds. In such communities, we expect pilferage to be reciprocal, or nearly equal among scatter-hoarding species, and thereby contribute to coexistence.