Cache pilfering in a granivore guild: Implications for reintroduction management

Niche partitioning in an assemblage of granivorous rodents, and the challenge of community-level conservation

Coexistence of competing species in the same foraging guild has long puzzled ecologists. In particular, how do small subordinate species persist with larger dominant competitors? This question becomes particularly important when conservation interventions, such as reintroduction or translocation, become necessary for the smaller species. Exclusion of dominant competitors might be necessary to establish populations of some endangered species. Ultimately, however, the goal should be to conserve whole communities. Determining how subordinate species escape competitive exclusion in intact communities could inform conservation decisions by clarifying the ecological conditions and processes required for coexistence at local or regional scales. We tested for spatial and temporal partitioning among six species of native, granivorous rodents using null models, and characterized the microhabitat of each species using resource-selection models. We found that the species’ nightly activity patterns are aggregated temporally but segregated spatially. As expected, we found clear evidence that the larger-bodied kangaroo rats drive spatial partitioning, but we also found species-specific microhabitat associations, which suggests that habitat heterogeneity is part of what enables these species to coexist. Restoration of natural disturbance regimes that create habitat heterogeneity, and selection of translocation sites without specific competitors, are among the management recommendations to consider in this case. More generally, this study highlights the need for a community-level approach to conservation and the usefulness of basic ecological data for guiding management decisions.

Pilfering of western juniper seed caches by scatter-hoarding rodents varies by microsite and canopy type

Scatter-hoarding rodents store seeds throughout their home ranges in superficially buried caches which, unlike seeds larder-hoarded in burrows, are difficult to defend. Cached seeds are often pilfered by other scatter-hoarders and either re-cached, eaten or larder-hoarded. Such seed movements can influence seedling recruitment, because only seeds remaining in caches are likely to germinate. Although the importance of scatter-hoarding rodents in the dispersal of western juniper seeds has recently been revealed, the level of pilfering that occurs after initial burial is unknown. Seed traits, soil moisture, and substrate can influence pilfering processes, but less is known about how pilfering varies among caches placed in open versus canopy microsites, or how cache discovery and removal varies among different canopy-types, tree versus shrub. We compared the removal of artificial caches between open and canopy microsites and between tree and shrub canopies at two sites in northeastern California during late spring and fall. We also used trail cameras at one site to monitor artificial cache removal, identify potential pilferers, and illuminate microsite use by scatter-hoarders. Removal of artificial caches was faster in open microsites at both sites during both seasons, and more caches were removed from shrub than tree canopies. California kangaroo rats were the species observed most on cameras, foraging most often in open microsites, which could explain the observed pilfering patterns. This is the first study to document pilfering of western juniper seeds, providing further evidence of the importance of scatter-hoarding rodent foraging behavior in understanding seedling recruitment processes in juniper woodlands.