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Interspecific competition and conservation of Pacific pocket mice (Perognathus longimembris pacificus)

  • Author(s): Chock, Rachel;
  • Advisor(s): Grether, Gregory F;
  • Shier, Debra M
  • et al.

Reintroduction programs for endangered species rarely take competitive interactions between species into account. The endangered Pacific pocket mouse (Perognathus longimembris pacificus) is being reintroduced to parts of its former range where multiple species of native rodents have overlapping diets. The species in this foraging guild compete for seeds both exploitatively and through direct interference interactions. I conducted simulated territory intrusion experiments between P. longimembris and four sympatric species, and determined that pocket mice, the smallest species, are subordinate to all larger species. Body size asymmetries strongly predicted dominance, regardless of phylogenetic relatedness or residency status. Repeated aggressive interactions from resident heterospecifics could lower the chances of reintroduced pocket mice establishing burrows during the critical settlement period. As such, temporarily reducing the density of competing species might be an advisable reintroduction strategy, in combination with other interventions, such as predator exclusion. However, the presence of other members of the seed-foraging guild could have a net benefit for P. longimembris, if pocket mice pilfer from the other species' seed caches more frequently than the other species pilfer from their caches. In a field experiment with dyed seeds I found that cache pilfering occurred infrequently, and a field-enclosure experiment revealed that none of the species use heterospecific scent to find (or avoid) seed caches. A year-long trapping study showed that species utilize spatial niche partitioning, but aggregate the timing of their activity, in areas with high levels of rodent activity. Species differed in the microhabitat they utilized, and although it is not clear if patterns of spatial niche partitioning are due to interspecific interactions or differences in habitat preferences, this study provided clear guidance for habitat management and release site selection for P. longimembris. Collectively, this research suggests that during the initial phases of reintroduction when Pacific pocket mice are establishing their burrows and foraging areas, they will benefit from a reduction of heterospecific competitors, who may displace them from optimal sites. However, during the later growth and regulation phases of a reintroduction, P. longimembris are expected to be able to persist without ongoing management of competitor species.

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