Body size, not phylogenetic relationship or residency, drives interspecific dominance in a little pocket mouse community

The role of interspecific aggression in structuring ecological communities can be important to consider when reintroducing endangered species to areas of their historic range that are occupied by competitors. We sought to determine which species is the most serious interference competitor of the endangered Pacific pocket mouse, Perognathus longimembris pacificus, and more generally, whether interspecific aggression in rodents is predicted by body size, residency status or phylogenetic relatedness. We carried out simulated territory intrusion experiments between P. longimembris and four sympatric species of rodents (Chaetodipus fallax, Dipodomys simulans, Peromyscus maniculatus, Reithrodontomys megalotis) in a field enclosure in southern California sage scrub habitat. We found that body size asymmetries strongly predicted dominance, regardless of phylogenetic relatedness or the residency status of the individuals. The largest species, D. simulans, was the most dominant while the smallest species, R. megalotis, was the least dominant to P. longimembris. Furthermore, P. longimembris actively avoided encounters with all species, except R. megalotis. One management recommendation that follows from these results is that P. longimembris should not be reintroduced to areas with high densities of D. simulans until further research is carried out to assess the fitness consequences of the interactions. Our finding that the species least similar in body size is the most serious interference competitor of P. longimembris highlights an important distinction between interference and exploitative competition in rodent communities.