Competition With Native Species Can Impact Reintroduction Success For The Endangered Pacific Pocket Mouse

As we enter into the Anthropocene, with the human impact on natural systems becoming ever greater, improving the success rates of conservation measures is more vital than ever. Methods that are being used with increasing frequency include translocation (moving wild animals from one area of their range to another in order to augment existing or create new populations) and captive breeding and reintroduction programs.

Unfortunately, these efforts have historically low success rates, with reintroduced animals fleeing from release sites, susceptible to higher rates of predation, or failing to reproduce in the wild. Research shows specific efforts to mitigate these risks can lead to higher success rates. For example, acclimation cages or pens can reduce dispersal and encourage animals to remain in the protected habitat; temporary predator exclusion gives newly released animals time to explore the habitat and find or build refuges; and release groups comprised of animals that have established social relationships reduces the time they spend fighting with one another and increases their reproductive rates. One factor that may be important, but has yet to receive much attention, is competition with other species at the reintroduction site.

The Pacific pocket mouse (Perognathus longimembris pacificus) is an endangered species historically found in the coastal sage scrub of southern California and northern Baja, Mexico. In 2012, with only three populations remaining, a captive breeding program was initiated with a goal of reintroducing the species to areas of their historic range and establishing additional wild populations. Pacific pocket mice naturally coexist with other species of rodents, but as the smallest members of the community (and one of the smallest mammals in North America, weighing in at just 6 grams), competitive interactions might play an important role in the critical early stages of reintroductions, when animals are more likely to flee from the release site.

Researchers set out to understand the relationship between Pacific pocket mice and each of the four competitor species that live at the reintroduction site. They wanted to know if one species was most dominant to Pacific pocket mice, and therefore be avoided or temporarily excluded from the reintroduction site. They also tested whether resident animals had an advantage during one-on-one interactions. If this were the case, temporarily removing competitors and allowing pocket mice to establish territories could allow them to maintain these territories as the other species returned to the area.

Understanding the patterns that predict dominance between species would also make this work relevant for other communities. Body size is expected to be an important factor, with larger species dominating smaller species; however, closely related species can be more similar in their diet and habitat needs, leading to increased competition, so relatedness (time since most recent common ancestor) was also tested as a predictor of dominance.

Researchers staged interactions between two individuals, one pocket mouse and one other species, by placing them in a plexiglass enclosure near the burrow where one of the animals (the resident) was caught. The enclosure had no bottom, so animals could see and smell their surroundings to determine if they were in a familiar area. The species rarely fought during the 5-minute trials, but they displayed multiple aggressive and submissive behaviors, such as approaching and displacing one another, which would create consequences for animals trying to access the same resources.

A dominance index was created by taking the number of aggressive displays, subtracting the number of submissive displays, and dividing by the total number of displays by each individual. Body size was the best predictor of dominance, as the largest animals were most dominant over the smallest, and species closely matched in body size had similar dominance scores. Neither relatedness or residency status influenced dominance interactions between species.

Pocket mice showed fewer aggressive displays than kangaroo rats, the largest species, but an equal number of aggressive behaviors as the three other species in this study. Pocket mice, however, displayed a higher number of submissive behaviors than all the larger species. Dominance hierarchies were maintained through the frequency of submissive behaviors by subordinate animals rather than high rates of aggressive behaviors by dominant animals. This result may be surprising, as we tend to think of aggression as the main component of dominance, but is similar to findings from studies of baboon social groups.

Researchers also studied patterns of pursuit and avoidance by measuring the distance between animals throughout the trials and assessing which species moved closer to or further from their opponent. Both size and relatedness were influential, with larger species pursuing and smaller species avoiding opponents, and there were more interactions between closely related species compared to those more distantly related. Avoidance of dominant species can be costly for pocket mice if it prevents them from using optimal shared habitat or reduces the time they have for other activities such as foraging or finding mates.

Wildlife managers are implementing these findings in ongoing reintroductions of captive-bred Pacific pocket mice. Although species can coexist in established communities, reintroduced animals face extra challenges of learning an unfamiliar area, building new burrows for shelter, avoiding predators they have never experienced, and establishing relationships with new neighbors. The highest rates of mortality occur in the first days to weeks following release, and minimizing aggressive interactions during this critical period could increase the chances of settlement and reintroduction success.

This research shows that kangaroo rats are the most dominant species, and could exclude Pacific pocket mice from preferred habitat or other limited resources during settlement. Selection of release sites that do not contain kangaroo rats, or avoiding areas with medium to high densities of kangaroo rats, may give reintroduced Pacific pocket mice a better chance of establishing a new wild population and improve the chances of survival for a species on the brink of extinction.

These findings are described in the article entitled Body size, not phylogenetic relationship or residency, drives interspecific dominance in a little pocket mouse community, recently published in the journal Animal Behaviour. This work was conducted by Rachel Y. Chock, Debra M. Shier, and Gregory F. Grether from the University of California Los Angeles and the San Diego Zoo Institute for Conservation Research (DMS).