Pocket mice in peril

The endangered Pacific pocket mouse (PPM) was historically distributed across a stretch of sandy soil habitat spanning from Tijuana to Los Angeles. After large-scale development began along the coast in the 1930’s, it was rarely seen, and was thought to be extinct before it was rediscovered in the 1990’s. PPM currently persists in three isolated populations in Southern California, cut off by an urban landscape.

Habitat fragmentation from urban development often leads to the loss of genetic diversity and inbreeding. We measured these indicators of population health by taking small tissue samples from wild PPM and genotyping them.

Indeed, genetic information from hundreds of wild PPM tell us that the three remnant populations are very small: the number of breeders ranges from about 3 to 50 individuals. One population of PPM had nine different genotypes (unique DNA sequences) at a maternally-inherited gene before 2004, but after 2012 all samples had the same, single genotype. This dramatic loss of diversity in less than two decades reveals the powerful effects of isolation on small populations.

Assisted migration to enable the exchange of genetic variation (gene flow) between fragmented populations can help preserve genetic diversity and reduce inbreeding. The positive effects of gene flow into isolated populations has been demonstrated in Florida panthers, Rocky Mountain bighorn sheep, prairie chickens, Scandinavian wolves, and European adders.

But this management strategy must be applied with caution, because interbreeding populations that have been separated for too long can actually make the problem worse. An extreme example of this problem comes from the mule, the hybrid offspring of a female horse and a male donkey, which is usually infertile. This is called outbreeding depression, the reduction in survival and reproduction of outbred offspring. Understanding its risks and causes can help us better conserve endangered species in the wild, including the Pacific pocket mouse.

The conservation breeding program gives some answers

To examine the risk of outbreeding depression from assisted migration in the wild, we looked to the PPM conservation breeding program at the San Diego Zoo. Ongoing threats in the wild prompted the establishment of the breeding program in 2012 to prevent extinction, preserve remaining genetic diversity and eventually reintroduce PPM back into unoccupied areas of their historic range. The breeding program also allowed us to test, over multiple generations, the reproductive success of PPM individuals with ancestry to the three wild populations.

The results showed a benefit to the introduction of new genetic variation through gene flow in all populations, but gene flow from certain populations was more successful than others.

This information is helping us to refine our recommendations about how these endangered populations should be managed to minimize the risk of extinction. In this way, PPM in the zoo are helping us by not only providing a source of individuals for reintroduction to areas where they have been lost, but also by helping us to better ensure the survival of the wild populations that persist.

Photo above: An endangered Pacific pocket mouse born in the Conservation breeding program. Photo credit: Daria Clark