Habitat Fragmentation in Small Vertebrates from the Sonoran Desert in Baja California

Abstract

Land conversion is one of the greatest threats to terrestrial ecosystems around the world, and understanding its impacts on the biota is crutial for the management and conservation of species in and around human-modified landscapes, particularly in those where local declines can quickly translate into the extinction of endemic species or Evolutionary Significant Units.I investigated how habitat loss and fragmentation impacted dispersal and extinction risk in three small vertebrates (a phrynosomatid lizard Urosaurus nigricaudus, and two heteromyid rodents Chaetodipus arenarius and Dipodomys simulans), in a highly fragmented agricultural valley from the Sonoran Desert in the Baja California Peninsula, Mexico, where reptiles and rodents show high endemism and phylogenetic diversity. After reconstructing the history of habitat loss at the valley during the last 60 years, my approach involved the development and genotyping of 10 DNA microsatellite loci in 800 individuals from the three species that were sampled from continuous and fragmented habitat and analyzed using various population genetic methods.Although genetic diversity was not significantly affected by habitat loss and fragmentation, I observed an increase in genetic structure, relatedness, the spatial scale of individual movement and reversal of sex-biased dispersal in the three species, compared to continous habitat. I found evidence of a large and spatially localized extinction debt in the lizard, that showed individual dispersal restricted to<400 m in the fragmented habitat, while the two heteromyids seemed capable of dispersing over distances of few kilometers. Several observations supported a higher extinction risk in kangaroo rats compared to pocket mice. Continuous areas surrounding the fragmented landscape where identified as important sources of individuals to habitat fragments located nearby. Even the vegetation associated with a narrow wash across the fragmented landscape appeared to act as a corridor as high levels of dispersing individuals were inferred in the three species over a scale of several kilometers. This study provided an approach to evaluate the effects of distinct landscape features in preventing or allowing individual dispersal in multiple co-distributed species towards their conservation in human-modified landscapes.