Recent declines in genetic diversity with limited dispersal among coastal cactus wren populations in San Diego County, California

Habitat loss and fragmentation can lead to smaller and more isolated populations and reduce genetic diversity and evolutionary potential. Conservation programs can benefit from including monitoring of genetic factors in fragmented populations to help inform restoration and management. We assessed genetic diversity and structure among four major populations of the Cactus Wren (Campylorhynchus brunneicapillus) in San Diego County in 2011–2012 and again in 2017–2019, using 22 microsatellite loci. We found a significant decline in heterozygosity in one population (San Pasqual) and a decline in allelic richness and effective population size in another (Sweetwater). Genetic diversity in the remaining two populations was not significantly different over time. Local diversity declined despite evidence of dispersal among some populations. Approximately 12% of genetically determined family groups (parents, offspring, siblings) included one or more members sampled in different territories with distances ranging from 0.2 to 10 km. All but one inferred dispersal events occurred within the same genetic population. Population structure remained relatively stable, although genetic differentiation tended to increase in the later sampling period. Simulations suggest that at currently estimated effective sizes, populations of Cactus Wrens will continue to lose genetic diversity for many generations, even if gene flow among them is enhanced. However, the rate of loss of heterozygosity could be reduced with increased gene flow. Habitat restoration may help bolster local population sizes and allelic richness over the long term, whereas translocation efforts from source populations outside of San Diego may be needed to restore genetic diversity in the short term. The Cactus Wren is a rare and fragmentation sensitive species in coastal southern California, USA. Two of four known aggregations of coastal Cactus Wrens on conserved lands in San Diego County showed significant declines in genetic diversity between 2011 and 2019 and all four aggregations have low effective population sizes. Forward‐time simulations show that assisted gene flow among these sites could slow the loss of heterozygosity but not overall allelic richness. Resource managers could use this information to assess whether conservation goals are being met and support management actions including translocation of individuals or eggs to preserve genetic diversity in the short term and support habitat restoration to b