Results of a recent study by WERC scientists are providing helpful information to resource managers as they work to protect important habitat.
Western Ecological Research Center research geneticist Amy Vandergast, research ecologist and songbird expert Barbara Kus, and ecologist Kristine Preston collaborated with the California Department of Fish and Wildlife, the San Diego Association of Governments, the USGS Ecosystem Mission Area Wildlife Program, and dozens of partners and collaborators who participated in surveys and provided site access and other logistical support to complete this study. Results are presented in a paper published in the recent issue of Nature Scientific Reports.
The paper, Distinguishing recent dispersal from historical genetic connectivity in the coastal California gnatcatcher, found that although the threatened California gnatcatcher populations are fragmented in southern California coastal sage scrub habitat, genetic signatures indicated that recent long-distance dispersal has occurred so that the species appears to be a single genetic unit throughout most of its US range. This is good news for resource managers who are interested in understanding California gnatcatcher populations as they grapple with problems of habitat loss and fragmentation.
Results indicated that California gnatcatchers retain genetic connectivity across most of the current distribution of coastal sage scrub fragments, with the exception of some outlying aggregations, which appeared to be more isolated. Specifically, the authors found a positive and non-linear relationship between genetic diversity and the availability of suitable habitat within 30-km surrounding aggregations. This means that genetic diversity, a measure closely linked to population size and evolutionary potential, declined steeply when suitable habitat within 30 km fell below 10%. In regions where habitat connectivity was greater, the authors found evidence of recent long distance dispersal between geographically widespread aggregations of gnatcatchers. This genetic approach led to the novel finding that the dispersal capabilities of California gnatcatchers appear to be much greater than previously estimated from banding studies.
Land and resource managers may be encouraged that gnatcatchers retain genetic similarity and a large effective population size across the majority of their range in southern California. Both are positive indicators that gnatcatchers could persist in southern California under current conservation and management strategies, which rely on a network of preserves. The detection of lower connectivity and diversity within the smallest aggregations surrounded by low proportions of suitable habitat is important information for resource managers because it suggests that further habitat degradation throughout the range could lead to future loss of diversity.
The authors note that the genetic signatures found among gnatcatchers differed dramatically from those found in a co-occurring songbird, the coastal cactus wren. In a related study in the same preserve network, the cactus wren was found to have declined dramatically in abundance and had stronger genetic signatures of isolation. These differences may be due to different habitat and dispersal behaviors. Compared to California gnatcatcher the cactus wren has a lower maximum observed dispersal distance (up to 10 km), and requires mature cactus stands for nesting, which are patchy within coastal sage scrub habitat. These differences highlight that the California gnatcatcher is not an appropriate monitoring surrogate for other, more fragmentation-sensitive species.
Genetic studies such as this are powerful tools for resource managers. Using this study’s results as a baseline, land managers can use future genetic monitoring to assess maintenance of gnatcatcher population connectivity, and identify additional conservation and management efforts to retain or improve connectivity. Genetic studies such as these build on our current understanding of the species, and supplement information from banding studies. The good news for those managing the California gnatcatcher is that current habitat conditions and management actions appear to support gnatcatcher population connectivity in much of southern California.
Amy G. Vandergast, Barbara E. Kus, Kristine L. Preston, and Kelly R. Barr. 2019. Distinguishing recent dispersal from historical genetic connectivity in the coastal California gnatcatcher. Scientific Reports 9:1355. https://doi.org/10.1038/s41598-018-37712-2
Kelly R. Barr, Barbara E. Kus, Kristine Preston, Scarlett Howell, Emily Perkins, and Amy G. Vandergast. 2015. Habitat fragmentation in coastal southern California disrupts genetic connectivity in the cactus wren (Campylorhynchus brunneicapillus). Molecular Ecology. 24(10):2349-2363. https://doi.org/10.1111/mec.13176