The temperate climate of the San Francisco Bay Area has always attracted immigrants — animals and humans — that have had unpredictable impacts on those already living in the area.
For the bay's Savannah sparrow, a subspecies that lives in salty tidal marshes, increased immigration of its inland cousins over the past century has definitely been bad news.
A new genomic analysis of Savannah sparrows (Passerculus sandwichensis) from around the state — many of them collected as far back as 1889, their pelts stored in the Museum of Vertebrate Zoology at the University of California, Berkeley — shows that over the past 128 years, the Bay Area's sparrow's adaptation to salt water is being diminished by interbreeding with inland sparrows adapted to fresh water.
The result is stable levels of genetic diversity among coastal Savannah sparrows in Northern and Central California, but a loss of the genetic variants that make them adapted to tidal marshes. This could affect the bird's ability to survive in tidal marshes, where it subsists on saltwater and salty crustaceans, something freshwater birds are ill-adapted to.
The surprise finding, published last week in the journal Global Change Biology, can be explained in part by the steep state-wide decline in tidal marshes, which are wetlands subject to the tidal influx of salty ocean water. The Bay Area has seen a 90% decline in tidal marshland since the 1800s, which has reduced Savannah sparrow populations to the level that interbreeding with immigrants is affecting their unique saltwater adaptation.
"There seem to be increasing levels of gene flow from eastern California into places like the Bay Area, potentially due to the local population becoming a sink where the local breeders can't really produce enough offspring to maintain a population," said Phred Benham, a postdoctoral fellow at UC Berkeley and leader of the study." So you get the influx of immigrants from another population. The migrants think it's a nice place to stay and try to set up a nest, and there's this opportunity for gene flow to occur into the residents."