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Savannah sparrows are losing their unique adaptations in the Bay Area 

The tidal marshes of the San Francisco Bay Area are witnessing a subtle yet profound ecological change. Savannah sparrows, which have been long adapted to the region’s saltwater marshes, are undergoing a genetic transformation and losing the very traits that make them unique. This shift is a consequence of habitat loss and increased interbreeding with their inland counterparts, according to the results of a new study. 

Disturbing trend 

For over a century, the Bay Area has been a haven for diverse wildlife, including the saltwater-adapted Savannah sparrows. However, a new genomic analysis, incorporating specimens dating back to 1889 from the Museum of Vertebrate Zoology at UC Berkeley, indicates a disturbing trend. These sparrows that were once distinct in their adaptation to the harsh saline environment are now showing a dilution of these specialized traits.

Genetic dilemma

The study highlights a stable genetic diversity among the coastal Savannah sparrows. Yet, it underlines the loss of specific genetic variants crucial for survival in tidal marshes. This change is attributed to interbreeding with freshwater-adapted inland sparrows, leading to a depletion of the saltwater-adaptive alleles. 

Such a genetic shift could severely impact the bird’s ability to thrive in its native marshlands, where its diet and survival heavily depend on saltwater tolerance.

Habitat loss 

A significant factor in this genetic blending is the steep decline in tidal marshes across the state. The Bay Area alone has seen a staggering 90% reduction in tidal marshland since the 1800s. 

This loss has reduced local Savannah sparrow populations to a level where interbreeding with immigrant birds has become more prevalent.

Influx of immigrants 

“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 who led 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.”

Coastal adaptation

As Professor Rauri Bowie pointed out, the inland birds are ill-suited for breeding in saltwater environments. “The inland areas where grasslands remain extensive are producing large numbers of these birds, and they have to go somewhere. And so the Bay Area is a destination,” said Professor Bowie. 

“They’re coming in, but they are not adapted to breed in that environment. So they don’t do well, and they’re introducing maladaptive alleles – freshwater-adapted alleles into saltwater-adapted populations. And some of these coastal populations are endangered. If you go to these habitats, you see lots of Savannah sparrows, but there’s something going on underneath the hood that’s a lot more complex in these kinds of specialized environments.”

Threatened species

The introduction of maladaptive genes into the local gene pool, known as “genetic swamping,” threatens the very existence of coastal populations, some of which are already endangered.

Among the 17 recognized subspecies of Savannah sparrows in North America, those adapted to saltwater marshlands are particularly threatened. Two such subspecies in California are the northern subspecies (P. s. alaudinus), listed as a “species of special concern,” and the federally protected southern subspecies, the Belding’s Savannah sparrow (P. s. beldingi).

Possible silver lining 

The good news for the northern marshland subspecies, noted Benham, is that tidal wetlands around the San Francisco Bay estuary are protected and growing, potentially allowing coastal Savannah sparrow populations to increase.

“There’s a huge amount of tidal restoration efforts underway in the Bay Area, and Savannah sparrows aren’t the only ones that depend on this. One of the most critically endangered species in the Bay Area is the salt marsh harvest mouse. There’s a lot of federal money going into protecting their habitat, which I think ultimately would benefit the Savannah sparrows,” said Benham. 

“I think it would be really great to revisit these populations in 10 years to see if this trend is ongoing or if tidal marsh restoration has allowed the locals to reestablish their populations and their dominance.”

Bowie said that continued monitoring of immigrant sparrows would give a sense of whether this genetic swamping is continuing and how quickly it’s happening. “I’d hope that as marshes are restored, that would be mitigated. But we don’t know that.”

The study is published in the journal Global Change Biology.

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