Sparrows particularly adapted to Bay Area marshes are losing their uniqueness

The San Francisco Bay Area’s temperate climate has always attracted immigrants – animals and humans – who have had unpredictable impacts on those already living in the area.

For the bay meadow sparrow, a subspecies that lives in salt marshes, the increased immigration of its inland cousins ​​over the past century has definitely been bad news.

A new genomic analysis of meadow sparrows (Passerculus sandwichensis) from across the state—many of which were collected as early as 1889, their skins preserved at the University of California, Berkeley’s Museum of Vertebrate Zoology—shows that over the Over the past 128 years, the Bay Area sparrow’s adaptation to salt water has been diminished by interbreeding with inland sparrows adapted to fresh water.

The result is stable levels of genetic diversity among the sparrows of the coastal savannahs of 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 feeds on salt water and salty shellfish, something freshwater birds are poorly adapted to.

The surprise discovery, published last week in the journal Biology of global change, may be explained in part by the precipitous statewide decline of coastal marshes, which are wetlands subject to the influx of salty ocean water through tides. The Bay Area has seen a 90% decline in intertidal marshes since the 1800s, which has reduced Savannah Sparrow populations to a level where interbreeding with immigrants affects their unique adaptation to salt water.

“There appears to be increasing levels of gene flow from eastern California to places like the Bay Area, potentially due to the local population becoming a sink where local breeders can’t really produce enough offspring to maintain a population,” said Phred Benham, a postdoctoral researcher at UC Berkeley and leader of the study. “So there’s an influx of immigrants from another population. Migrants think it’s a nice place to stay and try to establish a nest, and there’s this opportunity for gene flow to happen among the residents.”

This gene flow from freshwater-adapted sparrows to saltwater-adapted sparrows means that the genetic variants, or alleles, that allow tidal marsh sparrows to exist in a salt-rich environment – producing larger kidneys, for example – are lost.

“Inland areas where grasslands remain extensive produce large numbers of these birds, and they have to go somewhere. So the Bay Area is a destination,” said Rauri Bowie, professor of integrative biology at UC Berkeley and curator of birds at the museum. of Vertebrate Zoology (MVZ).

“They’re coming in, but they’re not adapted to breed in that environment, so they’re not doing 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 into these habitats, you see a lot of meadow sparrows, but there’s something going on under the hood that’s much more complex in these kinds of specialized environments.

A subspecies of particular concern

With 17 subspecies recognized throughout North America, Savannah Sparrows are common, Bowie said. But the rare subspecies adapted to living in salt marshes are considered threatened. Two occur in California, a northern subspecies (P. s. alaudinus), which is listed by the state as a “species of special concern”, and a federally protected southern subspecies called the Western Sparrow. Belding savannahs (P. s. beldingi), which occurs around San Diego and Tijuana, Mexico.

The good news for the northern marsh subspecies, Benham noted, is that tidal wetlands around the San Francisco Bay estuary are protected and growing, which could allow coastal populations of buntings to savannahs to increase.

“There are huge tidal restoration efforts underway in the Bay Area, and meadow sparrows aren’t the only ones depending on them. One of the most endangered species in the Bay Area is the salt marsh harvest mouse. There’s a lot of federal money going toward protecting their habitat, which I think would ultimately benefit the meadow sparrows,” he said. “I think it would be really great to look back at these populations in 10 years to see if this trend continues or if the restoration of tidal marshes has allowed people to reestablish their populations and dominance.”

Continued monitoring of immigrant sparrows, Bowie said, “would provide insight into whether this genetic flood is continuing and how quickly it is occurring.” I hope that as the marshes are restored this will be alleviated. But we don’t know that. “

Benham and Bowie, assisted by Jennifer Walsh of the Cornell Laboratory of Ornithology at Cornell University in Ithaca, New York, set out to ask an important evolutionary question: how does habitat loss affect the genetic diversity of animals who occupy this habitat?

“We wanted to test a very simple prediction, which is that there is a reduction in genetic diversity associated with habitat loss,” Benham said.

For genetic analysis, Benham obtained DNA samples from savannah sparrows captured across the state between 1889 and 2017. To obtain DNA from museum samples, he scraped skin cells from the pads of the toes of birds. For comparison, he collected blood or tissue samples from live birds he had encountered over the past decade while studying Savannah sparrow populations around the Bay Area for his doctorate. dissertation from the University of Montana.

Using a technique called exome capture, he profiled the DNA of 219 individuals and looked for genetic variations in areas of DNA previously identified as important for the saltwater adaptation of water sparrows. swamp. He had discovered some of these adaptations while studying these savannah sparrows.

For example, not only are their kidneys larger and therefore able to remove more salt from the birds’ urea, but their skin is less permeable to water. To compensate for the reduced reliance on evaporative cooling, such as sweating, to carry away heat, their bills have become larger, allowing more heat to be lost to the blood flowing through them. All of these adaptations compensate for the fact that songbirds never evolved a solution common to other seaside birds: the ability to excrete excess salt through their nasal glands.

Researchers found that the genetic diversity of the Southern California subspecies, the Belding Savannah Sparrow, had declined slightly, but was low even in the 1880s.

Genetic diversity of the Northern California subspecies, however, has remained relatively high over the past century, likely due to immigration from inland locations. But this immigration led to genetic alleles in regions of the genome involved in saltwater adaptation that more closely resembled alleles found in freshwater-adapted sparrows.

Bowie noted that the results address a major debate today in conservation biology: whether it is wise to bolster small, endangered inbred populations by crossing them with related populations. In the case of the Savannah Sparrow, such inbreeding has reduced adaptation to the environment.

“Here we show that if gene flow occurs across these steep environmental gradients, it can have significant negative effects on the local population,” Benham said.

“It’s actually quite difficult to demonstrate the effects of inbreeding,” Bowie pointed out. “Having access to historical sets of museum specimens from before the transformation of the salt marshes in the bay to compare them with modern birds helps you demonstrate that inbreeding depression has a significant effect on populations. Genetic diversity appears large, but it masks an effect of inbreeding depression.