It’s a year old, but I just read a paper on one of those species that pops up pretty frequently in environmental impact reports — the Mohave ground squirrel, Xerospermophilus mohavensis — whose conclusions add a little bit more depth to the Deep Time perspective of the desert around here. And by “depth” I mean “epic stories.”
And by “around here,” I mean “at least 45 miles northwest of here”: the Mohave ground squirrel’s range is all on the other side of the Mojave River, aside from a strip of land a few miles wide along the north desert foothills of the San Bernardino Mountains, where it mostly hasn’t been seen for the last few decades anyway. The squirrels’ historic range was somewhere in the neighborhood of 7,000 square miles or 20,000 km2, depending on which authority’s rounded-off guesstimate you use.
The Mohave ground squirrel is a foliage and seed eater — you may have guessed that second part from “Xerospermophilus,” which essentially means “desert seed lover.” It’s about nine inches nose to tail, short-haired, roughly ground-squirrel-shaped, ranges in color from pinkish to gray to cinnamon, in a more or less solid-colored coat. The species is rare throughout its range, which is shown in the U.S. Fish and Wildlife Service map embedded below:
That map is from the Federal Register, dated October 2011, in a “Finding” that the squirrel didn’t warrant protection as endangered or threatened under the U.S. Endangered Species Act. That finding was more politics than science. The squirrel is threatened by urban and industrial development, off-road vehicle use, and by climate change. Under CalESA, the California version of that law, the squirrel is listed as “Threatened”: the IUCN classifies the species as “vulnerable.”
The authors of the paper I mentioned up top, K.C. Bell and Marjorie Matocq, wanted to learn more about historic habitat colonization and connectivity among Mohave ground squirrel populations. They sampled nuclear and mitochondrial DNA from 258 individual squirrels collected in 13 locations throughout the species’ range, then charted the genetic diversity among the samples. They also sampled nearby populations of the closely related round-tailed ground squirrel, Xerospermophilus tereticaudus, and some suspected hybrids between the two species.
Bell and Matocq found that the Mohave ground squirrel species comprised three genetically distinct geographical populations — one from the northern part of the species’ range, one occupying the west-central part, and one in the range’s southern extension. The populations aren’t reproductively isolated: there’s unambiguous evidence of gene flow among all three groups, though the northern population seems more isolated from the other two. The southern population turned out to be more genetically diverse than the other two, which supported earlier researchers’ suggestions that the squirrels have inhabited the southern part of their range for significantly longer, evolving genetic diversity. Populations to the north would have been established by small “founder” groups, and would thus have less diversity.
When you talk about the Mojave Desert more than 11,000 years ago, you’re talking about a very different place than the Mojave Desert of today. For one thing, there were elephants and lions and saber-toothed cats and sloths the size of cows wandering around there. For another thing, there were huge lakes for them to wander past. 11,000 years ago the Mojave Desert was enjoying a pluvial period, with warm, wet weather, and all across the desert valleys were filled with lakes.
In the northern Mojave, the Owens River — now a pale shadow of even its 19th Century self — filled a chain of five gigantic lakes with runoff from the glaciated Sierra Nevada. At the end of the line was Lake Manly, 80 miles and 600 feet deep at its largest, filling Death Valley. Lake Manly was fed by overflow from Panamint Lake, in the next valley to the west. Panamint Lake was fed by overflow from Searles Lake, now the site of the trona mine at Trona, California. Searles Lake was fed by China Lake, now the site of a U.S. Naval Air Station. China Lake was fed by Owens Lake, up at the top of the chain.
Twelve thousand years ago the climate started getting drier. Sierra Nevada runoff didn’t run off the Sierra Nevada quite as abundantly. Lake levels dropped across the desert, and eventually — when the level of China Lake dropped below its spillover into Searles Lake — Searles Lake dried up.
Which was about the time Mohave ground squirrels started moving north, across land that had once been flooded, to the territory on what had been the far shore. After 7,000 years another wet cycle — about 3,500 years ago — filled Searles Lake again, isolating squirrels in the northern population from their cousins to the south. The lake lasted long enough that the northern and southern populations of ground squirrels evolved every so slightly away from each other.
And then the lake dried up again, and squirrels moved into the land where it had been, forming the central-western population.
If the lake had lasted for longer, two species of Mohave ground squirrel could have been the result. That’s almost exactly how the Mohave ground squirrel species itself came into being, split off from its sister species, the round-tailed ground squirrel, X. tereticaudus. I mentioned above that the Mojave River marks a major boundary of the Mohave ground squirrel’s range. There are ground squirrels on the other side of the river too: round-taileds. The river itself isn’t much of a barrier: for most of its 110-mile length, most of the time, it flows underground. There’s not enough of a barrier there to split a species: a squirrel could cross it 364.9 days out of 365 without getting its feet damp.
Up until 6,000 years ago, though, there was usually a lot more water in the Mojave. Fed by runoff from the Transverse Ranges in Southern California, it made its own chain of lakes across the southern Mojave Desert, which chain may also have ended in Lake Manly — at least in phenomenally wet years. But there were dry periods even then, and at one point during one of those prolonged droughts the ancestors of Mohave and round-tailed ground squirrels ranged across it all.
Somewhere around 1.6 million years ago the Mojave River filled back up again. The ancestors of Mohave ground squirrels stood on the northwest bank, looking across at the ancestors of round-tailed ground squirrels on the far shore. Eventually they evolved in different directions. The river dried up 6,000 years ago, and there’s been some movement of each species across the river since, with some evidence of interbreeding — though Bell and Matocq found that two of three putative hybrid squirrels kept in storage since the 1980s-90s were actually pure round-tails, with no Mohave ancestry — that interbreeding is rare enough to be insignificant. The two species have diverged.
So, reproductive isolation can cause genetic divergence and even speciation. Nothing new there. But I’ve long thought of the Great Divide between Mohave and round-tailed ground squirrels as one of the wonderful epic tales of Evolution: There was once a nation that was divided by a flood, and when they eventually reunited they had become irrevocably different. When I learned that story what had been a mere edge of a line on a range map became, in my mind, a saga millions of years in the unfolding. And there are stories like that everywhere you look in the natural world. Sometimes they’re obvious, an abrupt discontinuity between two related species. And sometimes, as with Bell and Matocq’s work, you need to dig stories out of mitochondria and read them in scatter plots.
But they’re there, and as old and jaded as I am I often wish I had a few more centuries left me in which to hear them all.
Bell, K. C. and Matocq, M. D. (2011), Regional genetic subdivision in the Mohave ground squirrel: evidence of historic isolation and ongoing connectivity in a Mojave Desert endemic. Animal Conservation, 14: 371–381. doi: 10.1111/j.1469-1795.2011.00435.x