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This Delta Rodent Has an Epic Story

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Riparian woodrat | Photo: B. Moose Peterson

An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with for all the project's stories.

They're gregarious. They live in matrilineal groups, and each of them often has more than one home. And they're a federally protected endangered species... or are they two endangered species? Meet an intriguing denizen of the Delta's riparian forests, the riparian woodrat.

The riparian woodrat is part of a large and diverse tribe of rodents that live in wildlands throughout the western part of North America. Known to scientists as Neotoma fuscipes riparia, the riparian woodrat is a subspecies of the more widely distributed dusky footed woodrat, Neotoma fuscipes.

Like its neighbor the riparian brush rabbit, the riparian woodrat is confined to just a few acres of riverside forest in the south Delta. There's a population in the brush rabbits' stronghold in Caswell Memorial State Park, and one a few miles away in the San Joaquin River National Wildlife Refuge.

Woodrats, also commonly called packrats, are engaging little things, and the riparian woodrat is no exception. Despite their common names, they aren't particularly closely related to the introduced rats to which California urbanites are probably more accustomed. Members of the taxonomic family Cricetidae, they're actually more closely related to hamsters.

There are a whole lot of woodrats all across California, from the desert woodrat in the southern part of the state to the bushy-tailed woodrat in the north. They share a habit of more or less social living, of building elaborate and well-defended dens out of available debris (mostly sticks), and of collecting random objects for seemingly inexplicable reasons -- thus earning the name "packrats." They're territorial, and they stick to home territories that are often rather small for most of their lives.

The riparian woodrat shares those traits, though it does seem to do just fine adapting existing shelters such as buildings and hollow trees for use as shelter, thus saving the time and energy that would go into building a house of sticks, twigs, and car keys. On the other hand, sometimes they'll build more than one stick house in their territories and split their time among them.

They're stocky little things, weighing in at adulthood somewhere between one-half and four-fifths of a pound, reaching 16 or 17 inches in length -- half of that length being the rat's fur-covered tail. Like their riparian brush rabbit neighbors, they're supremely adapted to life in the Delta's and Central Valley's lush riverbank forests. Able climbers, they tend to prefer forests with a complex vegetative structure consisting of both an understory of low shrubs and a mid-story of taller willows and vines, all of it beneath a canopy of oaks, sycamores, or other big trees.

And of course that sort of habitat is in very short supply in the 21st Century: about 95 percent of the Central Valley's original riparian forest has been cut down and replaced by farm fields, suburbs, flood control works, or other artificial landscapes. By the late 1990s, the only known population of riparian woodrats was, like the riparian brush rabbit, confined to Caswell Memorial State Park, a 258-acre tract on the Stanislaus River near the small community of Ripon.

Flooding during the late 20th Century threatened to wipe out the Caswell woodrats. In the wake of a devastating flood in January 1997, in which 85 percent of park was under between two and 10 feet of water for at least two weeks, the U.S. Fish and Wildlife Service proposed in November of that year to list the woodrat, along with the rabbit, as Endangered under the U.S. Endangered Species Act. Somewhat unusually, nobody objected to the listing, at least not in written comments provided to the Fish and Wildlife Service. That listing became official in February 2000, and the riparian woodrat became a federally listed Endangered species.

After the listing, a new, somewhat smaller population of woodrats was found in the San Joaquin River National Wildlife Refuge. Even with the new find, the population of riparian woodrats was still likely below 500 as of 2003, with most of the animals residing on Caswell State Park's 258 acres.

Unlike with the riparian brush rabbit, which has been the beneficiary of a captive breeding and relocation program, concrete measures to improve the chances that the woodrat might be with us in the 22nd century have been limited -- at least as of the Fish and Wildlife Service's 2012 review of the subspecies -- to restoration of likely habitat for the woodrats. While habitat restoration is difficult and crucial work, and it benefits a range of species other than the woodrat, it remains a first step in woodrat conservation.

That 2012 Five Year Review contained an intriguing mention of research into the riparian woodrat's genome, comparing the Caswell Memorial State Park population with the one five miles away at the San Joaquin River National Wildlife Refuge. According to contemporary work by wildlife biologist Marjorie Matocq, the two populations of riparian woodrat might actually belong to different species.

At the time of listing (and at this writing,) riparian woodrats are considered a subspecies of the dusky footed woodrat, Neotoma fuscipes. Dusky-footeds other than the riparian woodrat live in the coastal mountains from Santa Cruz through southwestern Washington, with interior populations in the Northern Sierra Nevada and in the inner Coast Ranges from the Bay Area south to Kern County.

There's a closely related species of woodrat, the big-eared woodrat (Neotoma macrotis), which occupies much of the rest of California, with populations along the coast from Monterey to Baja, and an interior band running from the Transverse Ranges near Los Angeles to the southern Cascades near Mount Lassen.

The two species are related closely enough that they can interbreed. In fact, Neotoma macrotis was considered a subspecies of Neotoma fuscipes until Matocq split it out in 2002.

According to the work cited in the 2012 Five Year Review, Matocq had found data in the woodrats' genomes that suggested the Caswell State Park population of riparian woodrats might be more closely related to the big-eared woodrat than the ducky-footed, which implied that that population might need to be redesignated a subspecies of Neotoma macrotis.

And that would essentially mean that there were two species of riparian woodrats, with the Caswell rats being a subspecies of the big-eared woodrats, while the San Joaquin River National Wildlife Refuge rats stayed within the dusky-footed species.

Subsequent work by Matocq added some nuance. Around 600,000 years ago, the Central Valley and Delta area were a gigantic inland freshwater sea, formed when runoff from the snowy Sierra Nevada collected in the Valley, which then had no outlet to the sea. Woodrats lived in the hills surrounding that sea, isolated enough from each other that they evolved into different species: Neotoma fuscipes on the western shore, and Neotoma macrotis on the east.

About 500,000 years ago that inland sea finally eroded a notch through the coast ranges just west of the present-day Delta, and that big inland freshwater lake raced out to the Pacific. But a chain of ephemeral lakes still formed almost every year up and down the center of the Valley, making the habitat unusable by woodrats.

About the only place accessible to woodrats on the valley floor would have been riparian forests, generally growing on natural levees formed by the valley's rivers as they dropped sediment along their banks, and providing the rodents an avenue of escape from floods in the form of climbable trees.

And until about 150 years ago, the woodrats took advantage of that habitat. As riparian forests cris-crossed the Valley and Delta area, they became corridors for genetic connectivity between the dusky-footed woodrats to the west and the big-eared woodrats to the east.

In that context, it makes sense that the Caswell woodrats, being slightly closer to the source of big-eared woodrat genes, would have more of them in their genome than the San Joaquin River woodrats do. Even with woodrats sticking to their home territories and not engaging in long overland treks in search of mates with novel genes, woodrats mated with woodrats from the east edge of their territory. Their offspring mated with rats from the west side. The genes must flow.

Whether that difference is sufficient to justify taxonomic rearrangement remains to be decided, but it's an excellent additional piece of information of just how we've changed the California landscape we inherited, in case we needed one. We found a thriving, interconnected landscape that wildlife of thousands of species were using and evolving in, and we laid it out on a rectilinear grid for the next season's profit, the wildlife be damned. How do we take those laudable riparian forest restoration efforts and extend them from one side of the Valley to the other, to restore that chain of connectivity that created the riparian woodrat in the first place?

Whether or not we have two species of woodrats, or just one, is likely the less important question. Especially since before long we may have none.

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