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The Bay Delta's Cutest Endangered Species Hangs On

Squee! A salt marsh harvest mouse | Photo: U.S. Geological Survey/Flickr/Creative Commons License

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 kcet.org/baydelta for all the project's stories.

Not all endangered species are majestic, or unusual-looking, or unfamiliar. Some of them look like they might merit legions of squeeing fans on YouTube. And one such critter still ekes out a living in San Francisco Bay's beleaguered but recovering salt marshes.

An oddly attractive plant called pickleweed grows in the Bay's salt marshes. Its segmented stems, often tinted a bright red (especially in cold winters), are usually covered in a frosty rime of salt. Pickleweed is able to grow in salt water partly because it can excrete that salt rather than accumulating it in its stems' tissue.

If an animal was to subsist on pickleweed, it would need to be able to cope with all that salt -- especially since most of the available drinking water would have salt in it as well. Fortunately for the salt marsh harvest mouse, this surprisingly cute endangered species is able to do just that.

Listed as Endangered under the Federal Endangered Species Act in 1970, the three-inch mouse -- Reithrodontomys raviventris -- relies almost completely on pickleweed as cover and as food.

Two subspecies of salt marsh harvest mouse live in the Bay's salt marshes. A southern subspecies, Reithrodontomys raviventris raviventris, inhabits what's left of the Central and South Bay's pickleweed marshes, while the northern -- Reithrodontomys raviventris halicoetes -- ekes out a continued existence in the marshes that fringe San Pablo and Suisun bays. You can distinguish between the two subspecies either by where you happen to see one, or by the color of their bellies. The southern subspecies has the reddish belly for which the species got the monicker "raviventris," while the northern mice have white bellies.

Close relatives of the plains harvest mouse of Mexico and the central U.S., salt marsh harvest mice are well-adapted to the specific conditions in the pickleweed. Their kidneys are highly efficient at removing salt from their bloodstream. The northern subspecies can actually subsist indefinitely on seawater. While the southern mouse can't quite commit that far to the saline life, it seems to prefer brackish water over fresh. And both subspecies eat large amounts of the salt-encrusted stems of pickleweed without apparent problems.

The mice are able swimmers, which helps at high tide, as well as any time that pickleweed cover across the slough looks tempting. That ability only goes so far, though. During high tides, when pickleweed stands might become completely submerged for hours on end, salt marsh harvest mice will often retreat to higher ground covered with salt-tolerant vegetation.

If there is any such "escape habitat" handy, that is. In the Central and South portions of San Francisco Bay tides are higher than they are farther north, and that -- combined with the aggressive shoreline development of much of that part of the Bay -- means high tides can leave the southern subspecies with nowhere to retreat to. That can mean losing nests of baby mice to drowning, as well as making it easier for predators to pick off adult mice who are forced out into the open.

Among the local animals likely to eat such exposed mice are feral cats, foxes, herons, egrets, gulls, and even Ridgway's rails, there being no solidarity among endangered species.

Fortunately, salt marsh harvest mice are pretty gregarious and unaggressive, meaning that a lot of them can share the same stretch of pickleweed habitat, if that habitat is big enough to support a viable population.

That's the rub, though: there's a size below which a pickleweed stand can't support a viable mouse population. And so habitat fragmentation of the Bay's saltmarsh, with once-broad stands of pickleweed broken up by roads, fill, riprap, commercial development, and other intrusions, have hit the salt marsh harvest mouse hard. Less than 10 percent of its original habitat exists at all, and much of that may be too fragmented, or too polluted or populated by imported predators such as cats, to do the mice any good.

As critically endangered species go, though, the salt marsh harvest mouse is luckier than some. As we wrote here, a burgeoning wetland protection movement in the 1970s changed public opinion about the salt marsh, and helped protect and even restore some of the last bits of the mouse's remaining habitat.

As that understanding spread, the mouse and its neighbor the Ridgway's rail became keystone species, in a sense: by protecting their habitat, we protect the habitat and the fates of other species dependent on the salt marsh, though few of them are as charismatic as a cute mouse.

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