by Kate Allan
The salt marsh harvest mouse is an elusive Bay Area native that you may have come across in nature guides, open space signage, or construction and restoration work in a salt marsh. It is a small brown mouse with a white or reddish belly found only in salt marshes around the San Francisco, San Pablo and Suisun Bays and their tributaries. With the prolific development that has occurred in the Bay Area an estimated 85% of this species’ habitat has been lost or degraded, which led to its listing as endangered under the federal Endangered Species Act and the California Endangered Species Act. The salt marsh harvest mouse has been designated as a “fully protected” species under California Fish and Game Code– this is essentially the highest level of protection afforded a wildlife species in California. With this high level of protection, the pressure is on for regulators to facilitate the recovery of this species by protecting and restoring habitat.
What does this mean for residential, commercial, recreational, and even restoration projects in mouse habitat? It may mean work restrictions, environmental review, and substantial permitting requirements for project proponents around the Bay.
Recently, the way that scientists and regulators define habitat for this species has changed. For decades, salt marsh harvest mouse has been strictly associated with pickleweed (Salicornia pacifica). People who have dealt with mouse restrictions during this time were so well trained that it is still common to hear: “There’s no pickleweed, so we don’t have to worry about finding salt marsh harvest mouse here, right?” It’s likely that sampling bias resulting from the removal of historic habitat, and the difficulty in distinguishing this species from its common neighbor, the western harvest mouse, led scientists to the conclusion that pickleweed was a necessary and dominant element of salt marsh harvest mouse habitat.
Now, recent studies are shifting the “pickleweed only” paradigm. Although this species is consistently found in pickleweed-dominated marsh, it has also been found in relatively high numbers in mixed salt marsh where pickleweed is not dominant, in brackish marsh with little or no pickleweed, and in the grasslands adjoining marsh – at least 100 meters from the marsh edge! They are also found not only in tidal marsh, but in disturbed and diked marshes as well. This expanded definition of salt marsh harvest mouse habitat means that agencies and researchers can more accurately count and better protect the mice more effectively, which in the future may lead to the down- or delisting of this species.
What does this mean for your project? In terms of biological constraints, it means that regulatory protection of this species has expanded spatially to encompass the additional habitat areas. In other words, certain areas in or near marsh and grassland which were once generally considered constraint-free may actually support mice and need to be protected. How might this new definition of habitat help you? To start, it allows agencies and consultants to give even better advice about how to avoid “take” (defined as harming, harassing or killing a protected species), reducing your liability and the likelihood that your project will be stopped due to an unexpected endangered species encounter. Also, as noted above, better protection for this species is expected to lead to its eventual removal from the endangered species list, meaning the mouse would no longer present a major constraint on projects in the Bay Area.
Although the presence of pickleweed is still a great indicator of salt marsh harvest mouse presence, it’s time for the “pickleweed only” paradigm to go and to usher in the salt grass and spearscale, alkali heath and gumplant, cattail and tule, common reed and cordgrass, and the grasslands adjacent to marsh habitat paradigm. With this new (although admittedly less-catchy and more complicated) paradigm to identify salt marsh harvest mouse habitat, and we ultimately aid in the recovery of this endangered species.