Salt Marsh Harvest Mouse

Salt Marsh Harvest Mouse

by: Maggie McGregor

photo: C.D.F.G., 2005

Background: The salt marsh harvest mouse is a tiny rodent; no more than three inches in length and weigh less than half an ounce. Their tails are at least the length of their bodies. The mice have a dusty brown colored back and a gray underbelly. They work with the tides of the marsh, and float or swim when the tide comes in. Harvest mice munch on pickleweed found in the tides. The mouse has two subspecies, which are both restricted to the marshes of three places: San Francisco, San Pablo, and Suisun Bay. One of the subspecies, southern, is extremely limited to just the central and south San Francisco Bay, which of course gives the mouse very little chance at a comeback, with such a limited scope of habitat. This subspecies in particular has suffered endangerment due to fragmentation. The second subspecies, northern, lives predominantly in the bays of San Pablo and Suisun. This species of the salt marsh harvest mouse has suffered less loss than the southern subspecies, but still struggles from degradation (U.S. Fish & Wildlife Service). 

photo: U.S. Fish & Wildlife Service

Geographic &Population Changes: The very maximum age for the salt marsh harvest mouse is only about twelve months. The females only birth four young per litter, once a year, which is a recipe for disaster. These mice are not strong in survival in their habitats. Populations seem to be suffering greatly from long ranges of high tides, and a lack of escape. Storms are consistently increasing with harshness and strength, as well as global warming, which could eventually lead to extreme flooding, wiping out the population of the tiny salt marsh harvest mouse. In the 1850’s, there were about 193,800 acres of the necessary tidal marsh. Now, scientists estimate 30,100 acres to remain. The population numbers of the harvest mouse have gone down with those of the tidal marshes (U.S. Fish & Wildlife Service)

Listing Date/Type: The mouse was listed as an endangered species of the United States List of Endangered Native Fish and Wildlife on October 13, 1970. Its recovery plan was drafted on February 10, 2010. The final recovery plan was revised and drafted on February 26, 2014 (U.S. Fish & Wildlife Service Environmental Conservation Online System). 

Endangerment continued

photo: USFWS
Salt Marsh Harvest Mouse munching on pickleweed

Main Threats to Existence: The main reason for the population decline in the harvest mouse is due to loss of habitat through destruction and vegetation change. The South San Francisco Bay is only two meters deep, while it used to be kilometers deep. Habitat loss tends to be associated with areas known for diking tidal marshes. Rising sea levels are proposing a very serious threat in the long term of the existence of the salt marsh harvest mouse, specifically in the San Francisco Bay (U.S. Fish & Wildlife Service).

photo: U.S. Fish & Wildlife Service
total cost of recovery plan of three marsh species

Recovery Plan: The main goal of the recovery plan is to be able to delist the species from the Endangered Species Act. The secondary goal is to recover the species enough so they can be downlisted from endangered to threatened. The plan to achieve such goals is as follows: 1. Secure the populations. 2. Eliminate the threats that are causing the harvest mouse to be endangered. 3. Restore and maintain a proper ecosystem for the mouse to be able to successfully function. To keep impacts of oil spills to a complete minimum, the San Francisco Bay and Delta Area section of the San Francisco-Area Contingency Plan will be placed on high priority due to the emergency protection of the salt marsh harvest mice. Appropriately sized ecotones are extremely important to salt marsh harvest mice and their wetland habitats. Human activities also need to be kept to the lowest possible minimum, as the recreational trails of outdoor activities: boating, dog-walking, hiking, and more, often attract predators to the harvest mouse (U.S. Fish & Wildlife Service).