Measuring the health of Mt. Tam

Maintaining a healthy, vibrant and diverse Mt. Tam begins with understanding how key ecological resources are faring, and how we can better care for this iconic and beloved place.

One Tam partners and Bay Area scientists have come together to try to answer the question: How healthy are Mt. Tam's natural resources?

Western Pond Turtle

Western Pond Turtle

Photo by Sami Kreling

Why Was This Indicator Chosen?

This state Species of Special Concern is a good indicator of freshwater aquatic conditions and, to some extent, grassland habitats as well. The small numbers of western pond turtles (Actinemys marmorata) seen at Muir Beach in the early 1990s are gone (PWA, 1994; Ely, 1993; Fong unpublished data, 2015), but years of monitoring on Marin Municipal Water District (MMWD) lands provides adequate data to understand how the remaining known populations of western pond turtles on Mt. Tam are faring.

What is Healthy?

The western pond turtle population in the One Tam area of focus should be stable or increasing, with an age distribution that indicates successful reproduction and juvenile survival.

What Are the Biggest Threats?

  • Predators like non-native American bullfrogs (Lithobates catesbeianus) and bass (Micropterus spp.), as well as certain overly abundant native predators such as skunks (Spilogale spp.), raccoons (Procyon lotor), ravens (Corvus corax), and crows (Corvus brachyrhynchos)
  • Competition or predation from red-eared sliders (Trachemys scripta elegans) and other non-native turtles that live in Mt. Tam’s ponds and reservoirs
  • Car strikes as the turtles travel between aquatic habitats or when migrating to nesting sites

    What is The Current Condition?

    Even though the Redwood Creek population has disappeared, the overall current condition is Good, as numbers of western pond turtles in the Olema Valley pond and MMWD sites are unchanged. The age structure of the MMWD population also appears stable and indicates good levels of juvenile survival.

    What is the Current Trend?

    Volunteer and biologists’ observations between 2003 and 2016 have shown No Change in the trend for western pond turtles.

    How Sure Are We?

    Our confidence is High, as multiple data sources support habitat occupancy assessments, and ongoing monitoring has yielded consistent results regardless of the level of effort.

Annual counts of native and non-native turtle species on MMWD lands (Hossfeld & Ettlinger, 2016)

What is This Assessment Based On?

  • MMWD monitoring since 2004
  • National Park Service inventory data from the One Tam area of focus from 1996 (Fong, 2002) and 2014–2015

What Don’t We Know?

Key information gaps include:

  • Population drivers such as egg and hatchling predation rates that affect abundance, survival, and recruitment; the cause of the decline and loss of western pond turtles from Muir Beach; and the influence of local coastal climatic conditions on breeding ecology
  • Demographic data on age/size structure of sustainable western pond turtle populations in coastal California
  • Surveys including presence/absence data for historic populations


MMWD Turtle Observer ProgramMt. Tamalpais Watershed Western Pond Turtle Study
Western Pond Turtle Indicator Overview

Learn More


Ely, E. (1993). Sensitive species herpetological survey - Golden Gate National Recreation Area 1993. Prepared for the Golden Gate National Park Association. np.

Fong, D. (2002). Western pond turtle inventory, Golden Gate National Recreation Area. Report for the Golden Gate National Recreation Area, 29 pp. (Unpublished).

Hossfeld, D., & Ettlinger. E.  (2016). Turtle Observer Program Report 2016. Corte Madera CA: Marin Municipal Water District. Available from:

Philip Williams and Associates (PWA), Moss Landing Marine Laboratory, Smith, J., Northmore, J., Roberts and Associates,& Hornor, N. (1994). Preliminary Environmental Assessment of Wetland Restoration Alternatives for Big Lagoon at Muir Beach, Marin County. Prepared for California Department of Transportation, District IV.