Non-native, Invasive Species on Mt. Tam

Non-native, invasive species in Marin County come in myriad forms, including water molds, plants, invertebrates, fish, amphibians, birds, and mammals. The major threats posed by invasive species include changes in fire frequency or intensity, groundwater depletion, changes to soil chemistry, competition with native species, and a loss of native species diversity.

Plants

While all natural areas on Mt. Tam face some degree of threat from invasive plant species, some are more resistant to invasion than others due to varying soil types, moisture levels, and canopy density. Furthermore, small, patchy habitats have more gaps for invasive species to take hold, and more peripheral areas that may be exposed to invasion.

There are currently over 300 documented non-native plant species on Mt. Tam, representing about a third of the known plant species diversity on the mountain. Out of those, around 60 are priority species targeted for early detection, mapping, and control by the One Tam Conservation Management Team.

Highest priority species are those that are not currently widespread in Marin County or on Mt. Tam, but that have demonstrated a capacity to do harm to ecosystems in other regions or adjacent counties. Suitable habitat for these species is found on Mt. Tam, thus finding and managing incipient populations in the early stages is critical. In 2016, the One Tam Conservation Management Team was able to treat all instances of these highest priority species found during early detection surveys.

Other non-native, invasive species that are often widespread in the county and/or on Mt. Tam are managed by existing agency vegetation programs with staff and volunteer support. These species may become high priorities for removal when they are found far from source populations in small amounts.

Animals

Non-native invasive animals compete with native species for food, shelter, and nest/den sites, and some also prey directly on native species. There are 27 known non-native animal species on Mt. Tam. Species of particular concern include:

• American bullfrogs (Lithobates catesbeianus) compete with and prey upon other amphibian species including federally threatened California red-legged frogs (Rana draytonii) as well as foothill yellow-legged frogs (Rana boylii), which are a federal and state species of concern
• Signal crayfish (Pacifastacus leniusculus) prey upon juvenile foothill yellow-legged frogs
• Red-eared sliders (Trachemys scripta elegans) and other non-native turtles compete with and prey upon native aquatic wildlife
• Domestic and/or feral cats (Felis catus) prey on native birds, rodents, and reptiles
• Wild turkeys (Meleagris gallopavo) eat seeds, invertebrates, small vertebrates, and other food needed by native species, damage native vegetation, and cause soil disturbance and erosion through foraging