The health of the biodiversity of Mt. Tam is represented by a collection of key taxonomic groups. Each group has one equal segment, and each segment was given its own condition, trend, and confidence score by aggregating the metrics of the species within them (Figure 23.1). All vegetation indicators have been combined into the group “plants,” which may bias the analysis in favor of the wildlife guilds included.
For plants, the aggregated condition and trend was calculated from our knowledge of key biodiversity characteristics. Four plant lists were utilized:
- Likely extirpated plants from the One Tam area of focus, developed by comparing what we currently have with what was known to exist on Mt. Tam (see Appendix 7)
- The status of rare plant populations (see Appendix 6)
- The status of locally rare plant populations (as defined as plants with three or fewer populations or plants at the edge of their range/disjunct population)
- An assessment of the number of non-native species compared to native
For birds, the aggregated condition and trend was calculated from the species trait-status database (Appendix 4), these species were chosen as good representatives of the overall bird population on Mt. Tam. First, the individual status and trends of the indicator species were rolled up to habitat community level (e.g., riparian areas, oak woodlands), then aggregated again to obtain an overall condition and trend.
The other taxonomic groups were assessed by aggregating the condition and trends of the individual indicators within that group. For example, the mammal grouping included native mammal diversity, American badgers (Taxidea taxus), and North American river otters (Lontra canadensis).
Future work to refine this overall biodiversity assessment could include looking at extirpated fauna species, as well as other important taxonomic groups currently lacking in data, such as fungi, lichens, and invertebrates.
Overall biodiversity on Mt. Tam is in a “Cautionary” condition. Some taxonomic groups have experienced local or global extirpations and include species that are in perilous condition, while other taxonomic groups are faring better with limited extinctions and generally healthy populations of extant species. The trend in overall biodiversity is “Declining” as we have lost native species and have not gained new ones; some taxonomic groups are faring better than others.
Biodiversity for plants has a “Cautionary” condition and a downward trend. Currently, there are over 750 known native plant species, but also over 300 non-native species. While some non-native species have limited distribution and impact, many others are noxious invasives that are impacting native species and processes. These impacts include outcompeting and displacing native species, altering habitat, altering the fire regime, and huge costs associated with control and eradication (Mack & D’Antonio, 1998; Hobbs & Mooney, 2005; Pimentel et al., 2005).
In addition, there have been 68 documented likely plant extirpations (Appendix 7). Of the known extant native plant species, over 40 are considered rare, threatened, or endangered. These special status plants are susceptible to stochastic events and existing stressors that could lead to further imperilment and even local or global extinction.
Despite the loss of species, and threats from non-native species and other stressors, the floristic biodiversity of the area is high, and supports an equally high diversity of habitats that host dozens of wildlife species. These vegetation and habitat types are defined by high variability in topography, temperature, precipitation, and soils within the One Tam area of focus.