The ascent to peak health: Measuring the state of a mountain’s natural resources

By Michelle O'Herron

Forested Mount Tamalpais rises above lowland fog.
Forested Mount Tamalpais rises above lowland fog.

Photo by One Tam/Rachel Kesel

The distinctive silhouette of Mount Tamalpais, stretching gracefully across the skyline just north of the Golden Gate Bridge, ranks among the top iconic landmarks of the San Francisco Bay Area. A mosaic of public open spaces and protected areas, Mt. Tam, as it is known to locals, extends from the shores of the Pacific Ocean, up to 2,500 feet elevation at its highest point, before sloping back down to the shores of the San Francisco Bay to the east.

The mountain’s folded flanks and multiple peaks yield a remarkably varied topography, which, combined with a major marine upwelling zone on one side and a large estuary on the other, creates an incredible array of microclimates. A wide range of soil types within these spaces have allowed diverse plant communities to form, including several species found nowhere else in the world.

Mt. Tam is also home to threatened and endangered wildlife, including the northern spotted owl, California red-legged frog, and coho salmon and steelhead trout populations, and it provides a welcome respite for migrant birds along the Pacific Flyway (fig. 1). As a critical link in a larger regional network of open spaces, the mountain is a natural refuge for both humans and wildlife, and its reservoirs are a source of drinking water for almost 200,000 nearby residents.

see caption
Figure 1. Located between the ocean and the bay and just north of the Golden Gate Bridge, Mt. Tam’s blend of microclimates has given rise to tremendous ecological diversity, including (A) oak woodland habitat, (B) coho salmon, (C) northern spotted owl, and (D) manzanita.

(A) Marin Municipal Water District, (B) NPS/Jessica Weinberg, (C) NPS/Heather Jensen, (D) Marin Municipal Water District/Andrea Williams

That Mt. Tam’s 36,000 acres (14,569 ha) of designated open space exist right on San Francisco’s doorstep is thanks to the hard work and foresight of early local conservationists; however, the mountain is a patchwork of open spaces that were protected at different times and for different purposes. As a result, its public lands are managed by four different agencies: the National Park Service, California State Parks, Marin County Parks, and the Marin Municipal Water District (fig. 2).

Even though they live in these protected areas, Mt. Tam’s plants and wildlife are not immune to the threats of climate change, invasive species, habitat fragmentation, altered fire regimes, plant diseases such as sudden oak death, and noise, light, and air pollution. Interactions among these stressors (e.g., between climate change and fire frequency) further compound their effects and make managing them much more challenging. Recreational pressures are another concern for this much beloved mountain, which has more than 200 miles (322 km) of trails and receives about five million visitors per year.

While Mt. Tam’s four land management agencies had worked together in the past to address these issues, they largely operated independently without a shared, comprehensive, strategic approach to resource management. That changed in 2014 when they joined forces with the nonprofit Golden Gate National Parks Conservancy to form the Tamalpais Lands Collaborative (TLC). The TLC focuses on priority conservation and restoration projects, coordinated education and volunteer programs, and increased volunteerism and stewardship. One Tam, the public engagement initiative of the TLC, helps galvanize community support to achieve these goals.

The TLC Area of Focus includes lands managed by four different public agencies: Marin Municipal Water District, Marin County Parks, California State Parks, and the National Park Service
Figure 2. The TLC Area of Focus includes lands managed by four different public agencies: Marin Municipal Water District, Marin County Parks, California State Parks, and the National Park Service.

Building an ecological health assessment

As the TLC partners began to delve into their collective conservation and stewardship goals, it became clear that a baseline understanding of the mountain’s overall ecological heath was needed to help inform their mutual priorities and to articulate a clear case for public support. It was also clear that creating such a comprehensive health assessment was going to require a collaborative, iterative, and multidisciplinary approach.

This exciting and daunting task began with an intentionally small and scientifically diverse group of staff from the TLC partner organizations, along with Point Blue Conservation Science—a key ecological monitoring and restoration partner. Limited membership in this “Health of Mt. Tam’s Natural Resources Advisory Committee” (advisory committee) increased its efficiency and reduced the overall burden on agency resources. While keeping the team relatively small was important, having team members who represented a range of biological expertise also proved to be invaluable.

Recognizing that they were not the first group to attempt a large-scale ecological health assessment, the advisory committee reached out to others around the country who had conducted similar work, including the National Park Service, Chicago Wilderness Society, Conservation Lands Network, San Francisco Estuary Partnership, and the San Francisco Bay Area Wetlands Ecosystems Goals Project.

In particular, the advisory committee wanted to understand how these groups had determined their project goals, scope, scale, and process; how they defined and quantified ecological health; how and why certain health metrics were selected; and how their work had been received by various audiences. While the experiences of these groups varied widely, common themes for structuring an ecological health assessment process emerged:

  1. Choose indicators that are ecologically meaningful and measurable; those that are highly valued by the public are also important to consider.
  2. Engage appropriate subject-matter experts through a structured and well-organized framework to gather necessary information while maintaining the scope and scale of the project.
  3. Base the initial report on existing data, as the time and expense of collecting new information can be prohibitive; data gaps will identify important areas of future study.
  4. Create scientifically based, clear, meaningful, and engaging communications to share the findings.

The advisory committee ultimately decided to follow a methodology similar to that used by the National Park Service Natural Resource Condition Assessments (NRCAs). As with NRCAs, the final report was not intended to be a management document, although it did include research, monitoring, and management considerations for each ecological health indicator. Also like NRCAs, the Mt. Tam ecological health assessment relied on existing information to determine trends and conditions, confidence levels, stressors and threats, and critical data gaps; however, it also incorporated expert observation and opinion.

While starting with the right framework was critical, the last item (4) from the list of suggestions above proved to be more impor­tant than initially expected. The advisory committee worked hard early on to define the ultimate purpose and audience for this project: synthesizing and distilling the best available knowledge about Mt. Tam’s resources in a way that would be useful to managers and clear and compelling to the public. Being able to return to this shared purpose was essential as the team navigated the complexities that lay ahead.

Metrics, conditions, and tends, oh my!

How to define ecological health on a mountain-wide scale?

Each of the four primary land management agencies on Mt. Tam is similarly tasked with preserving, maintaining, and maximizing biodiversity and natural processes; however, each does so under different missions, policies, and regulations. Any definition of ecological health for the entire mountain needed to encompass this range of approaches and goals, and yield a product that was useful to managers and understandable and compelling to the public.

A full and varied cache of literature exists on the many ways to define and measure ecological health, but with these goals in mind the advisory committee chose parameters that spoke to ecological function, biodiversity, species richness, resiliency, and natural processes, as follows:

  1. The full complement of plants, animals, and other life-forms are present, can reproduce, and are able to find food, shelter, and water for as long as climate conditions allow them to persist on Mt. Tam.
  2. Natural processes occur in a manner and frequency considered “normal” based on either historical evidence or the ability of these processes to maintain ecological functions and adapt under changing climate conditions.
  3. Mt. Tam’s ecosystems are resilient (able to function or recover despite disturbances, changes, or shocks).

What constitutes a “meaningful and measurable” health indicator?

If defining ecological health was challenging, figuring out how to measure it was another matter entirely. Good indicators are measurable and reveal things about other aspects of ecosystem health. The advisory committee created a comprehensive list of 37 potential ecological indicators that spoke to different aspects of ecological health and were meaningful to the partners involved in this process. These included species, taxonomic groups, communities, and ecological processes. Suspecting that many of these would prove useful, but not knowing exactly what the final assessment might include, the committee was hesitant to cut down the list early on. However, not every species, community type, or process on Mt. Tam could or should be included.

Based on the aggregated definition of ecological health above, one or more factors from the following list drove the selection of the indicators that were ultimately put forth for consideration:

  • It is present in the One Tam area of focus (fig. 2).
  • It is useful for measuring an important aspect of the health of the mountain (e.g., an indicator of biological integrity and biodiversity, natural disturbance regimes, or habitat quality).
  • Information or expert opinion is available to draw upon to try to determine its condition or trends.
  • It is a federally or state threatened, endangered, or rare species that, if lost, would have an impact on the mountain’s health by the above definitions.
  • It is especially iconic or charismatic, can be used to build public affinity and interest, or can be used to help gauge the health of the mountain by the above definitions.

What do we actually know about Mt. Tam’s health?

The advisory committee decided to follow advice to base this health assessment on existing data and other resources. However, distinct agency priorities, missions, and budgets meant that the TLC partners had largely conducted their monitoring, inventories, research, and data management independently. Using existing data meant finding ways to reconcile information at different levels of detail, collected over different timescales using different protocols, and maintained in different formats and locations.

The advisory committee very quickly realized what a major undertaking this would be and brought on a new team member specifically to gather all available existing information and organize it into combined databases and bibliographies. This aggregated information was distilled into summary worksheets for each indicator that included important elements borrowed from the NRCA reports, including

  • a preliminary assessment of the condition and trend,
  • the confidence level in these assessments,
  • a rationale for choosing that indicator,
  • a description of the resource and its significance to the health of Mt. Tam,
  • current and desired future conditions,
  • proposed goals and metrics by which to measure condition and trend,
  • key ecological stressors,
  • existing information sources (e.g., research data, monitoring, restoration projects),
  • known information gaps, and
  • future planned and desired management.

Engaging the broader scientific community

Of the initial 37 proposed indicators, 24 (fig. 3) were ultimately used as the basis for a one-day workshop with approximately 40 natural resource staff scientists from all of the TLC land management agencies, the Golden Gate National Parks Conservancy, Point Blue Conservation Science, the National Park Service Inventory and Monitoring Program, and Point Reyes National Seashore. Participants broke out into facilitated, subject-specific groups to review the summary worksheets, discuss the state of agency knowledge and data sources, identify information gaps, and provide feedback on the list of proposed indicators, metrics, and condition and trend assessments.

Figure 3. Ecological health indicators selected for the final report that were measurable, revealed something about the broader ecological health of the mountain, and had sufficient existing data or expert opinion. Information gaps identify important areas for future inquiry.

Plant Communities
  • Coast redwood (Sequoia sempervirens) forest
  • Sargent cypress (Hesperocyparis sargentii)
  • Open-canopy oak woodlands
  • Shrublands: Coastal shrub and chaparral (including serpentine chaparral)
  • Maritime chaparral
  • Grasslands
  • Seprentine barren community endemics
Broad Ecological Themes
  • Overall Mt. Tam biodiversity condition and trend
  • Climate-vulnerable plant communities
  • Climate-vulnerable bird communities
  • Shrubland ecosystems
  • Grassland ecosystems
  • Open-canopy oak woodland ecosystems
Wildlife Species and Groups
  • California red-legged frog (Rana draytonii)
  • Foothill yellow-legged frog (Rana boylii)
  • Western Pond Turtle (Actinemys marmorata)
  • Northern spotted owl (Strix occidentalis caurina)
  • Osprey (Pandion haliaetus)
  • American badger (Taxidea taxus)
  • North American river otter (Lontra canadensis)
  • Anadromous fish
  • Birds
  • Mammals

Gaps in Data
  • Seeps, springs, and wet meadows
  • Riparian woodlands and forests
  • Hardwood forests and woodlands
  • Douglas-fir forests
  • Lichens as an indicator of health (climate and air quality)
  • Soils
  • Hydrologic functions
  • Insects
  • California giant salamandas (Dicamptodon ensatus)
  • Small mammals, especially bats

Internal vetting was essential to helping refine and validate the proposed definitions and measures of ecological health. It also built critical understanding and investment in the process and the goals before the partners incorporated external input. Two more workshops that included 60 local academic and agency scientists relied upon the existing data and information that had been painstakingly gathered. Where data were scarce or nonexistent, participants were asked to use their best professional judgment to make a statement about goals, conditions, and trends. They also identified data gaps and areas of uncertainty, and the research or monitoring necessary to fill those gaps.

As with the other stages of this process, keeping the agreed- upon end goals in mind was the key to success. Every workshop attendee understood that the final product was not meant to be a scientific research paper, but rather a scientifically based decision support and public engagement tool. They knew that they had to be willing to make a statement about conditions and trends—even if they lacked 100% certainty. And, they were clear that the focus was on defining the desired condition of each indicator and the actions needed to reach that condition.

Using the feedback from these workshops, the advisory committee set the following definitions and parameters for condition, trend, and confidence levels for each ecological health indicator.

Desired Condition: The qualities land managers and other experts consider necessary for a particular indicator to maintain its ecological function(s) and the threshold or state it should be in to be considered healthy.

Condition: The current condition of the indicator based on the aggregation of its metrics.

  • Good: The condition goal is 75–100% met.
  • Caution: The condition goal is 26–74% met.
  • Significant Concern: The condition goal is 0–25% met.
  • Unknown: Not enough information is available to determine condition.

Trend: The change in condition of the indicator based on current versus previous measure(s), independent of status (e.g., a resource may be “Declining” but may still be in “Good” condition).

  • Improving: The condition is getting better.
  • No Change: The condition shows no consistent trend over time.
  • Declining: The condition is deteriorating or getting worse.
  • Unknown: Not enough information exists to state the trend.

Confidence: The amount of certainty with which the condition and trend are assessed.

  • High: Measurements are based on recent, reliable, suitably comprehensive monitoring.
  • Moderate: Monitoring data lack some aspect of being recent, reliable, or comprehensive; however, measurement is also based on recent expert or scientist observation.
  • Low: Monitoring is not sufficiently recent, reliable, or comprehensive; but either some supporting data exist or measurement is also based on expert or scientific opinion.

Bringing it all together

Seven individual wildlife species, three wildlife taxonomic groups, and seven plant communities were selected as indicators primarily because they had sufficient information or opinion consensus to set metrics and assess condition and trends (fig. 3). The final report clearly notes where data gaps for each indicator required the use of best professional judgment. Indicators that were deemed important, but for which there was not enough information or expert opinion, were included as needs statements for future research or monitoring (fig. 3). Seven broad, landscape-level themes were also evaluated thanks to early work to create a species-traits database that encompassed things like plant community associations, climate vulnerability, and sensitivity to ecological stressors for each indicator (fig. 3). Everyone who had participated in the scientist workshops had the opportunity to provide the technical review for the final report.

Not only has this process provided an invaluable ecological baseline for managers, but it also created excitement for the project by bringing together agency staffs and other scientists, revealing untapped synergies, and leading to new ideas for research, monitoring, and management. Completed in the fall of 2016, Measuring the Health of a Mountain: A Report on Mount Tamalpais’s Natural Resources is being used by managers to focus their monitoring and research work, implement shared data collection, and better align their planning and budgets to support common needs.

The report and its related communication materials have also achieved the initial goal of bringing the best available information together in a clear and publicly meaningful way. A summary video, brochure, and interactive web dashboard distill the report’s findings in accessible and compelling formats. Additionally, hundreds of members of the public, along with the local scientific and conservation communities, have attended two annual symposia on the outcomes of the project.

These efforts have been invaluable for engaging stakeholder groups and other members of the public, and helping to illuminate how Mt. Tam’s multiple land managers use science to protect and restore its important and unique resources. They have also created pathways for collaboration and understanding as managers plan for the mountain’s future.

About the author

Michelle O’Herron is an independent science communication and landscape partnerships consultant ( For more information about this project please visit the One Tam Peak Health website at, or contact Sharon Farrell or Bill Merkle.

Golden Gate National Recreation Area

Last updated: March 19, 2021