Assessing Vulnerability of Tidal Marsh Birds to Climate Change through the Analysis of Population Dynamics and Viability

The vulnerability of species at risk from climate change is recognized as an important issue in California as well as globally. Assessing vulnerability requires information on the long-term viability of populations and understanding the influences on that viability, due to environmental drivers as well as impacts of management action. We developed population-dynamic models to assess and better understand the long-term population viability of four key, tidal marsh-dependent species, under a variety of environmental conditions, including climate change impacts. In the San Francisco Estuary, each species is represented by one or more subspecies that is entirely or mainly confined to the tidal marsh habitat in the region: California Black Rail (Laterallus jamaicensis coturniculus), a California Threatened species, California Clapper Rail (Rallus longirostris obsoletus), a Federally Endangered species, Saltmarsh Common Yellowthroat (Geothlypis trichas sinuosa), a California Species of Special Concern, and three tidal marsh subspecies of Song Sparrow, all of which are California Species of Special Concern: Alameda (Melospiza melodia pusillula), Samuel’s (M. m. samuelis), and Suisun (M. m. maxillaris).

Availability of habitat is a prerequisite for long-term viability of marsh bird populations and this has been modeled in a companion California Landscape Conservation Cooperative project (Veloz et al. 2011). However, habitat alone will ensure neither resilience nor recovery of depleted and threatened populations. Two important population bottlenecks considered here are reproductive success and over-winter survival. An important component of reproductive success with respect to maximizing long-term viability and enhancing population resilience is the survival of clutches and broods from egg-laying to fledging; in particular, nest survival may be affected by flooding of nesting habitat leading to inundation of nests. The second bottleneck is overwinter survival which may be compromised when tidal marsh species are exposed to predators (e.g., herons and egrets) during the highest tides, if refugia for marsh birds are not available.

The study assessed future population trends and long-term viability due to anticipated changes in important climatic variables: precipitation, temperature, and maximum high tides. Detailed, mechanistic models were developed for one species, tidal marsh Song Sparrows. For these populations we drew on extensive, long-term field studies and our previous demographic analysis.

Whereas our results are of special relevance to wildlife resource managers in the San Francisco Estuary, the methods we have developed and described are applicable to estuarine bird populations anywhere, and thus the value of our approach extends beyond the specific species we have modeled. Insights from our modeling can help prioritize future action for a range of species and wetland habitats.