Soft Bird's Beak Population Census, Status, and Response to Conservation Actions - Brenda Grewell
photo by Brenda Grewell
Soft bird’s-beak is hemiparasitic plant endemic to high elevation tidal marshes of Suisun Marsh and the North Bay of the San Francisco Estuary. Historic accounts indicate this species is an anthropogenic rarity that is endangered due to loss and degradation of tidal wetlands. The goal of this study was to provide critical ecological data to habitat managers for effective conservation and recovery of this rare tidal marsh plant. Specifically, the project objectives were to 1) conduct a population census to determine current distribution and population size and compare this information to historic census data, and 2) conduct demographic monitoring to evaluate threats to critical life stages, diagnose causes of population performance, and refine a life cycle model for conservation management.
Results of the field census revealed a wider distribution and greater number of soft bird’s beak population patches at BSRA than were previously known. An experimental seed bank study was established to quantify emergence, persistence, and death rates of the seed life stage to better understand the life history of the species and to refine a population life cycle model. This demographic model is recommended as a tool to test potential conservation management actions and soft bird’s beak population response to conservation management. Results show that soft bird’s-beak maintains at least a short-term persistent seed bank that can greatly influence population dynamics and persistence.
Demographic monitoring and survivorship models indicate that survivorship and fecundity were high within a BSRA sub-population in frequently flooded habitat. At a second muted tidal site at the upland transition of BSRA, fecundity of the population was low due to pre-dispersal seed predation and high seedling mortality. At Rush Ranch, the 4 th generation of the experimentally reintroduced soft bird’s-beak population in the Solano Land Trust’s Spring Branch restoration site has grown exponentially, but demographic monitoring revealed a severe drop in fecundity in 2004 that is best explained by declines in habitat quality and signals a need for management intervention.
Major threats to the long term sustainability of both native and restored populations of soft bird’s-beak include 1) unsuitable host associations with invasive sickle grass (Hainardia cyclindrica) and rabbitsfoot grass (Polypogon monspeliensis) that lead to high seedling mortality; 2) displacement by invasive weeds including perennial pepperweed (Lepidium latifolium), wild celery (Apium graveolens), and black rush (Juncus gerardi); and direct destruction of soft bird’s-beak habitat by feral hogs (Sus scrofa) at Rush Ranch. Prioritization of funds for control of invasive species that threaten the viability of soft bird’s beak, and tidal wetland restoration and enhancement projects at Rush Ranch and BSRA to improve habitat quality and facilitate recovery of this endangered rare plant are recommended.
