Dissertation/Thesis Abstract

Tidally-induced limits to California clapper rail ecology in San Francisco bay salt marshes
by Overton, Cory Tyler, Ph.D., University of California, Davis, 2013, 151; 3614256
Abstract (Summary)

The state and federally endangered California clapper rail ( Rallus longirostris obsoletus) has declined in abundance and been reduced in range and now occupies fragmented intertidal saltmarsh only within San Francisco Bay. Historically extensive salt marsh habitats existed in San Francisco Bay and today, remnants are largely restricted to the water's edge with dikes and levees separating marshland from modified habitats unsuitable for clapper rails. Clapper rail population abundance has roughly tracked a series of positive and negative impacts including market hunting at the turn of the 20th century, widespread habitat reduction and fragmentation, and invasive species introduction and eradication programs. Despite these changes, rail populations have been subject to the ebb and flow of the tides, which regularly inundate salt marsh habitats. The influence that tides have on vertebrate species living in intertidal saltmarsh should be substantial, but the relationship between tide and California clapper rails is poorly understood. This research identified important ways in which tides influenced demographic processes, space use, and resource selection in California clapper rails. Tidal inundation in San Francisco Bay saltmarshes creates zonation in plant communities, typically with tall monocots in the low marsh (Spartina sp.), short pickleweed (Sarcocornia pacifica) in mid-elevation ranges, and gumplant (Grindela humilis) in the high marsh. Invasive Spartina (Spartina foliosa x alterniflora ) grows taller and thicker than native Pacific cordgrass ( Spartina foliosa). Invasive Spartina also grows lower onto mudflats, further up into pickleweed areas, and provided both nesting habitat and tidal refuge for clapper rails. In Chapter 1, I examined survival rates of California clapper rails. Specifically, I investigated whether seasonal patterns observed in the early 1990s were still evident and assessed the influence that Invasive Spartina and the degree of tidal inundation on weekly survival rates in four South San Francisco Bay salt marshes. Between January 2007 and March 2010, California clapper rail annual survival was 73% greater in Spartina-dominated marshes (Ŝ = 0.482) than in a control marsh dominated by native vegetation (Ŝ = 0.278). Lower survival also occurred during periods when tide height was greatest and during the winter. Survival patterns were consistent with Invasive Spartina providing increased refuge cover from predators during tidal extremes which flood native vegetation, particularly during the winter when the vegetation senesces. Tide heights also strongly influenced selection for artificial habitats provided adjacent to one marsh during the winters of 2010-2011 and 2011-2012. Ten floating islands equipped with canopies providing cover were monitored using time-lapse cameras for evidence of clapper rail use. Clapper rails regularly used artificial islands once tides reached heights equal to the average surface elevation of the marsh. When tides had inundated the marsh plan, observed use of the artificial islands was more than 300 times expected use based on the surface area provided. Probability of use varied among the islands and low levels of use were observed at night. Endemic saltmarsh species are increasingly at risk from habitat change resulting from sea-level rise and development of adjacent uplands. Escape cover during tidal inundation may therefore need to be supplemented if species are to survive. I developed a new method to estimate space use accounting for individual movement phases within non-stationary relocation datasets using simulated radio-telemetry data. To define movement phases I used a nonparametric, multivariate test to detect change points in the mean or variance of a sequence of x,y coordinates. Once all phases (change points) were identified, Gaussian kernel density analysis was used to estimate space use during each phase, which I termed change-point utilization distributions (CPUDs). One advantage of this technique is that the location of change points can subsequently be tested for relationships with conditions that might trigger a change in how individuals use space. Change points in clapper rail movement were associated with a variety of environmental and biotic variables including high tides, nesting activity, intrusion by neighboring clapper rails, and transient movements outside the home range. Change points occurred more than twice as frequently during the highest observed tides relative to all other tide heights. Another use of CPUDs is that space use patterns of adjacent individuals can be evaluated for joint overlap only during specific time periods when overlap occurs. I used CPUDs developed for California clapper rails and identified the point within overlapping space-use estimates where each individual had priority access to the resources within its utilization distribution (i.e. the lowest kernel density isopleth that was common to two overlapping individuals). This provided an estimate of the spatial region at which individuals exhibited territoriality. During the breeding season, space use distributions overlapped less and average territory size increased relative to the non-breeding seasons. Population density implied by these territory sizes (1.38 birds/ha) is comparable to density estimates during the 1970s and 1980s. Together these findings show the great degree to which clapper rail behavior and demography can be influenced by the tides that populations experience. It is my hope that conservation efforts for this species, particularly in the arena of habitat restoration may benefit from this research.

Indexing (document details)
Advisor: Holyoak, Marcel
Commitee: Eadie, John M., Strong, Donald R.
School: University of California, Davis
Department: Ecology
School Location: United States -- California
Source: DAI-B 75/07(E), Dissertation Abstracts International
Subjects: Wildlife Conservation, Wildlife Management, Ecology
Keywords: Clapper rail, Habitat, San francisco bay, Space use, Survival, Tide
Publication Number: 3614256
ISBN: 9781303792441