Landscape use by herons and egrets in the San Francisco Estuary
Spatial Metrics for Wetland Restoration: Developing Fine-scale Spatial Models for Avian Population Parameters in San Francisco Bay Tidal Marshes
Summary: As part of a multi-investigator, interdisciplinary, wetland monitoring project (www.irwm.org), we have developed a suite of spatial metrics linking tidal marsh processes with observed patterns in geomorphology, vegetation, and landscape composition. These metrics, derived from multi-spectral aerial imagery, have provided a basis for fine-scale spatial modeling of wildlife habitat potential. Using empirical data on avian abundance and nesting success, we have developed predictive models that may be extrapolated to similar wetland sites around the bay. Metrics evaluated include 50-m radius moving window statistics for channel area and density, habitat diversity, vegetation cover classes, and NDVI; as well as inverse-distance weighted proximity to channels, habitat edges, and other landscape features. The maps resulting from these predictive models provide resource managers with current information on species abundance and reproductive success within restored and mature marshes, and can help assess the conservation and restoration efforts within the region. In addition, these analytical tools allow us to locate areas or types of marsh where our predictions are less certain (i.e., where the model performance is poor), and therefore will benefit from additional sampling and research. Here we present the model results as well as spatial predictions for three avian species (Song Sparrow, Common Yellowthroat, and Black Rail).
Map: Predicted density (birds/ha) of Common Yellowthroats at six San Francisco Bay tidal marsh study sites. (Diana Stralberg)
Landscape use by herons and egrets in the San Francisco Estuary
Summary: We measured the colony size, productivity of successful nests, and nest survivorship at 45 known heronries within 10 km of historic tidal-marsh of San Pablo Bay and Suisun Bay. The data were used to evaluate landscape associations of heronries and the potential use of tidal marshes by herons and egrets. We analyzed landscape associations based on the aerial extents of NOAA land cover types (from Landsat images, 2000-2002) and several wetland-patch (FRAGSTATS) metrics within 1, 3, 5, 7, and 10 km of heronries. First, we compared land cover and wetland patch metrics at all distance scales with those of randomly selected, unoccupied sites. These comparisons revealed selection of nesting areas with more estuarine emergent wetland and more open water within 1 km of heronries than expected. Fourteen-year patterns of productivity (pre-fledging brood size) of successful nests were significantly related to landscape conditions at relatively large spatial scales. Great Blue Heron brood size increased with the extent of estuarine emergent and open-water wetlands within 10 km; Great Egret brood size increased with the extent of estuarine emergent wetlands within 10 km and greater wetland edge complexity within 5 km. Our results contribute to the development of techniques to measure the effects of landscape conditions on local ecological processes and, specifically, on the use of marshes by an important group of predators.
Map: Predicted Great Blue Heron brood sizes (several models averaged). (Diana Stralberg)
California Migratory Bird Habitat Assessment
Summary: In collaboration with the Nature Conservancy, PRBO is working to help prioritize areas and habitats of importance for migratory waterbirds in California. Focusing primarily on Central Valley shorebirds, we have analyzed three years of comprehensive survey data from our Pacific Flyway Project with respect to GIS-based land use / landcover and climate data for California’s Central Valley. Preliminary findings highlight the value of managed wetlands (mostly USFWS refuges) for all shorebird species analyzed. Rice fields and other agricultural types were also found to be important for several species, particularly when interspersed with managed wetlands.
Map: Central Valley dowitcher density by sub-basin, overlaid with refuge land locations. (Diana Stralberg)
Monitoring Point Reyes Snowy Plovers
Summary: PRBO has conducted intensive monitoring of the Western Snowy Plover (Charadrius alexandrinus) population within the Point Reyes National Seashore (PRNS) since 1977. The Pacific coast population of the Western Snowy Plover was listed as threatened by the U.S. Fish and Wildlife Service (USFWS) due to loss of habitat by encroachment of non-native vegetation, predation pressure, and disturbance due to recreational use of beaches. We have helped the National Park Service to reach the Draft Snowy Plover Recovery Plans goal of 64 breeding birds within PRNS by recommending management actions and monitoring the birds� response to those actions. Maps of changes in vegetation and Snowy Plover nests over time have helped illustrate the need for dune restoration. (Authors: K. Peterlein, L.E. Stenzel, G.W. Page)
Map: Cumulative number of Snowy Plover nests found per hectare at Kehoe Beach, 1997-2003.
Map by Chris Rintoul
Modeling potential impacts of non-native Spartina spread on shorebird populations in south San Francisco Bay
Summary: San Francisco Bay holds 70% of California�s mudflats and provides habitat to more wintering and migratory shorebirds than any other wetland along the Pacific coast of the contiguous U.S. The Bay�s mudflats are currently threatened by the spread of a non-native cordgrass, Spartina alterniflora, and associated hybrids, which grow at lower elevations than the native S. foliosa and can render large mudflat areas effectively unavailable to shorebirds for foraging. Using shorebird and benthic invertebrate survey data, tidal benchmark data, and GIS-based habitat data, we analyzed the potential effect of S. alterniflora on shorebird habitat in the South Bay, creating grid-based spatial models of shorebird habitat value and potential S. alterniflora spread. We developed 12 potential scenarios of habitat value loss for shorebirds, based on assumptions about invertebrate density, inundation tolerance of S. alterniflora, and temporal availability of mudflat resources. Predictions of habitat value loss ranged from 10% to 70%. We identified the upper mudflats, due to their greater exposure time, and the east and south shore mudflats, due to the high numbers of birds detected there, as the areas of highest value to shorebirds in the South Bay. These areas also coincide with the areas of greatest Spartina invasion potential. (Authors: D. Stralberg, V. Toniolo, G.W. Page, L.E. Stenzel)
Map: Predicted extent of Spartina alterniflora spread based on a 42% inundation tolerance and overlap with tidal mudflats, classified according to their potential value for shorebirds.
Map by Diana Stralberg and Viola Toniolo
Mapping California inland-breeding seabird colonies
Summary: The California Inland-Breeding Seabird Atlas maps were designed to document the current status and highlight the conservation needs in California of a group of seven species of colonial or semi-colonial waterbirds (�inland-breeding seabirds�): the American White Pelican, Double-crested Cormorant, Ring-billed Gull, California Gull, Caspian Tern, Forster�s Tern, and Black Tern. Although the majority of survey work was conducted from 1997 to 1999, the atlas maps show the locations of colonies of the two species of gulls active from 1994 to 2000 and those of the other five species active from 1997 to 2000. Colony sites were mapped on 1:100,000 scale USGS topographic quads. Interactive maps were created for three general regions within California � Northern, Central, and Southern � using the HTML ImageMapper 3.0 extension for ArcView 3.x.
Map: Screen snapshot from the Inland-Breeding Seabird Colony Atlas.
Map by Chris Rintoul, Diana Stralberg, and Viola Toniolo
Quantifying spatial and temporal heterogeneity in reproductive success of tidal marsh Song Sparrows (Melospiza melodia): the importance of �edge effects� and habitat configuration
Summary: San Francisco Bay tidal marshes are naturally fragmented by topography, but in recent years this fragmentation has been exaggerated by the filling of tidal wetlands and development of surrounding uplands. Previous results suggest that the abundance of adult Tidal Marsh Song Sparrows (three endemic subspecies of Melospiza melodia) is lower in small marshes surrounded by urban development than in larger, less disturbed marshes. We next wanted to look at the effects of marsh fragmentation on this species� nest success. Because nest predation is a main cause of nest failure, we investigated the effects of edge proximity on predation rate and overall nest success. For five marshes, we looked at the fate of more than 1,800 Song Sparrow nests monitored between 1996 and 2000. We found that, comparing across study sites, nests that were located closer to upland edges fledged young significantly less often. Because we found differences in the proportion of nests surviving to hatch, but no significant association with nestling survival, we infer that increased egg predation is the primary culprit. However, examining variation in nest success within a given marsh, we found no significant effect of upland edge proximity. In other words, nest success was related to edge characteristics of the entire marsh but not the location of nests within a marsh. Although further investigation is needed, we suggest that landscape-level conservation planning is needed to identify conservation and restoration sites that are most hospitable to Song Sparrows and other marsh-dependent species.
Map: Location and fate of nests monitored at China Camp State Park in 2001. Shown is an example of distance to nearest water and upland edge for an individual nest.
Map by Diana Stralberg
Predicting the effects of habitat change on South San Francisco Bay bird communities: An analysis of bird-habitat relationships and evaluation of potential restoration scenarios
Summary: The recent acquisition of 5,000+ hectares of salt evaporation ponds by state and federal agencies provides an unprecedented opportunity to restore large areas of contiguous tidal wetlands in San Francisco Bay. The conversion of existing ponds to tidal marsh will create valuable habitat for a range of marsh-dependent species. At the same time, many waterbird species now depend on salt ponds as a replacement for natural shallow water habitats that have been lost since European settlement. In light of these apparent trade-offs, we evaluated the potential effects of restoration on wetland bird communities. We used a combination of standardized bird surveys, salt pond salinity and bathymetry, aerial photos, and land use data to develop empirical spatial models that predict the effects of specific restoration scenarios on bird diversity and potential numbers (as represented by available habitat). Our models suggest that waterbird habitat potential can be improved by maintaining a wetland mosaic that includes tidal marsh in various successional stages interspersed with salt ponds managed for appropriate depths and salinities. This has led to the current development of a spatial optimization model that will identify optimal tidal marsh-salt pond configurations and management regimes for a range of conservation targets.
Map: Channels and ponds mapped by PRBO (2001-2002) and USGS (1996), overlaid on color infrared aerial photos (PRBO, August 2001). Newark Slough, Dumbarton Marsh East and West, Audubon Marsh East and West.
Map by Diana Stralberg