|Ninety percent of salt marshes in southern California have been lost, mainly due to habitat loss and fragmentation caused by coastline urbanization (Zedler 2001). San Diego Bay's coastline is extensively developed with only the southern portion of the Bay retaining remnants of natural estuarine habitat. The beaches of the Silver Strand, south San Diego Bay, and the Tijuana Estuary are the few remaining areas in which seabirds and shorebirds may forage in a natural environment. The Silver Strand remains relatively natural due to the protection of naval resources, while south San Diego Bay is protected as a National Wildlife Refuge. The Tijuana Estuary is one of the least fragmented salt marshes in southern California and is one of the few estuaries in southern California retaining its ocean connection (Zedler 2001). With only ten percent of historical marshes remaining, the functional integrity of these remaining areas is essential in supporting resident species.
One of the biggest concerns for the preservation of natural resources is the ability to determine whether an ecosystem is healthy and functioning properly. One such method for evaluating ecosystem health is to identify trophic interactions within the system and determine if the interactions are balanced and properly functioning. Understanding and anticipating nonlinear dynamics are important aspects of adaptation planning since responses of biological resources to changes in the physical climate system are not necessarily proportional and sometimes inherently nonlinear (Burkett et al 2004). With the concern of global climate change and its effects upon dynamic systems, it is important to identify bio-indicators that can signal change within an ecosystem. Recent research has highlighted the utility of using seabirds as indicators of ecosystem health, e.g. evaluating heavy metal pollutants and trophic interactions in an ecosystem (Burger and Gochfeld 2005; Hilty and Merenlender 2000).
The South Bay Unit of the San Diego Bay National Wildlife Refuge is nesting grounds for six seabirds with conservation status including the California Least tern, which is classified as federally endangered. Currently, two projects in the Conservation Ecology Lab are evaluating seabirds as ecosystem indicators in San Diego Bay. The first is evaluating foraging dynamics of the Gull-billed tern, a top generalist predator in the San Diego Bay system; the second is determining contaminant loads in the strictly-piscivorous Black Skimmer.
In 1999, Gull-billed terns were first documented to be depredating the chicks of California Least terns and Western Snowy plovers, both of which are protected under the US Endangered Species Act as Endangered and Threatened, respectively. Information on this depredation, termed "intraguild predation" (IGP), has been mainly provided through anecdotal sources, e.g. observations and comments provided by colony monitors. The focus of this research is to investigate the prevalence of Gull-billed tern mediated IGP on California Least terns and Western Snowy plovers using radio telemetry and stable isotope analysis. Manual and automated telemetry will characterize GBTE movements, identifying core foraging areas and describing Gull-billed tern visits to California Least tern and Western Snowy plover breeding locations within San Diego Bay. Stable isotope analysis will provide insight on Gull-billed tern adult and chick diet to determine the importance of particular prey items to the Gull-billed tern diet.
Increased anthropogenic disturbance in and around the San Diego Bay has led to large accumulations of contaminated urban runoff. Persistent organic pollutants – including organohalogen compounds, DDT, and flame-retardants (PBDEs) – are known to bioaccumulate, causing reproductive impairments and detrimental effects to birds and other wildlife. The San Diego Bay hosts the largest western colony of black skimmers, with approximately 350 breeding pairs. Yet, while colony size has held steady and even increased over the years, reproductive output has remained relatively low. Initial studies both locally and nationally have provided evidence that black skimmers accumulate contaminants in higher amounts than conspecifics (Burger 2002). Furthermore, as fish specialists, skimmers may be at an increased risk of bioaccumulative effects. Working in collaboration with SDSU's Graduate School of Public Health (Dr. Eunha Hoh), this study combines contaminant analysis with stable isotope analysis to link contaminant concentrations with diet composition. We will also analyze non-targeted compounds to determine if any other substances, either naturally-occurring HNPs (halogenated natural products) or man-made, represent a potential threat to skimmers and other wildlife dependent on the bay.
Photo credit to Matt Sadowski