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Elegant Tern migration may help species withstand effects of El Niño

Dr. Andres Aguilar, California State University, Los Angeles
Dr. Michael H. Horn, California State University, Fullerton

April 28, 2016

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Elegant Terns delivering prey fish to their chicks. South Bay Salt Works, San Diego. Photo credit: K. Goodenough.

The Elegant Tern (Thalasseus elegans) is an abundant, coastal tern with one of the smallest breeding ranges of all seabirds. Elegant Terns feed on a variety of nearshore fishes and are indicators of both the abundance of prey items and changes that may occur in prey communities. The species is considered ‘near threatened’ due to its narrow breeding range and population fluctuations due to climatic effects.

The great majority of the total global Elegant Tern population usually nests only on one island, Isla Rasa, in the midriff group of the north-central Gulf of California (Baja California). Their numbers fluctuate periodically at Isla Rasa, primarily because of El Niño Southern Oscillation (ENSO) conditions and shifting abundance of forage fish, such as Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax). Terns often fail to nest during strong El Niño events, such as those that occurred in 1982-83 and 1997-98, due to the low abundance of forage fish. Failure to nest during these years could contribute to population declines for this species.

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Elegant tern nesting sites in Southern California and Baja California.​​

The Elegant Tern’s range expanded in the 1950s: it first began nesting in south San Diego Bay in 1959, then moved north to Bolsa Chica Ecological Reserve in 1987 and Los Angeles Harbor in 1998. Although the southern California population represents a small proportion of the total species abundance, its size has increased steadily in nest numbers over the last 20 years. In 2014 there were an estimated 54,106 nests among the three southern California nesting sites. Despite the overall increase in numbers in southern California that started in 1993, nest counts have varied among the three sites over the years, perhaps in response to local oceanographic conditions, prey availability, and/or predator disturbance.

Given the recent colonization of southern California by Elegant Terns, we wanted to explore the possibility that these populations are still connected via migration to the larger breeding population in Baja California. If these breeding colonies are still connected, then the species may be better able to persist during regional disturbance events, like an El Niño, and management decisions should be made on a regional scale that includes breeding areas in southern California and Baja California. If there is little to no migration between southern California and Baja California, then future conservation efforts should consider management plans that focus on each breeding area independently.

To determine the degree of connection between the southern California and Baja California Elegant Tern populations, we investigated genetic differences between them during El Niño (2014) and non-El Niño (2013) years. We collected DNA samples (blood and egg shells) from approximately 24 Elegant Terns from each of the three southern California breeding sites and blood samples from 48 terns at Isla Rasa. Our research is part of a larger study that involves collaboration with consulting biologists Kate Goodenough and Robert Patton in San Diego and Dr. Enriqueta Velarde, a professor at Universidad Veracruzana in Mexico.

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Lizbeth Pliego and Russel Bacosa (CSULA undergraduates) per​forming DNA extractions on Elegant Tern samples. Photo credit: A. Aguilar.

We utilized a relatively new approach in population genetics, restriction site associated DNA sequencing (RADseq), an approach that allows us to survey thousands of regions across the genome to determine if there is any temporal and/or spatial genetic structure in Elegant Terns. This approach is useful for highly mobile species where detecting population structure may be difficult. Our sequencing efforts yielded over 20,000 single nucleotide polymorphisms (SNPs) to evaluate population structure in this species. A power analysis suggests that the loci we have used in this study should be sufficient to detect even low levels of population structure if it exists.

Analysis of the SNP data reveals there is little population structure among individuals that nest on Isla Rasa and those that nest in southern California. This result suggests that either the composition of the two main nesting areas is very fluid (meaning individuals readily migrate between Baja California and southern California to breed) or that not enough time has elapsed since the establishment of southern California nesting colonies in the 1950s to detect divergence from the Baja California population. However, because the southern California population continues to increase in size, we believe that individuals are migrating up from Baja California. The establishment of colonies in southern California and a high degree of migration between the Baja California and southern California populations may be critical in allowing the Baja California Elegant Tern population to withstand the local impact of El Niño events. Thus, the conservation of both populations may be paramount to the long-term viability of this species, particularly in light of a predicted increase in El Niño event frequency and intensity over the coming century as a result of climate change.

Dr. Andres (Andy) Aguilar​ is an Assistant Professor in the Department of Biological Sciences at CSU Los Angeles. Dr. Michael Horn​ is a Professor in the Department of Biological Science at CSU Fullerton. COAST provided funding for this project: Rapid Response Funding Program Award# COAST-RR-2014-002, December 2014.