Nesting Ecology and Conservation of Least Terns in St. Croix, USVI

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Title: Nesting Ecology and Conservation of Least Terns in St. Croix, USVI
Author: Lombard, Claudia Danielle
Advisors: Theodore R. Simons, Committee Co-Chair
Kenneth H. Pollock, Committee Member
Nick Haddad, Committee Member
Jaime A. Collazo, Committee Co-Chair
Abstract: Little is known about the Least Tern (Sterna antillarum antillarum) in the Caribbean, including the U.S. Virgin Islands. Concerns about predation rates and human disturbance on St. Croix prompted a study of their nesting ecology and a population viability analysis to determine their status and help formulate conservation strategies. From 2003 to 2006 I monitored 56 colonies (4,640 nests) built on salt ponds, sandy beaches, offshore cays, and an industrial park. Daily nest survival rates averaged 0.92 ± 0.04. Rainfall significantly and negatively influenced daily survival rates. The impact was most severe when precipitation events exceeded 190 mm. In densely human populated areas, mammalian predation was more pervasive. Five colonies were protected from predation (exclosure) and flooding (elevated platform) to determine the gains in nest survival. These colonies yielded a daily nest survival of 0.957 ± 0.02, or a gain of ˜100% in nest survival (0.24 to 0.50). Chick survival rates were 0.30 ± 0.11 (n = 44). Estimated breeding productivity was 0.08 (i.e., female fledglings⁄female). Breeding productivity needs to be at least 0.33 to attain a   1. This threshold assumed daily nest survival rates of at least 0.957, chick survival rates  0.5, and medium survival rates (i.e., 0.56 for S1, 0.81 for S2 and S3 and 0.92 for S4). A viability analysis predicted persistence of the St. Croix population only under the most optimistic age-specific survival rates or when immigration from other populations was allowed. Increasing reproductive output by implementing nest protection measures was not an effective tool to reduce extinction risks. Implementing a 25% management effort improved time to quasi-extinction by only 8.4 months when compared to a scenario with no management. Applying management to all nests (100%) increased the time to quasi-extinction by only 6 years. The Least Tern population nesting on St. Croix is the largest known in the Caribbean. The annual number of nests ranged from 919 to 1341. Three explanations can be advanced to reconcile the observed level of breeding activity and outcomes generated by model scenarios. One is that the species is indeed facing a precarious outlook. Least Terns are long-lived species and time lags may mask the population's downward trend in persistence. A second is that observed levels of reproductive activity might be maintained by immigration from source populations elsewhere in the Caribbean. Both of these possibilities are consistent with a population acting as a sink. The third explanation is that the status of the species is not as precarious because age specific survival rates are higher than assumed in my "base" model (i.e., medium survival rates). This possibility is plausible because many survival estimates for Least Terns were based on return rates, which are biased-low. Conservation efforts in St. Croix should be aimed at protecting existing and historical Least Tern breeding habitat, particularly the most isolated sites, and at minimizing egg and chick mortality. Improved reproductive rates alone, however, will not prevent a trajectory of decreasing persistence. My work underscored the importance of immigration and age-specific survival rates in maintaining a viable population. Estimates for these parameters are not available in the Caribbean. Reducing parameter uncertainty is necessary to set local management targets and formulate an integrated, multi-scale conservation strategy. Initially, efforts should focus on colonies closest to St. Croix to generate estimates of age-specific survival and movement rates. A focal geographic area facilitates addressing important study design considerations (e.g., sample size, sampling frequency) necessary for precise estimates. Subsequently, similar efforts could be extended to other focal areas. Molecular genetic approaches could help discern patterns of population connectivity and areas of conservation importance.
Date: 2007-05-30
Degree: MS
Discipline: Zoology

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