2017 Caymans Field Season
and Glenn Gerber (San Diego Zoo Global)
Overview. The first major objective of this project was to assess trends in Little Cayman’s breeding population of Sister Islands Rock Iguana (SIRI) in light of a third consecutive year of nest counts and Capture-Mark-Release-Recapture (CMRR) data. We maintained regular surveys of eight known nesting sites on the West End, accounting for shifts or declines in nest densities as well as return rates of tagged females. Intensive island-wide sampling was carried out to monitor turnover in the adult population.
In addition to evaluating trends in the adult population of Little Cayman, another major objective of this study was to investigate recruitment and age-dependent survivorship. By nearly doubling our hatchling sampling efforts between 2015 and 2016, and expanding our regular survey areas in 2017, we anticipated greater precision in our estimate of initial cohort size, which should illuminate reproductive success in the population.
Finally, a major objective of this summer was to apply research methodologies developed on Little Cayman to identify possible nesting areas on Cayman Brac and evaluate hatching success and inbreeding load. To achieve this goal, we organized a preliminary three-week trip to Cayman Brac at the peak of nesting season to identify nests, which will be sampled for hatchlings later in the season.
Resolve population trends on Little Cayman. From May through June 2017, we conducted daily surveys of major nest sites on the West End of Little Cayman to note fresh activity and maintain records of any marked or unmarked females encountered at the communal nesting aggregations. These eight sites, one originally identified by Glenn Gerber in 1992 (Preston Bay) and seven additional sites identified by Matt Goetz in 2010, have been surveyed continuously since 2015 to assess current trends in population recruitment and habitat use. In addition, we expanded our regular survey extent to include three new sites along the southern coastline. This groundwork will facilitate our goal of radio-tracking hatchlings from a wide distribution of the island during emergence season in August.
Although 2017 can be characterized as a particularly wet summer, final nest counts reflect overall stability at most sites over the last three years. However, we have noted a marked decline in reproductive effort at the island’s largest communal nesting site, Preston Bay. This site was estimated to have supported over 60 nesters in 2010 and 25 in 2015, but contained fewer than 15 nests by our final tally in 2017. Our CMRR data obtained on our daily roadside transects, also suggest a steady decline in the adult breeding population. Of 208 adults tagged over the last two summers, only 85 individuals remained unaccounted for by the end of our intensive surveys in 2017. However, at least 17% of individuals “lost” between 2016 and 2017 can be positively traced to road mortality, and less than 25% of the nearly 180 iguanas marked by Goetz between 2007 and 2011 have ever been recaptured. Taken together, these data suggest that longevity may be more limiting in this population than originally presumed, and that despite the appearance of robust densities, attrition in the adult breeding population may be a driving gradual population reduction.
Evaluate population recruitment. Population surveys in 2017 followed intensive sampling of neonates in August 2016, yielding over 420 tagged hatchlings. Under the assumptions of the Lincoln-Peterson model including equal catchability of marked and unmarked animals, our maximum likelihood estimate (MLE) of recapture probability for the 2016 cohort was 11%, generating an initial cohort size of 3,722. Provided SIRI’s large average clutch size and high hatching success, fewer than 300 breeding females could have produced a cohort of this size. Although the 95% confidence intervals around this estimate are very large — approximately 2,000 to 21,000 — we suspect annual reproductive output is quite robust on Little Cayman.
Although the behavioral ecology of adult rock iguanas is relatively well studied, our current understanding of juvenile ecology is limited. By intensively sampling neonates between the 2015 and 2016 hatching seasons, then recapturing yearlings the following season, we have started to shed some light on this critical life stage. Results thus far indicate high reproductive success and overall high dispersal among neonates, further upholding the major focus of SIRI management planning — nest site protection — for maintaining healthy densities of iguanas across Little Cayman.
Identify active nesting sites on Cayman Brac. Identifying active nesting sites on Cayman Brac represents an important first step in outlining conservation priorities for this reduced sister population. Before starting, no communal nesting sites were confirmed to remain on Cayman Brac due to intense development pressures. We spent three weeks on the island surveying for nesting sites and tagging adult iguanas. We were pleasantly surprised to observe high traffic by nesters along a large stretch of southern coastline, as well as patchy digging in a community park area. By the end of the period, we captured nearly 50 iguanas. Because Cayman Brac supports a large human population, it is important to communicate the vital role these areas serve for iguanas and to discourage development.
Having marked and excavated a number of nests, we anticipate high success at sampling hatchlings in August, as we have done successfully on Little Cayman for the past two summers. Additionally, the majority of our captures on Cayman Brac were new captures, despite tagging efforts that have been ongoing for the past five years. This suggests that the Cayman Brac SIRI population, which is presumed to have declined significantly in recent years, is reproducing successfully and may in fact be rebounding by some margins. We intend to evaluate our collected samples for any genetic sign of inbreeding depression in this population.
Future. Over 300 adults and nearly 900 hatchling SIRI have been tagged on Little Cayman since 2015. Additionally, we anticipate at least 100 adults harboring tags will survive on Cayman Brac, and hatchlings were marked for the first time this month. In order to maximally capitalize on these years of effort, we propose making at least one annual return trip to the islands over the next two to three years. A short (2–3 week) sampling and surveying trip around the peak of nesting would allow us to address questions of nest-site fidelity by females and hatchlings and to uphold ongoing mark-recapture efforts, both of which would be valuable to our understanding of population ecology. Continuing to expand research efforts onto Cayman Brac would also be advantageous for informing conservation management across the range of SIRI. Finally, addressing the upsurge of invasive Common Green Iguanas and hybrids in the Sister Islands over the last three years has become a conservation priority, demanding regular research and monitoring. Our hope is to continue to cultivate and expand our partnerships with the Cayman Islands Department of Environment and the National Trust to better manage these emerging threats.