Report submitted by Jeanette Moss, Mississippi State University
Little Cayman (LC) is a 28-square kilometer island of relatively undisturbed xeric forest supporting the most robust population of Sister Islands Rock Iguanas (SIRI; Cyclura nubila caymanensis). While a cursory census suggests good population health, mounting threats associated with human development are suspected to be driving a steady decline for this endemic reptile. Conservation planning continues to prioritize the protection of coastal nesting sites, however, because these do not constitute suitable year-round habitat for iguanas, the survival of future generations will likely depend on the maintenance of intact forest, shrubland, and other associated habitats in the island interior. Last year, the IIF supported fieldwork on LC to initiate long-term monitoring efforts and to investigate nesting ecology questions. Our chief objectives this year were: 1) continue to monitor population trends on LC via mark-recapture and nest site surveys; 2) assess the respective importance of site fidelity and site proximity in predicting nest site selection by female SIRI; and 3) investigate the distribution of home territories among iguanas in the West End of LC by tracking a representative sample of nesting females and evaluating habitat associations.
Over the course of nine weeks, 62 nests were identified in the West End of LC, which is comparable to the 78 nests identified in 2015 but still far short of the 120 identified by Matt Goetz in 2010. Nearly 75% of observed nesting occurred over a three-week period between 25 May and 15 June. Preliminary trends indicate that seasonal precipitation may play an important role in the timing of nest closures on LC, however more long-term monitoring is needed to assess the extent to which environmental factors may affect nesting patterns. Mark-resight efforts thus far indicate a high degree of site fidelity among adults, as the majority of recorded long dispersal events were associated with nesting females or juveniles. Indeed, 34 of 72 adults tagged in roadside surveys in 2015 were re-sighted in 2016, and 73.5% of re-sights occurred within 0.5 km of their original capture location. Only three of the 228 hatchlings marked in 2015 were recaptured in 2016. The comparatively large number of new yearling captures (n = 38) suggests the cohort size was quite robust. Based on one-year growth data obtained from small samples of hatchlings and juveniles, we can deduce that SIRI reach sexual maturity in 2 to 3 years (SVL = 28-30 cm) and that the highest rate of growth occurs in the second year. Our preliminary data also suggests that juveniles disperse very long distances. Of particular note is one yearling that was recaptured 14.9 km from its natal site in 2016, and a hatchling that was observed to travel 2.96 km from its natal site in a single day. This year, a total of 118 new adults and subadults, and 423 hatchlings were tagged to continue long-term monitoring.
To investigate the respective roles of female philopatry and home range proximity in individual site selection, we employed a combination approach of surveying by mark-resight and direct radiotracking of individual nesters. Initial appraisal of the final locations of 18 tracked individuals suggests fairly random dispersal patterns by nesters. On average, females traveled just over 1.1 km to their home territories. Females sharing general home territories did not necessarily migrate to the same nest sites, but none migrated farther than 2.9 km. Since Cyclura are known to be capable of migrations up to 15 km, our data suggest that females likely select for site proximity when suitable nesting habitat is not scarce, as in the West End of LC. Of the 23 females captured and marked at nesting sites in 2015, at least 65% were found to exhibit site fidelity in 2016. This data, in combination with individual tracking datasets, suggests that intrinsic behavioral mechanisms also play a large role in driving selection of nesting habitat. In addition to recaptures, we captured and tagged an additional 20 unmarked nesters this year, which will facilitate greater resolution of return nesting events in following field seasons.
The movement paths of tracked individuals in our study indicate that females are fully utilizing their natural, intact habitat to move between sites during nesting season and that few barriers to dispersal exist on Little Cayman. However, point sampling reveals that only 32% of final tracking locations fall within undisturbed areas of dry shrubland, according to the habitat classification scheme laid out in the Cayman National Biodiversity Action Plan. Meanwhile, 53% of home territories fall within habitat classified as “man-modified” or “urban.” If our sample of females is representative of the West End population, this observation suggests that over half of iguana home territories are concentrated around edge habitat.
Two years of field research on LC have thus far resulted in the tagging of over 900 adult and hatchling SIRI for long-term population monitoring. Recaptures in this year alone have provided much insight into dispersal and habitat use patterns. Chiefly, our data suggest that adults exhibit a high degree of fidelity to home territories and that juveniles tend to disperse unpredictably and often over long-distances. Moreover, our recapture data recapitulates important details of this taxon’s natural history, including growth rates and age at sexual maturity. Continued monitoring over additional field seasons is expected to provide valuable insight into long-term population trends and recruitment rates.
Our finding that both philopatry and proximity are important predictors of nest site selection in the West End suggest that traditional communal sites will continue to be important over the long-term and that areas of coastline accessible to the densest populations of iguanas should recruit the highest number of first-time nesters. Unfortunately, downward trends in nest counts from year to year are consistent with a reduction in the adult breeding population as well as possibly the availability of quality nesting habitat. Therefore, long-term management plans may need to consider the preservation of more coastal shrubland in the West End.
While the protection of nesting habitat is critical to maintaining LC’s SIRI population, the results of this study have also forewarned the dangers of over-developing the island’s interior. Preference for disturbed edge habitat is expected to place iguanas at heightened day-to-day risk of road collisions and predation by feral cats and dogs. Fortunately, many of the iguanas tracked in this study returned to retreats in isolated, undisturbed tracts of dry shubland. In order to curtail encroaching development and maintain these natural populations, it is increasingly critical that large protected areas be established in LC’s interior while high-quality foraging habitat is still abundant.
Ongoing research on this population involves investigations of population genetic health. The Welch lab is in the process of scoring multilocus genotypes for over 500 adult and hatchling blood samples collected on Little Cayman in 2015 and 2016 to assess the degree of inbreeding depression and investigate whether genetic structuring is evident at any communal nesting site. As yet, there is no evidence for natal philopatry in a Cyclura species, however, analysis of genetic relatedness among nesters may provide some insight. Long-term monitoring of hatchlings marked during this three-year study should also directly address the question of natal philopatry. Future research efforts will include investigations of female dispersal and home territory distribution in the eastern side of the island, as well as patterns of natal dispersal and mortality rates. Continuation of mark-recapture and long-term monitoring efforts on the island are expected to provide much more insight into patterns of habitat use, turnover, and survivorship in the population.