2015 Sister Isles Rock Iguana Report
Update on SIRI population status, actions taken, challenges faced
Report submitted by Jeanette Moss, Mississippi State University
Photos by Jeanette Moss unless otherwise noted.
The work conducted on Cayman Brac and Little Cayman this year, with the support of the International Iguana Foundation, represents a preliminary assessment of the current Sister Isles Rock Iguana (SIRI) population and its nesting activities. IIF funds were combined with a generous grant from the Mohamed bin Zayed Species Conservation Fund to support Jen Moss’s travel to Cayman Brac for a 10-day sampling trip in March 2015, as well as travel (Jen Moss, Matt Goetz, and Glenn Gerber) to Little Cayman over SIRI’s nesting season, from early May through end of June.
Since Matt Goetz’s first island-wide nesting surveys were conducted on Little Cayman from 2007-2010, the Cayman Islands National Trust has made great strides towards protecting their native reptiles. Thanks to massive fundraising success in the local communities, new protected areas have been established, iguana crossing signs have been distributed on all the roads to limit road-kills, and a new nature boardwalk has been opened at Preston Bay to encourage an appreciation for natural beach communities. At Mahogany Bay, a roadside site where tourists once flocked for the experience of feeding iguanas, harmful activities have been put to a stop and the resident iguanas have apparently receded back into their natural habitats. Further yet, Mike Vallee (Little Cayman dive master) has recently raised the funds to launch a “Green Iguana Be-Gonna” campaign, which will aim to control the spread of invasive green iguanas on the island by using drones to locate the animals in otherwise impassable swamps and dense forest.
Despite these efforts, preliminary observations of the Little Cayman iguana population are suspected to reflect a further decline since Goetz’s study. Large tracts of shrubland have been cleared for planned subdivisions, though the overgrown and deserted roads have yet to see any further development. Plans for a new airstrip that would supposedly have increased tourism to the island were thwarted by multiple sinkholes, but the land was left highly disturbed. Feral cat sightings (including at active nest sites, where one was even trapped unintentionally), and road-kills due to speeding cars (six were reported during our study period, of which one was a still-gravid female) are sadly still regular occurrences. Mating and nesting season (early spring through late summer) see particularly high road mortalities as animals are moving out of their home ranges (frequently crossing roads from the interior onto the coast) in search of mates and nesting areas. Rallying concern for these problems within the local community has been a constant struggle due to the disproportionately dense congregations of animals around areas of human activity. The old museum lot – where for years iguanas were fed on a daily basis – is a special case, supporting over 50 animals within its artificially small territory. Occurrences such as these unfortunately lead to gross misrepresentations of the true population status.
According to local accounts, noticeably fewer iguanas are seen roaming the island compared to five to ten years ago, when Goetz began his work. More concerning is the low representation of younger age classes among the animals sampled (approximately 20-30%), though the population’s true recruitment success remains to be investigated. The density of nesting iguana appears at first glance to be lower at revisited sites, and the habitat suitability of large communal nest sites (in particular, Preston Bay) appears reduced by excessive palm leaf litter and coconut debris. While coconut palms have been part of the natural plant communities since their Polynesian introduction hundreds of years ago, their high density within the beach communities is suspected to be an artifact of human colonization. Indeed, pervasive habitat degradation and disturbance to Caribbean ecosystems has the added negative side effect of allowing early-successional plants (such as Acacia and Love Vine) and non-natives like Australian Pine to rise to dominance, spelling out long-term problems for habitat restoration. Additionally disconcerting, is the expanding market for beach properties along the entire coastline, including lots adjacent to important protected areas like Preston Bay. It will be important to determine what kind of toll these developments have taken, and might take, on the long-term viability of the species.
On the other hand, new nest sites were identified this summer that may reflect a shifting of nesting activity correlated with changes (likely both natural and artificial) in suitability of island habitat. While Preston Bay remains the most densely utilized communal site on the island (23 nests), significant activity at other coastal sites such as the Airstrip site (9 nests) and the Northwest site (5 nests) where little activity had been previously recorded, suggest that more island-wide surveys may be necessary to identify important nesting areas. Further, two nests were identified at an interior shrubland site on Spot Bay Road, whose resident iguanas were previously thought to disperse to the coast to nest. This observation demands closer attention to small patches of suitable substrate in the island’s interior, as these areas may support more nesting activity than was originally presumed. The use of high-resolution aerial imagery and scouting drones, combined with radio-telemetry data from dispersing iguanas, may facilitate these findings.
Another take-away from the close daily monitoring of nest sites throughout this study has been to shed new light on the entirely individualistic behaviors of these animals. Investigations of disparate nesting strategies found that individual behavior is near impossible to predict based on site data alone, and that more thorough investigations will be needed to understand SIRI nesting strategies. Specifically, the number of test digs made, the re-purposing or sharing of existing digs and nests, the construction of unique entrance tunnels, and the time spent sealed underground and/or guarding are all poorly understood SIRI behaviors that warrant further study, perhaps by use of trail cameras and/or geological ultrasounds.
While very successful in terms of total numbers surveyed and sampled, this field season was not without its challenges. Fewer inland nest sites were identified than anticipated due to their insular and patchy distribution and the impenetrable nature of Little Cayman’s interior shrubland habitat. Next season’s efforts should incorporate conservation drone technology and high definition aerial imagery to identify patches of nesting substrate in the island’s interior.
Study sites were visited only once or twice daily to limit disturbances, and new activity was recorded. Ongoing observations of individual behavior were made possible only by capturing and marking individuals with unique bead combinations. Many captures were thwarted at active nest sites due to limited personnel and unpredictable behavior of nesting females. It was difficult to trace a particular female to her nest, since not all females stayed behind to guard after closing. Many animals were very skittish and evaded capture, so efforts were made to capture animals opportunistically wherever possible. Occasionally, females were still gravid when captured or had already dispersed from their nesting sites. Thus, logging the activity of individual nesters was limited to around 25 iguanas, and many data points (days sealed inside, days guarding, etc.) were incomplete.
Excavation efforts posed an additional challenge to this year’s study. The intention was to excavate all identified nests to investigate tunnel dimensions, locate egg chambers, and collect clutch data. However, many nests could not be fully excavated due to the length and depth of the entrance tunnels and the time commitment that would be required. Further, many excavations were postponed until the end of the study to limit disturbances to active communal sites, but by that time it was discovered that excessive rain and age had made tunnel identification impossible. In the end, 20 partial excavations were carried out and a total eight egg chambers were found.
- Cayman Brac (March 7–14):
- 24 iguanas processed (10 recaptures, 14 new; 4 <yearlings)
- 23 blood samples taken
- Little Cayman (May 3–June 30):
- 137 iguanas processed (28 recaptures, 109 new; 26 <yearlings)
- 128 blood samples taken
- 6 Road mortalities observed
- 74 nests identified (found closed between May 9 and June 29)
- 24 nests with known female IDs, 16 with precise guarding times collected
- 20 nests partially excavated
- 8 egg chambers found (5 with known female IDs) with clutch sizes between 3 and 20 and egg masses between 50 and 71 grams
Future Work Planned:
- Analyses of genetic data (2015 Ongoing)
- Hatchling success study (August 2015)
- Identification of more inland sites using conservation drones, HD aerial imagery (summer 2016)
- Dispersal studies (summer 2016)