Endemic rock iguanas under threat

Little Cayman’s indigenous rock iguana (Cyclura nubila caymanensis) population has declined by more than half within the past five years, data compiled by Department of Environment (DoE) scientists show.

The rock iguana population was surveyed in late 2019 using a distance sampling method developed in conjunction with the DoE by the U.S. Fish and Wildlife Service’s Dr. Frank Rivera-Milan. The survey data put the estimate of the mean number of rock iguanas at 1,786 (averaged between a low of 1,474 and a high of 2,098)

That’s a 39 percent decline when compared to the most recent Little Cayman rock iguana population survey in 2015, which showed an estimated population of 2,915 rock iguanas (averaged between a low of 2,339 and high of 3,491). It represents a 54 per cent decline from 2014, when the estimated population was surveyed at 3,847 rock iguanas.

“These are deeply worrying losses for the Sister Isles Rock Iguana population on Little Cayman,” said Fred Burton, manager of the DoE’s Terrestrial Resources Unit. “The causes of the decline are already known: the rock iguanas are being struck by vehicles and their young are falling prey to feral cats. The rock iguana population now consists mainly of mature adults, with the young age classes very scarce due to the unnatural and extreme predation pressure.

“Road kill and feral cats are challenging issues to address, but there are solutions, and it makes much more sense to address these now than to wait until the rock iguanas are on the brink of extinction requiring a multi-million dollar rescue operation like we had to do with the blue iguanas on Grand Cayman.”

The DoE has long advised Little Cayman and Cayman Brac motorists to drive more slowly and cautiously, particularly during the spring and summer rock iguana breeding season when the large reptiles tend to venture into the roads more often. As Little Cayman’s road surfaces are improved, driving speeds have increased and roadkill rates are far beyond what the rock iguana population can sustain.

Feral cat predation on endemic species has been a long-term, serious concern throughout the Cayman Islands, affecting not only the indigenous rock iguanas, but also nesting seabirds such as the brown and red-footed boobies. Feral cat predation on native species is not a Cayman-specific problem. Feral cats have been named among the world’s 100 worst invasive species by the Invasive Species Specialist Group and, according to a 2011 study, can claim 14 per cent of all modern-era bird, amphibian and mammal extinctions in island countries. (*)

Unresolved legal action has hamstrung both the Ministry of Environment’s and the Ministry of Agriculture’s efforts to resolve the problem.

“As Minister for Environment, I urge Sister Islands motorists to take it easy on local roads – we’re losing at least 40 rock iguanas to vehicles a year on Little Cayman alone,” said Environment Minister, the Hon. Dwayne Seymour. “It’s also clear that something has got to be done about feral cats. These small islands are just losing too many rock iguanas and seabirds each year to sustain those important populations.”

DoE scientists are planning the department’s first distance sampling survey of the Cayman Brac rock iguana population later this year. The same key threats are prevalent on all three of our islands.

For more information about the Sister Islands Rock Iguana study, please contact DoE’s Terrestrial Resources Unit manager Fred Burton, at Frederic.Burton@gov.ky or on 244-5995.

(*) Source: https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2011.02464.x