Saba Iguanas Receive Attention
The Caribbean Netherlands island of Saba is home to a unique but relatively unknown iguana population. Given the many threats faced by islands worldwide, including the Lesser Antilles and Saba, fieldwork was conducted in 2021 in order to bring attention to this unique species and the threats it faces. As on St. Eustatius, the most urgent threats appear to be low survival of hatchlings, limited nesting sites, and the presence of non-native iguanas.
Precise taxonomic status in discussion
Saba black iguana. Photo source: Thijs van den Burg
The iguana population on Saba forms part of a recently described species, Iguana melanoderma. Although there is no doubt about the unique appearance of the iguanas, experts are still deliberating over its exact taxonomic status. Despite this, it is clear that the population requires urgent protection given the number of threats it faces. The biggest concern is the presence of and possible further influx of non-native iguanas from nearby St. Maarten.
Thankfully more iguanas than originally thought
A recent study estimated the iguana population at just 200-300 individuals. This number is worrying and would be extremely concerning for the long-term survival of the population. However, because the data in the aforementioned study were preliminary, additional fieldwork was conducted on Saba in 2021. During this time, 38 transects were surveyed multiple times using distance sampling methodology, which allowed the population to be estimated more accurately. An unexpected positive result of this analysis is that the population is at least ten times larger than originally estimated, in the thousands rather than hundreds.
Saba black iguana. Photo source: Thijs van den Burg
More melanism at higher altitudes?
It is widely believed that black reptile species have evolved this color in order to be able to get warmer faster in colder climates, given that they are cold-blooded. This may also apply to the iguanas on Saba, which boasts the highest mountain in the Kingdom of the Netherlands: Mount Scenery (887m). This high volcano ensures that Saba is often cloudy, which could make it difficult for reptiles to get warm. However, we did not find any relationship between the extent of melanism and the elevation at which iguanas were found. In other words, iguanas that live higher up the volcano are not darker than iguanas at lower elevations.
Is there another explanation for why the iguanas on Saba are black? One hypothesis is that the original iguanas that established on Saba were already black. This is in line with the existence of partially black iguanas in Venezuela, which appear to be closely genetically related to the iguanas on Saba.
Few nest sites or juvenile iguanas
In order for successful reproduction and conservation of an iguana population, good quality nest sites are essential. It is well known that goats can destroy iguana nest sites through overgrazing, which leads to habitat destruction. Furthermore, goats can trample iguana nests and destroy the embryos inside eggs.
On Saba there is an enormous goat population (several thousands) which negatively impacts the island’s vegetation and causes erosion. During fieldwork we searched for iguana nest sites to evaluate their availability, distribution and quality. While this was not the main goal of the research, we were disappointed to find only four nest sites.
Juvenile iguana. Photo source: Thijs van den Burg
Another area of concern was the low number of juvenile iguanas found during fieldwork. Despite finding over 600 individuals, just 2.4% of these were juveniles or hatchlings. While the exact reason for this is unknown, one possible cause could be the feral cat population. Previous research on Saba demonstrated that feral cats exist in the lower altitudes of the island where they hunt for prey, which are exactly the same areas where iguanas prefer to nest. In fact, iguana remains were found in 9% of the cat scats examined.
Unfortunately, the situation for the melanistic iguana on Saba appears to be similar to that of the Lesser Antillean iguana on St. Eustatius, where the presence of cats and goats and an absence of suitable nest sites are well-known issues.
Remaining areas of concern
Despite the larger than expected iguana population on Saba, there are still many causes for concern. Clearly the species needs better protection as well as continued knowledge-building. Pressing concerns for the long-term existence of this species are the presence of free-roaming goats and feral cats, as well as the low number of juveniles and absence of suitable nest sites. However, by far the biggest threat is the presence of non-native iguanas which, though competitive hybridization, can mate with and thereby suppress the native population. During fieldwork, some individuals were found that looked different to the native iguana, and preliminary genetic research has confirmed that these animals were indeed invasive. This situation therefore requires the authorities to take immediate action in order to halt the invasion of non-native iguanas.
Published in BioNews 56.