Nature today | Putting Saban iguana on the map

As in St. Eustatius, the most immediate threats to saba appear to be the low growth of baby iguanas, the limited amount of suitable nesting sites and the presence of non-native iguanas.

The exact classification mode is under discussion

Saba iguana assemblages are part of a newly described species Iguana melanoderma† While the unique appearance of iguanas is indisputable, experts still debate their exact taxonomic status. Despite this debate, it is clear that protecting the population is urgent, given the number of threats the population faces. The main concern is the presence and possibility of a further influx of non-native iguanas from neighboring Sint Maarten.

Fortunately more iguanas than expected

A recent study estimated that the iguana population ranged between only two hundred and three hundred adults. This number is very alarming and would seriously threaten the long-term survival of the population. Because the data used in this preliminary estimate were only preliminary, a new fieldwork project was carried out in Saba in 2021. During this period, 38 CTs were monitored multiple times. This allowed the use of statistical methods to more accurately estimate the size of the population. An unexpected positive result is that the population is at least ten times larger than previously thought, and therefore concerns several thousand animals.

The higher the level of melanin?

It is usually thought that black (or melanic) reptiles have adapted their color to cold environments so that they can warm themselves more quickly; After all, they are cold-blooded. This can also apply to saba iguanas. There is the highest mountain in the Kingdom of the Netherlands, the volcano is called Mt. Mashhad (887 meters). This high volcano means that Saba is often in the clouds, making it difficult for reptiles to warm up. However, we found no relationship between the degree of melanism and the height at which the iguana occurs. So a mountain iguana is no darker than a lower elevation iguana.

Is there another explanation why the iguanas in Saba are black? Another hypothesis is that the original iguanas that settled in Saba were actually black. This is consistent with the occurrence of the partly black iguana in Venezuela, which appears to be closely related genetically to the saba iguana.

Few nesting sites and juvenile iguanas

Quality nesting sites are essential for successful breeding and maintenance of iguana populations. Goats are known to destroy iguana nesting sites through overgrazing, which leads to habitat degradation. Goats can also destroy the nest with their own steps. This causes the nest to collapse and the embryos to die.

A large number of goats (several thousand animals) in Saba severely affects the vegetation of the island and causes erosion. Therefore, during the research, the nesting sites were also searched for in order to assign availability, distribution and quality. Although not the main target of the study, the results are unfortunate, with only four nesting sites identified.

iguana events in saba

An additional concern is the low number of baby iguanas that have been found during fieldwork. Of the more than 600 people observed, only 2.4 percent were juveniles or younger. Although the exact cause of this is unknown, the number of feral cats is one possible cause. Previous research in Saba showed that cats live mainly in the lower parts of the island, where they can hunt better. This is also where iguanas prefer the nest. Iguana remains were also found in at least nine percent of the cat litter studied.

This makes the situation of the Saba iguana similar to that of the Antillean iguana in Saint Eustatius, where the presence of cats and goats and a lack of nesting sites are known. Research and management measures are urgently needed on both islands.

Continuing to worry

Although there is a larger population of iguanas in Saba, there are several reasons for concern. The need for better protection and continued collection of knowledge is clear. The main concerns for the survival of this species are the presence of stray cats and goats, the reduced number of iguanas, and the reduced availability of nesting sites. However, by far the greatest threat comes from the presence of non-native iguanas, which can mix with and suppress indigenous populations through competitive crossbreeding. During the field work, animals quite different from the local ones were seen and preliminary genetic research has already shown that these animals are not really indigenous. This situation forces the authorities to take immediate action to suppress the infestation of non-native iguanas.

more information

For more information on project implementation, for example as part of an internship, please contact dolfi.debrot@wur.nl and thijs.burg@gmail.com.

Read the full report here.

Text: Thijs van den Burgh and Dolphie Debrot
Photos: Thijs van den Burg (Main image: Adult iguana on a rock in Saba)

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