A population assessment and habitat description of the Alsophis rufiventris on the Quill and Boven National Park, St. Eustatius

The genus Alsophis used to be very abundant in the Lesser Antilles. Numbers have declined ascribed to anthropogenic causes and the introduction of invasive species to their habitat. Most Alsophis species are classified as endangered or critically endangered by the IUCN. Alsophis rufiventris, a species native to the islands Saba and St. Eustatius, is facing a constant threat of the Javan mongoose being introduced in its habitat. Also dogs, cats, rats and humans actively influence the abundance of A. rufiventris. Though all these threats, the population of A. rufiventris seemed stable and was classified as vulnerable by the IUCN in 2016. In 2017 hurricane Irma and Maria impacted on Saba and St. Eustatius. It was expected that the A. rufiventris population was altered. Therefore a new population assessment was done in the Quill and Boven National Park on St. Eustatius in 2018 and was repeated in this study. In 2018 the population size in the study area was 165.

Distance sampling with line transects was used to estimate the population size of A. rufiventris. In this study 1068 line transect surveys resulted in 60 snakes recordings. As the snake count was sparse, N-mixture models were used as well.

Distance sampling resulted in a population size of 464 and the N-mixture models resulted in a population size of 178. These results are preliminary, as covariates had no influence on the model in the program DISTANCE. While, according to earlier research by Savit et al. in 2015 the covariates should influence the results. Multipliers have to be added in the program DISTANCE to get more accurate results. From this study the habitat preference of A. rufiventris cannot be described, as the findings on habitat categories were not significantly different from each other.

A. rufiventris is becoming more rare in its native habitat and that is a cause for concern. As many threats to the species are present, but also actively influence their abundance, the species might become, or effectively already can be categorized as (critically) endangered. It is advised to continue monitoring of this species not only on St. Eustatius but on Saba as well. Also conservation actions like establishing biosecurity and conducting rodent control might be beneficial. As these conservation actions might not happen in the near future, a breeding program in a zoo might be beneficial to preserve the genetic variance in the population.