invasive species control

The Saba black iguana, Iguana (guana/melanoderma) is a Lesser Antillean endemic variety of the green iguana that is (likely) only found on Saba and Montserrat. The species is the largest native land vertebrate of Saba. Hybridization with the introduced Green Iguana is now an imminent threat to this unique black island variety of iguana.

During fieldwork in 2021 non-native Green Iguanas were seen, captured, culled and genetically confirmed for Saba. If more animals remain on the island or have hybridized with the native Saban black iguana, there is an imminent threat that the latter will be displaced and disappear. A “Rapid Resonse Removal Campaign” is urgently needed to quickly and hopefully completely remove the invasive Green iguana. This Kennisdeskvraag therefore is aimed to swiftly design and rigorously execute such a campaign in close cooperation with local stakeholders. The project will be modelled after a similar successfully-executed campaign for Statia (Debrot et al. 2022).

As the number of Green iguanas is likely still very small, a Rapid Response Action is feasible and called for. The Ministry Agriculture Nature and Food Quality of the Netherlands carries ultimate responsibility for endangered species in the Caribbean Netherlands. In light of the local lack of capacity and expertise the ministry decided to commission the execution of a removal campaign by WMR. 

Together with local and Dutch expert iguana spotters two thorough surveys will be conducted this year in key risk areas to eliminate all iguanas that have Green Iguana or hybrid characteristics. The project will provide practical experience and lessons learnt that will be of great value to the further implementation of the Invasive Alien Species Joint Action Plan as previously also commissioned by the same ministry.

Culling can be an effective management tool for reducing populations of invasive species to levels that minimize ecological effects. However, culling is labour-intensive, costly, and may have unintended ecological consequences. In the Caribbean, culling is widely used to control invasive Indo-Pacific lionfish, Pterois volitans and P. miles, but the effectiveness of infrequent culling in terms of reducing lionfish abundance and halting native prey decline is unclear. In a 21-month-long field experiment on natural reefs, we found that culling effectiveness changed after the passage of a hurricane part-way through the experiment. Before the hurricane, infrequent culling resulted in substantial reductions in lionfish density (60–79%, on average, albeit with large uncertainty) and slight increases in native prey species richness, but was insufficient to stem the decline in native prey biomass. Culling every 3 months (i.e., quarterly) and every 6 months (i.e., biannually) had similar effects on lionfish density and native prey fishes because of high rates of lionfish colonization among reefs. After the hurricane, lionfish densities were greater on all culled reefs compared to non-culled reefs, and prey biomass declined by 92%, and species richness by 71%, on biannually culled reefs. The two culling frequencies we examined therefore seem to offer a poor trade-off between the demonstrated conservation gains that can be achieved with frequent culling and the economy of time and money realized by infrequent culling. Moreover, stochastic events such as hurricanes can drastically limit the effectiveness of culling efforts.