The Isle of Misfit Species Ecology of the invasive Boa on St. Croix, U.S. Virgin Islands

Non-native species pose a threat to biodiversity across the globe. The majority of modern extinctions occur on islands, and competition and predation by invasive species is believed to be a leading cause. Non-native breeding populations of Boa spp. are present on the Caribbean Islands of Aruba, Cozumel, Puerto Rico, and St. Croix. While the extent of the ecological harm Boa cause in the Caribbean is largely unknown, there is a real fear that Boa may place stress on native, endemic species only found on these islands.

The goal of this study is to detail a more comprehensive ecological profile of the invasive Boa population on the island of St. Croix in the US Virgin Islands. The primary objective is to compare DNA analysis on fecal samples and stomach contents from captured snakes supplemented by reported prey data from a social media page dedicated to the St. Croix Boa. This will lead to a more comprehensive prey list. A second objective is to use yearly capture and location data to map Boa expanse on the island and identify patterns of habitat use. Both a prey list and estimated range map will provide additional ecological data on the St. Croix Boa population that may be used to guide future management and removal efforts.

Boa captures were collected from the Facebook group “St Croix Snakes” between December 2018 and June 2022. All Boa capture data that mentioned an observed prey interaction or prey recovered during dissection were catalogued. Additionally, we combined thirty-nine snakes we captured and received from St. Croix frozen for diet analysis in 2021. Once thawed, samples were collected from the fore, mid, and hind gut and preserved in ethanol. DNA was extracted using a modified phenol-chloroform ethanol extraction and replicated using a 12S RNA primer. The resulting sequences were matched to prey using BLAST. Of the 39 samples collected, only 19 were successfully amplified using PCR. Of the 19 successful amplifications, only 7 were v matched to non-Boa species. The prey comprised 14.29% birds, 28.57% mammals, and 57.14% reptiles. These proportions were compared to the prey data proportions from Aruba during the early invasion, peak encounters, and decline in captures phases. Chi-square analysis suggest that the prey consumption between Boa on St. Croix and Aruba was not significantly different at all stages of the Aruba invasion. Correspondence Analysis suggests that the St. Croix Boa population, at the time of this study, is most similar to the 2013-2015 decline in capture phase on Aruba, due to the similar proportions of reptile prey consumed. In addition, “St Croix Snakes” provided prey records not present in the DNA analysis, including two native bird species, Zenaida Dove (Zenaida aurita) and the Lesser Antillean Bullfinch (Loxigilla noctis). The combined list of identified prey includes 11 species.

Boa capture data from Google Earth were georeferenced into ArcPro, and the minimum bounding geometries tool was used to create seven estimated range maps from 2012 to 2022. Kernel densities were created at 50% and 10% confidence intervals and laid over a vegetation layer to estimate habitat preferences over time. For each of the seven years, the most prominent habitat cover is deciduous, followed by disturbed habitats across a range of 50% and 10% confidence intervals. Chi-square analysis suggests that each consecutive year is statistically different from the year prior at both confidence intervals. The results produce an average yearly range expansion of 17.76 km2 /year. However, it is important to note that the range expansion that occurred during 2016 increased the Boa range by a magnitude of 10, from 8.49 km2 in 2015 to 80.52 km2 .

Overall, this study established a preliminary prey base for Boa on St. Croix and documented their spread from 5.5 km2 to 147.59 km2 over 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, and 2022. This study furthers the current understanding of Boa on St. vi Croix, but there remains much to learn before the impacts of the population are fully understood or any meaningful elimination effort can be implemented. Further research should utilize other prey identification methods, such as the visual identification. Ground truthing and active field surveys should be conducted to enrich our current model for Boa range and habitat preference. More concerted efforts should be used to inform the public of findings as they become available, and a standardized way of reporting captures and sightings should also be implemented. The results of this study do not indicate the ecological disassembly that we see in other cases of invasions, but more groundwork must be conducted to measure the full scope of the effect of the invasive Boa on the island of St. Croix.

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