Boa constrictor

Abstract

Giant constrictor populations in Florida and the Caribbean have top-down impacts on prey that are still poorly understood. We report opportunistically documented predation events by introduced Boa imperator (Central American Boa Constrictor) on four species of native birds on St. Croix, U. S. Virgin Islands, compare these to more detailed dietary studies from Aruba, and encourage detailed studies of Boa imperator diet on St. Croix to better understand population and community-level impacts of predation by this invasive snake.

https://bioone.org/journals/caribbean-journal-of-science/volume-53/issue...

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.

Dutch below

Likely introduced via the exotic pet industry, boa constrictors have had an island-wide population on Aruba since 2005. The impact of boa constrictors on Aruba has led to the implementation of control measures, including removal of snakes from the wild, but their continued presence highlights the need for continues eradication methods to avoid further unbalancing the island’s delicate ecosystems. 

(Photo credit: Vandy Louw)

The boa constrictor is a non-venomous snake species that is native to South America and the Caribbean islands of Trinidad, Tobago and Isla Margarita. Due to their popularity as exotic pets, these snakes have been introduced to other parts of the world, including Aruba. The first boa constrictor was found on Aruba in 1999, and despite efforts to curb their expansion, an island wide population was established by 2005. Between 1999 and 2016, over 4,520 constrictors were captured and removed from the island, however even with these measures, the local boa population has continued to thrive. These snakes can reach lengths over 4m (14 ft) long and can weigh up to 27 kg (60lbs). Although their coloration can vary, they are typically brown, gray and cream patterned, helping them camouflage within tree canopies. Boas rarely interact with humans but have been known to strike when they are threatened, and although not deadly, their bite can be very painful. 

Generalist Diet 

(Aruban burrowing owl (Athene cunicularia arubensis). Photo credit: Christian König)

Boa constrictors are apex predators, meaning they are at the top of the food chain in their ecosystems. In their native habitats, they play an important role in regulating populations of rodents and other small mammals. However, as an invasive species, their generalist diet means they could be a potential concern for a wide variety of native populations.  A 2021 study investigated the stomach contents of over 500 captured boas from Aruba.  Here, researchers identified over 400 different types of prey, with a nearly even split between mammals, lizards and birds.  In fact, with the exception of the seven different bat species known to inhabit Aruba, almost every other type of vertebrate was observed within the stomach contents evaluated. 

Rapid reproduction 

Boas are of significant concern because they mature quickly (within three years), have a long lifespan (40 years) and produce large litters of offspring (50+) every two years.  Left unchecked, this species is able to rapidly reproduce and outcompete native species. Of particular concern is the impact of these snakes on declining native bird populations including the burrowing owl (Athene cunicularia), Aruban parakeet (Eupsittula pertinax), and crested bobwhite (Colinus cristatus).  In fact, one boa dissected had four burrowing owls in its stomach, which when you consider the native population is estimated at around 200 pairs, is significant.  

Island wide Impact  

(Learning about the invasive boa constrictor. Photo credit: Suzanne Hendrik)

Aruba has already tried a wide variety of control strategies.  Organized bounty programs were found to be ineffective, as boas can be very difficult to track for inexperienced hunters. There was also a pilot effort to lure snakes into traps using live birds and chicken broth, however there was limited success given the boa’s preference for ambush predatory behavior. Other methods which have worked in other places, since as the temporary release of mongoose or the intentional introduction of a targeted disease have been dismissed as ecologically irresponsible. 

Overall, the impact of boa constrictors on the island of Aruba has been significant, highlighting the potential consequences of introducing non-native species to new environments. While these snakes may seem harmless in their natural habitats, their introduction to new ecosystems can have far-reaching and unintended consequences.  This is especially true for small islands already facing unsustainable threats from rapid urban development and climate change. 

DCNA 

The Dutch Caribbean Nature Alliance (DCNA) supports science communication and outreach in the Dutch Caribbean region by making nature related scientific information more widely available through amongst others the Dutch Caribbean Biodiversity Database, DCNA’s news platform BioNews and through the press. This article is part of a series of articles on ‘Invasive Alien Species in the Dutch Caribbean”. This article contains the results from several scientific studies but the studies themselves are not DCNA studies. No rights can be derived from the content. DCNA is not liable for the content and the in(direct) impacts resulting from publishing this article. 

Waarschijnlijk geïntroduceerd via de exotische huisdierenindustrie, hebben boa constrictors sinds 2005 een eilandwijde populatie op Aruba. De impact van boa constrictors op Aruba heeft geleid tot de implementatie van controlemaatregelen, waaronder het verwijderen van slangen uit het wild, maar hun voortdurende aanwezigheid benadrukt de noodzaak van voortdurende uitroeiingsmethoden om te voorkomen dat de kwetsbare ecosystemen van het eiland verder uit balans taken.

 (Boa constrictor. Foto: Vandy Louw)

De boa constrictor is een niet-giftige slangensoort die inheems is in Zuid-Amerika en de Caribische eilanden Trinidad, Tobago en Isla Margarita. Vanwege hun populariteit als exotische huisdieren zijn deze slangen geïntroduceerd in andere delen van de wereld, waaronder Aruba. De eerste boa constrictor werd in 1999 op Aruba gevonden en ondanks pogingen om hun uitbreiding te beteugelen, was er in 2005 een eilandwijde populatie gevestigd. Tussen 1999 en 2016 werden meer dan 4.520 constrictors gevangen en van het eiland verwijderd, maar zelfs met deze maatregelen is de lokale boapopulatie blijven bloeien. Deze slangen kunnen lengtes bereiken van meer dan 4m en kunnen tot 27kg wegen. Hoewel de kleur kan variëren, hebben ze meestal een bruin, grijs en crème patroon, waardoor ze zich beter kunnen camoufleren in boomtoppen. Boa’s hebben zelden interactie met mensen, maar het is bekend dat ze toeslaan wanneer ze worden bedreigd, en hoewel ze niet dodelijk zijn, kan hun beet erg pijnlijk zijn.

Generalistisch dieet

 (Arubaanse holenuil (Athene cunicularia arubensis). Foto: Christian König)

Boa constrictors zijn toproofdieren, wat betekent dat ze aan de top van de voedselketen in hun ecosystemen staan. In hun oorspronkelijke leefomgeving spelen ze een belangrijke rol bij het reguleren van populaties van knaagdieren en andere kleine zoogdieren. Als een invasieve soort betekent hun generalistische dieet echter dat ze een potentieel gevaar kunnen zijn voor een grote verscheidenheid aan inheemse populaties. Een studie uit 2021 onderzocht de maaginhoud van meer dan 500 gevangen boa’s op Aruba. Hier identificeerden onderzoekers meer dan 400 verschillende soorten prooi, met een bijna gelijke verdeling tussen zoogdieren, hagedissen en vogels. In feite, met uitzondering van de zeven verschillende vleermuissoorten waarvan bekend is dat ze Aruba bewonen, werd bijna elk ander type gewervelde waargenomen in de beoordeelde maaginhoud.

Snelle voortplanting

Boa’s zijn een grote zorg omdat ze snel volwassen worden (binnen drie jaar), een lange levensduur hebben (40 jaar) en elke twee jaar grote nesten nakomelingen produceren (50+). Als dit niet wordt tegen gehouden, kan deze soort zich snel voortplanten en inheemse soorten overtreffen. Van bijzonder belang is de impact van deze slangen op afnemende inheemse vogelpopulaties, waaronder de Arubaanse holenuil (Athene cunicularia), maïsparkiet (Eupsittula pertinax) en kuifbobwhite (Colinus cristatus). In feite had één ontlede boa vier holenuilen in zijn maag, wat aanzienlijk is als je bedenkt dat de inheemse bevolking wordt geschat op ongeveer 200 paren.

Eilandwijde impact

Leren over de invasieve boa constrictor. Foto: Suzanne Hendrik

Aruba heeft al een breed scala aan controlestrategieën uitgeprobeerd. Georganiseerde premieprogramma’s bleken niet effectief te zijn, aangezien boa’s erg moeilijk te volgen kunnen zijn voor onervaren jagers. Er was ook een proefpoging om slangen in vallen te lokken met behulp van levende vogels en kippenbouillon, maar er was beperkt succes gezien de voorkeur van de boa voor roofzuchtig gedrag in een hinderlaag. Methoden die op andere plaatsen hebben gewerkt, zoals de tijdelijke vrijlating van mangoest of de opzettelijke introductie van een gerichte ziekte, zijn afgedaan als ecologisch onverantwoord.

Over het algemeen is de impact van boa constrictors op het eiland Aruba aanzienlijk geweest, wat de mogelijke gevolgen benadrukt van het introduceren van niet-inheemse soorten in nieuwe omgevingen. Hoewel deze slangen in hun natuurlijke leefomgeving misschien onschuldig lijken, kan hun introductie in nieuwe ecosystemen verstrekkende en onbedoelde gevolgen hebben. Dit geldt met name voor kleine eilanden die al te maken hebben met onhoudbare bedreigingen door snelle stedelijke ontwikkeling en klimaatverandering.

DCNA

De Dutch Caribbean Nature Alliance (DCNA) ondersteunt wetenschapscommunicatie en outreach in de Nederlandse Caribische regio door natuurgerelateerde wetenschappelijke informatie breder beschikbaar te maken via onder andere de Dutch Caribbean Biodiversity Database, DCNA’s nieuwsplatform BioNews en via de pers. Dit artikel maakt deel uit van een serie artikelen over ‘Invasive Alien Species in the Dutch Caribbean’. Dit artikel bevat de resultaten van verschillende wetenschappelijke onderzoeken, maar de onderzoeken zelf zijn geen DCNA-onderzoeken. Aan de inhoud kunnen geen rechten worden ontleend. DCNA is niet aansprakelijk voor de inhoud en de indirecte gevolgen die voortvloeien uit het publiceren van dit artikel.

Published in BioNews 63

Curaçao and Bonaire form part of the Netherlands Antilles, while Aruba has a “status aparte” within the Kingdom of the Netherlands. All three islands are relatively arid compared to a typical Caribbean island, with mean annual rainfall of 409-553 mm, and experience several periods of drought lasting two or more years each century. A short history of the islands is given, and protected areas are described. The laws and regulations protecting amphibians and reptiles are complex, with general laws originating from the Kingdom of the Netherlands participation in international conventions (such as CITES) together with supplemental laws of the Netherlands Antilles and individual islands. Sea turtles are generally well protected, although their nesting beaches would be vulnerable to a rise in sea level. Among the terrestrial herpetofauna, only the Aruba Island rattlesnake (Crotalus unicolor) is on the IUCN Red List, being Critically Endangered. The status of this species and others of particular interest is described. The Curaçao Island snake (Liophis triscalis) should probably be included as Vulnerable or even Endangered, though there is insufficient information at present. Iguana iguana populations on the different islands, and the Curaçao whiptail (Cnemidophorus murinus murinus) on Klein Curaçao, are distinctive and significant for conservation. An overview is given of introduced amphibians and reptiles and their possible effects on the native fauna. The arid climate of the islands may hinder the establishment of invasive species, which are often not able to survive in the bush and thus reduces their impact on native species.

Boa constrictor was first documented on the Caribbean island of Aruba in 1999. Despite intensive efforts to eradicate the snake from the island, B. constrictor has established a stable, reproductively successful population on Aruba. We generated mitochondrial sequence and multilocus microsatellite data for individuals from this population to characterize the origins and means of introduction to the island. Phylogenetic analyses and measures of genetic diversity for this population were compared with those for invasive B. constrictor imperator from Cozumel and B. constrictor constrictor from Puerto Rico. Cozumel populations of B. c. imperator had significantly higher number of alleles and significantly higher values for FIS than the Puerto Rico and Aruba populations. Observed, expected, and Nei's unbiased heterozygosities, as well as effective number of alleles, were not significantly different. The effective population sizes from Aruba and Puerto Rico were generally lower than those for either of the Cozumel populations; however, there were broad confidence intervals associated with published estimates. We conclude that the present B. constrictor population on Aruba probably was not established from the introduction of a single gravid or parthenogenic female but instead most likely resulted from the release or escape of a small number of unrelated captive snakes. This study adds to the growing body of evidence suggesting the ease with which a small number of relatively slow-maturing B. constrictor can quickly invade, become established, and avoid eradication efforts in a new location with suitable habitat.