Red-Bellied Racer

The Red-bellied Racer (Alsophis rufiventris) is a harmless snake species that is native to the St. Christopher and Saba Banks. Today, it unfortunately only occurs on the islands of Saba and St. Eustatius, which represent just 11 % of the species’ original range. Previously the snake was also found on St. Kitts and Nevis, but was driven to extinction by the mongoose (Herpestes javanicus) following its introduction in the early 1900s. Little is known about the racer, which makes it an important species to study. However, studying snakes in their natural environment can be challenging due to their camouflage, secretive nature and cryptic behaviour.

On Saba and St. Eustatius, the racer plays a key role in the islands’ ecosystems by regulating small reptile and amphibian populations. Anoles, especially the endemic and highly abundant Anolis sabanus on Saba, and whistling frogs (Eleutherodactylus johnstonei) form an important food source for racers. 

Red-bellied Racer on Saba (Photo: L. Mielke)

Despite the presence of invasive species such as black rats (Rattus rattus) and domestic cats (Felis catus), racer populations on Saba and St. Eustatius were previously described as “robust” and “abundant”, and were considered stable. For that reason, the species was downgraded from Endangered to Vulnerable by the International Union for the Conservation of Nature in 2016. However, in September 2017, category 5 hurricanes Irma and Maria caused extensive damage to the natural ecosystems on both islands, resulting in habitat loss, reduced prey abundance and increased racer mortality, which raised concerns for the conservation status of this species.  

In 2011, a group of students from the University of Puerto Rico visited St. Eustatius for one week, where they conducted some racer surveys in the Quill and Boven National Parks. In 2018 and 2019, RAVON interns Kevin Verdel and Brent Kaboord conducted extensive surveys in the parks to monitor racer populations following hurricane impacts. Until this year, however, no quantitative data had been collected on Saba. Between July and September 2021, Hannah Madden from the Caribbean Netherlands Science Institute and volunteer Lara Mielke joined forces to conduct racer surveys on Saba. Replicating the methods used on St. Eustatius allowed us to compare the results from both islands and create a baseline for future research and conservation efforts.  

Important food source for racers on Saba – the endemic Anolis sabanus (Photo: L. Mielke).

In cooperation with the Saba Conservation Foundation, Lara Mielke conducted line transect surveys along existing hiking trails, covering six different vegetation types and a range of elevations, from 100m to the top of Mount Scenery (870m). Occupancy modelling revealed that racers were present in all vegetation types and were more likely to be present at elevations over 400m. Distance analysis revealed a density estimate of 10.9 racers/hectare on Saba, which across the entire survey region (438.6 ha) gives an estimate of 4,917 racers. These results are similar to those from St. Eustatius, where racers were present in all vegetation types surveyed and mean abundance increased in line with elevation. However, density estimates from St. Eustatius were lower (9.9/ha in 2018 and 7.3 in 2019), with a population estimate of 3,915 racers across the study region (540 ha) in 2019. This suggests that the racer population on Saba is healthier than that on St. Eustatius, but this may be due to the fact that surveys on Saba were conducted later, giving the population more time to recover.  

Red-bellied Racer along the Sandy-Cruz Trail on Saba (Photo: L. Mielke).

Unfortunately, few detailed studies exist on the ideal pre-hurricane density of A. rufiventris, however a 2016 study from Guana Island, British Virgin Islands, on the Puerto Rican Racer estimated 19/ha. While racer densities are likely to be species- and habitat-specific, based on literature describing pre-hurricane populations on Saba and St. Eustatius as “thriving”, it is highly likely that these suffered significant declines as a result of the hurricanes. The encounter rate of racers from St. Eustatius based on fieldwork in 2011 was estimated at 16/hour, which dropped to just 0.41/hr in 2019. On Saba, the current encounter rate estimate is 1.28/hr, thus while the species may have recovered since 2017, it is unlikely to be close to pre-hurricane levels.  

Besides hurricanes, racer populations face a suite of additional threats on both islands, including ongoing habitat destruction and predation by invasive species. We are extremely concerned about the impacts of free-roaming domestic cats on racers, which appear to be especially pervasive on Saba. Saba residents continue to import pet cats from St. Maarten, despite this being prohibited by a local invasive species ordinance. Consequently, enforcement of legislation and prevention of local extinction should be a top conservation priority on both islands, especially given the species’ extremely limited range.  

Domestic cat preying on a Red-bellied Racer on Saba (Photo: B. Noort).

Article published in BioNews 49

Sint Eustatius was hit by two major hurricanes, Irma and Maria, during the high-intensity season of 2017. Two new studies aid our understanding of how local populations deal with these destructive events. 

Caribbean flora and fauna have always dealt with the destructive forces of hurricanes. However, as climate change is leading to an increase in their frequency and strength and many species have decreased in abundance due to anthropogenic causes, a better understanding of how hurricanes effect local populations is essential. 

The Caribbean hurricane season of 2017 was the most intense recorded to date. Both Irma and Maria, category-5 hurricanes, closely passed Sint Eustatius and caused major destruction on the island; reported on in this Nature Today article. Although immediately after the storms it became clear that trees were heavily affected and mostly defoliated, understanding how and which species were affected requires time for data collection and comparison. Since 2017, several studies have provided pieces of information in order to understand how local populations coped, or not, with the 2017 hurricane season. 

Since 2017, researchers have found that >90% of all trees were defoliated by more than ¾, and that especially trees at higher elevations (such as on the Quill volcano) were affected more severely. Another study that focused on the endangered Bridled Quail-dove (Geotrygon mystacea), demonstrated that the population declined by 77% in 2019 compared to pre-hurricane levels. A follow-up study in 2021 (not yet published) recorded a further decline to just 125 individuals, and the Bridled Quail-dove will likely be re-assessed by the IUCN. 

Focusing on reptile species, two novel studies further aid our understanding of the ecosystem-wide impact that the 2017-hurricane season had on Statian biodiversity: 

Red-bellied racer. Photo credit: Henkjan Kievit

The first study demonstrates that the Red-bellied Racer (Alsophis rufiventris), indicated as Vulnerable by the IUCN, a harmless snake species now found only on St. Eustatius and Saba, was dramatically affected by the hurricanes. Compared to pre-hurricane density estimates (9.2 racers per hectare), post-hurricane estimates decreased by almost 50%. Encounter rates of individual racers have dropped from 16.0 snakes/hour in 2011 to less than 0.5 snakes/hour (post-hurricane). Given the current small size of the remaining population and the fact that the species’ current range is just 11% of its original extent, local extirpation is a real risk. 

Lesser Antillean Iguana. Photo credit. Philippa King

The second study, focusing on the Critically Endangered (Iguana delicatissima), shows that its population decreased by at least 20% during 2017. Comparing sighting and survey data from 2017–2018, the authors found a decrease in both the abundance and population size of the iguana species. Importantly, no recovery was observed in 2019, suggesting this small population needs multiple consecutive years without major hurricanes to recover. Interestingly, similar to Statia’s forests, iguanas at higher elevations were found to have been affected more severely.  

Small islands such as Sint Eustatius are home to declining populations of rare and endangered species. In many cases, these isolated populations are unable to migrate between islands and thus populations can only locally increase in size. These new studies highlight the need to improve habitat quality and lower anthropogenic threats to optimize the natural recovery of both species. Ideally, at least for Iguana delicatissima, population increase could be aided by a local head-starting project where baby iguanas are nourished and released once they are larger and more likely to survive.  

Recently published articles: 

Find me if you can: Pre- and Post-hurricane Densities of the Red-bellied Racer (Alsophis rufiventris) on St. Eustatius, and a review of the genus in the Caribbean. Read Report

Hurricane-induced population decrease in a Critically Endangered long-lived reptile Read Report

Article published in BioNews 46

We estimated occupancy, abundance (lambda), detection probability, density/ha and abundance of a regionally endemic snake in the Colubrid family on the Dutch Caribbean island of Saba in 2021, four years after hurricanes Irma and Maria impacted the island. Line transect surveys were conducted at 74 sites covering 6.7 ha. The proportion of sites occupied was estimated at 0.74 (min 0.48, max 0.90), with occupancy varying between vegetation types and across elevational gradients. Similarly, lambda was estimated at 1.61 (min 0.7, max 3.7) but varied between vegetation types and elevational gradients. Detection probability was estimated at 0.15 (min 0.10, max 0.21). Using Distance sampling, we estimated 10.9 (min 7.3, max 16.2) racers/ha, with a total population estimate of 4,917 (min 2,577, max 6,362) across the entire study region (438.6 ha.) Based on anecdotal observations from Saban residents and prior literature describing the pre-hurricane population as “abundant” (at least 2.0 racers/hour), we posit that the population experienced a hurricane-induced decline but may have since recovered, though not to previous levels (1.28 racers/hour). Nevertheless, our results suggest that racer densities on Saba are currently higher than those on St. Eustatius. Despite this, given the species’ extremely limited extant range and the presence of invasive species on both islands, prevention of local extirpation should be a high conservation priority.

Abstract

We estimated population densities of the red-bellied racer (Alsophis rufiventris) on the Caribbean island of St. Eustatius in 2011, 2018 and 2019 to determine the likely influence of hurricanes Irma and Maria (September 2017), in addition to evaluating abiotic parameters which may be correlated with its presence. Surveys were conducted at seven sites in 2011 prior to the hurricanes, and at 81 and 108 sites in 2018 and 2019 respectively posterior to the hurricanes. A total of 8.2 ha was surveyed in 2011, and 11.42 ha in 2018/2019. The pre-hurricane (2011) racer density estimate was 9.2/ha (min 7.3 - max 11.6); post-hurricane estimates were 4.6/ha (min 3.4 - max 6.0) in 2018 and 5.0/ha (min 3.8 - max 6.5) in 2019. The pre-hurricane encounter rate of individual racers was 16.0 snakes/hour compared to 0.34 snakes/hour in 2018 and 0.41 snakes/hour in 2019 (post-hurricane). The decrease in encounter rates between 2011 and 2019 implies a negative impact of the hurricanes on racer abundance. Based on calculations of detection probability (0.02 in 2018 and 0.03 in 2019), post-hurricane lambda estimates were 1.82 (95% CI 0.66 - 5.01) in 2018 and 1.60 (95% CI 0.39 - 6.65) snakes/ha in 2019. Given the current small size of the remaining population and the presence of invasive species across the snake’s range, this species could be at risk of local extirpation. We suggest conservation actions such as invasive species management and habitat restoration to enable further recovery.

In 2018 and 2019, RAVON/CNSI interns conducted a study on St. Eustatius to better understand the red-bellied racer population. Threatened by habitat loss, invasive species and hurricane
damage, little is known about this snake, which now only inhabits less than 11% of its historical range. This study will help shape the conservation efforts for this species going forward.

This article was published in BioNews29

More Information: Kaboord, B. 2019. A population assessment and habitat description of the Alsophis rufiventris on the Quill and Boven National Park, St. Eustatius. Research Report.