Hannah Madden

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

Two years ago we shared the worrying news that the Bridled Quail-dove population had suffered a significant decline on St. Eustatius (Statia) following hurricanes Irma and Maria in 2017. Almost four years on, has the situation improved? It is time for an update. 

Bridled Quail-dove in the Quill National Park (photo by Hannah Madden)

The Bridled Quail-dove (Geotrygon mystacea) is a shy, ground-dwelling species that is endemic to the Caribbean. It spends its day foraging the forest understory for fruits, seeds and the occasional gecko. Quail-doves prefer undisturbed forests with a closed canopy, and are sensitive to changes in their natural habitat. Unfortunately, the impacts of human-induced climate change mean hurricanes are becoming more intense and are occurring more often, which could spell trouble for this vulnerable species.  

I began surveying Bridled Quail-doves in the Quill National Park in 2016. The Quill is a dormant volcano that rises to 600 meters with a large, accessible crater. It is the only habitat on Statia where the species occurs. Pre-hurricane, quail-doves were relatively common and observant birders could almost certainly spot one or two while hiking along the trails. The species breeds in May, when its mournful and unmistakable ‘whooo’ can be heard echoing through the forest.  

Overgrazed understory in the Quill National Park, St. Eustatius (photo by Hannah Madden)

Hannah Madden during fieldwork in May 2021. Some 100 meter transects took up to 20 minutes to complete due to the steep and challenging environment (photo by Oliver Jones)

In September 2017 the forest habitat of the Quill suffered extensive damage from hurricanes Irma and Maria. Trees were stripped of leaves and fruits, branches were broken, and some trees did not survive. The effects of the hurricanes are still evident in some parts of the forest. In addition, we have a goat problem in the park; a very serious goat problem that has been pervasive for decades. Non-native, free-ranging goats overgraze already degraded habitats, resulting in a limited food supply and reduced understory cover for the quail-dove (as well as other forest-dependent species). Feral chickens disturb ground cover and compete for the same food source. Nest predators (invasive black rats and feral cats, both of which are present in the Quill) prey on eggs and chicks. This leads to lower reproduction and/or survival for the Bridled Quail-dove. There is no evidence of immigration of adults from nearby islands, which means populations are physically and genetically isolated. Based on the above we became concerned for the welfare of the Bridled Quail-dove and, thanks to support from many generous donors, BirdsCaribbean was able to provide funds to conduct post-hurricane surveys.  

I conducted surveys of the Bridled Quail-dove in the Quill National Park. This involved walking previously established transects within the dove’s range (~150 to 600 m). We surveyed during peak breeding season (May) to allow for audio as well as visual detections. Once I saw or heard a quail-dove, I measured the distance of the bird from the center of the transect. I also recorded elevation and canopy height to assess their influence on Bridled Quail-dove presence. Once surveys were complete, I pooled all data from 2016 – 2021 to obtain abundance and density estimates per year. 

Results 

As shown in the graph below, effort has increased from 1,200 m of transects in 2016 to over 15,000 m in 2021. This means survey effort has increased 13-fold to detect around one third of the number of doves that were counted in 2016. As you can imagine, repeating so many surveys is physically demanding. 

Number of detections (left axis; red line) and survey effort (right axis; blue line) between 2016 and 2021 (no surveys were conducted in 2020).

Unfortunately, abundance estimates of the Bridled Quail-dove on St. Eustatius have declined significantly since 2016, and the current estimate is just 123 individuals (min 72 – max 210). This is less than 50% of the 2019 estimate, and less than 5% of the 2016 estimate.  

After assessing the influence of covariates on Bridled Quail-dove presence, I found that doves were more likely to be present at higher elevations in habitats with a higher canopy (such as inside the crater, along the rim, and on the upper outer slopes of the Quill). In contrast, I found a strong negative effect of year on dove presence. This means that in the years following hurricanes Irma and Maria, doves were less and less likely to be present in the survey area.  

What now for the Bridled Quail-dove on Statia? 

We are extremely concerned about the Bridled Quail-dove on St. Eustatius. The combination of habitat degradation and predation by invasive species is likely responsible for the continued decline of this species. Its long-term survival is now uncertain unless urgent conservation actions are implemented. Even if the dove does persist, such a small and isolated population faces additional risks such as inbreeding. Since hurricanes are likely to increase in frequency and intensity, this could be catastrophic for the Statia population, and others in the region. Recently the International Union for the Conservation of Nature proposed that the Bridled Quail-dove be reassessed based on our work on Statia. This means the classification could be changed from Least Concern to Near Threatened or Vulnerable, but data are still lacking from many islands. We encourage enthusiastic birders to conduct their own surveys so that local populations can be assessed.  

Passive acoustic monitoring device (source: www.wwf.org.uk)

We will be working with local conservation NGO St. Eustatius National Parks (STENAPA) to create an Action Plan for the Bridled Quail-dove. To effectively protect the species locally, it is likely that multiple conservation actions will be required: goat and cat removal, rodent control, and feral chicken removal from the park. In order to achieve this we need the support of the local government and the community. Because of the tremendous effort now required to monitor Bridled Quail-doves on Statia, we also suggest trialing passive acoustic monitoring (PAM) devices during the breeding season. This involves the placement of recording units that can be placed in the field for up to a month to record and interpret calls. Using these devices will allow us to collect data in less accessible areas, as well as increasing the scale of our study. Hopefully by combining conservation efforts with field surveys and PAM we will have better news in the coming years.  

Hannah Madden is a terrestrial ecologist with the Caribbean Netherlands Science Institute, based on St. Eustatius. She is also a member of the IUCN Pigeon and Dove Specialist Group.  

Article published in BioNews 47

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

Abstract.– Intraspecific diversity is among the most important biological variables, although still poorly understood for most species. Iguana iguana is a Neotropical lizard known from Central and South America, including from numerous Caribbean islands. Despite the presence of native melanistic I. iguana populations in the Lesser Antilles, these have received surprisingly little research attention. Here we assessed population size, distribution, degree of melanism, and additional morphological and natural history characteristics for the melanistic iguanas of Saba, Caribbean Netherlands based on a one-month fieldwork visit. Using Distance sampling from a 38- transect dataset we estimate the population size at 8233 ±2205 iguanas. Iguanas mainly occurred on the southern and eastern sides of the island, between 180-390 m (max altitude 530 m), with highest densities both in residential and certain natural areas. Historically, iguanas were relatively more common at higher altitudes, probably due to more extensive forest clearing for agricultural reasons. No relationship was found between the degree of melanism and elevation, and few animals were completely melanistic. Furthermore, we found that body-ratio data collection through photographs is biased and requires physical measuring instead. Although the population size appears larger than previously surmised, the limited nesting sites and extremely low presence of juvenile and hatchling iguanas (2.4%), is similarly worrying as the situation for I. delicatissima on neighboring St. Eustatius. The island’s feral cat and large goat population are suspected to impact nest site quality, nest success, and hatchling survival. These aspects require urgent future research to guide necessary conservation management.

Abstract

The Invasive Alien Green Iguana (IAGI), Iguana iguana, has spread worldwide via the pet trade, as stowaways and via other means and has become a pest species of global concern. It also represents a major threat to the endemic Lesser Antillean Iguana, Iguana delicatissima, on St. Eustatius. Following the capture of an adult female IAGI on St. Eustatius in early 2016, we conducted a Rapid Response Removal Campaign (RC) from April 2016 to January 2017. Three sets of directed visual surveys totaling 409.5 observer hours and covering a combined trajectory of 114.2 km realized only a single detection of a hybrid that was later removed. During the remainder of the campaign period, an additional four IAGI hybrids were opportunistically detected and removed thanks to park staff or community involvement. Since the end of the campaign, eight additional detections and removals have been realized, three of which were IAGIs caught while offloading freight in the harbour and five of which were hybrids caught in surrounding suburban areas. We suggest that at least four distinct IAGI introductions to St. Eustatius occurred between 2013 and 2020. Our results show the value of motivating and mobilizing stakeholders and the public at an early stage of an invasion. Since the program’s initiation, eight of the 13 iguanas detected for culling were thanks to public and key stakeholder support and involvement. Four years after our campaign, the number of IAGIs and their hybrids still appear to be limited and concentrated in and around inhabited areas. Additional removal campaigns should be initiated as soon as possible, firmly based in public outreach, motivation and engagement. New legislation is needed to prohibit the importation, possession and harbouring of IAGIs or hybrids and to provide a framework for long-term structural funding required for effective control and removal. Routine fumigation and rigorous inspection of arriving cargo to eliminate the risk of stowaway IAGIs are also recommended. Culling of IAGIs is recommended for the port of St. Maarten, which serves as a major point of dispersal of IAGIs to St. Eustatius and likely also other islands in the region.

ABSTRACT

Catastrophic events, like hurricanes, bring lethal conditions that can have population-altering effects. The threatened Caribbean dry forest occurs in a region known for its high-intensity hurricane seasons and high species endemism, highlighting the necessity to better understand hurricane impacts as fragmentation and clearing of natural habitat continues. However, such studies remain rare, and for reptiles are mostly restricted to Anolis. Here we used single-season occupancy modeling to infer the impact of the intense 2017 Atlantic hurricane season on the critically endangered Lesser Antillean Iguana, Iguana delicatissima. We surveyed 30 transects across eight habitats on St. Eustatius during 2017-2019, which resulted in 344 individual surveys and 98 iguana observations. Analyses of abundance and site occupancy indicated both measures for 2018 and 2019 were strongly reduced compared to the pre-hurricane 2017 state. Iguanas at higher elevations were affected more profoundly, likely due to higher wind speeds, tree damage and extensive defoliation. Overall, our results indicate a decrease in population estimates (23.3-26.5%) and abundance (22-23.8%) for 2018 and 2019, and a 75% reduction in the number of opportunistic sightings of tagged iguanas between 2017-2018. As only small and isolated I. delicatissima populations remain, our study further demonstrates their vulnerability to stochastic events. Considering the frequency and intensity of hurricanes are projected to increase, our results stress the urgent need for population-increasing conservation actions in order to secure the long-term survival of I. delicatissima throughout its range.

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.

Abstract

Free-roaming livestock constitute a major threat to the terrestrial and marine ecosystems of St. Eustatius. In anticipation of a government-led culling program, we repeated population surveys of feral livestock from November 2019 to February 2020. Our goal was to compare current population estimates with those presented from 2013 by Debrot et al. (2015). Population densities of goats, sheep and chickens were estimated along 33.5 km of permanent roads and trails, representing six different habitats. Each of the 13 transects was surveyed twice. The results show that densities of goats and sheep have increased significantly compared with 2013, and chickens have increased slightly. The island population estimate (± SE) based on habitat-specific detection curves for goats is 7,602 ± 1,555; for chickens the island population estimate is 2,668 ± 417.

Detections of cows were too low to be included in the analysis, however these were present in town, lower Quill and grasslands. Given that sheep were primarily restricted to grasslands, the island population estimate is less exact at 4,316 ± 2,140. Nevertheless, the densities of freeroaming goats and sheep rose significantly between 2013 and 2020, and are now at levels considered extremely unsustainable considering the island’s size. Our estimates for goat densities per hectare and combined population size in the terrestrial protected areas are D = 5.93 ± 1.35 and N = 5,171 ± 1,182, compared with D = 1.09 ± 0.27 and N = 1,323 ± 329 in 2013. This suggests that the numbers of goats have increased fourfold in the parks over the past seven years, especially the northern hills. Such excessive densities of feral goats increase soil erosion and degradation, reduce organic matter, and reduce water retention in vulnerable landscapes.

Feral chickens were present in all habitats but especially prevalent in urban areas. In addition to being aggressive omnivores, chickens can transmit diseases and therefore constitute a public health risk. Our results suggest that there is an urgent need to drastically reduce numbers and implement effective management of free-roaming livestock on St. Eustatius. 

Referenced in BioNews 33 article "Survey of Free Roaming Livestock on St. Eustatius"