State of Nature in the Caribbean Netherlands: St. Eustatius
Wageningen Research recently published an alarming report on the natural resources of the three Caribbean Netherlands islands (Bonaire, Saba and St. Eustatius), commissioned by the Ministry of Agriculture, Nature and Food Quality. All 33 experts that worked on this report concluded that the "Conservation status[1] of the biodiversity in the Caribbean Netherlands is assessed as moderately unfavorable to very unfavorable”.
The island of St. Eustatius has an incredible range of tropical habitats from cloud forest at the top of The Quill to rich coral reefs below. While many of these habitats have been considered pristine in the past with high species diversity, the Wageningen Research report paints a bleak picture of the current state of St. Eustatius’ ecosystems, stressing that things will only get worse if the island does not significantly increase its investment in conservation. If the current rate of habitat degradation is maintained, important ecological functions such as erosion control and storm protection will be lost and the economic repercussions will be disastrous. Recent TEEB[2] research found that nature/ecosystem services on St. Eustatius have an annual economic value of 25.2 million USD, which represented 24% of the island’s gross domestic product (GDP) in 2013 (Cado van der Lely et al., 2014).
Tropical cloud, tropical rainforest and dry tropical forest
The degradation of St. Eustatius’ biodiversity affects all habitats and a wide range of species that depend on them. On land, both the tropical cloud and rainforest at the top of The Quill and the dry tropical forest found at lower elevations are now severely damaged and assessed as “very unfavorable”. This means that if no further action is taken, the critical ecological processes provided by the island’s vegetation – notably helping keep coastal waters clean and free from sediment – will be lost. While a lack of long-term monitoring data makes it difficult to estimate just how much forest has been impacted, comparison of vegetation maps from 1956 (Stoffers, 1956) and 2012/2016 (Freitas et al., 2012, 2016) shows that the cloud forest on the edge of The Quill crater has almost disappeared, and likely suffered further damage during the 2017 Hurricane season.
This is very concerning as the rainforest is a critical habitat to populations of several endemic and/or endangered species of plant, bird and reptiles. As much as 50% (900 ha) of St. Eustatius’ dry forest is now severely degraded to grass and bush land. While this forest type had recovered from past intense agricultural use, the large-scale presence of loose cattle has had disastrous effects. Invasive insects and plants such as Corallita (Antigonon leptopus) are further accelerating the degradation process, as is abandonment of agricultural land and the recent increase in urbanization to accommodate growing numbers of residents and tourists.
Beaches
Unlike most other Dutch Caribbean islands, St. Eustatius is unique in that it has very limited beach area. It also does not have mangroves or salt ponds that are typical of some of the other islands. The intertidal zone is dominated, in most places, by steep rocky cliffs interspersed with a few bays and sandy beaches.
The beaches may be few but they are significant sea turtle nesting grounds within the Caribbean Netherlands (Esteban et al. 2015). While a lack of quantitative information means that the island’s sea turtle population can be estimated as “stable at best”, the nesting beaches are under pressure from coastal development, illegal sand excavation and climate change effects. The incubation temperature in sand surrounding a clutch of eggs determines the sex of a turtle hatchling. Eggs incubating at cooler temperatures produce male turtles and eggs incubating at warmer temperatures produce females. This has led to concerns that, in the context of climate change and shallower nest depth due to sand mining, warming temperatures may lead to female-biased sea turtle populations. Laloë et al. (2016) showed that incubation temperatures are relatively high on St. Eustatius (mean of 31°C) so that the majority of turtle hatchlings born at these beaches have been female- biased during for the past few decades. Therefore, there is therefore a real concern that not enough male hatchlings would be born on the beaches in the future to sustain the local population. St. Eustatius’s beaches now face the additional threat of sargassum influxes which have inundated beaches throughout the Dutch Caribbean since 2011. In 2015, an especially large influx of sargassum resulted in one of the island’s sea turtle nesting beaches being covered with a thick layer of seaweed, potentially trapping sea turtle hatchlings and preventing females from successfully laying their eggs on the beach (Maurer et al., 2015).
Seagrass and algae fields
St. Eustatius’ seabed habitats are faring no better. The island has the second largest area of seagrass fields in the Dutch Caribbean, with an estimated surface area of 120 ha. It also has the largest area of algae fields, with an estimate of 570 ha. These fields provide important habitat to many fish species, sea turtles and queen conch (Lobatus gigas). A lack of monitoring data means that no clear trend can be established, however, recent studies have highlighted the alarming rapid spread of the invasive seagrassHalophila stipulaceadue to an increase in eutrophication of St. Eustatius’ shallow waters, to the detriment of the nativeH. decipiens. Additionally, seagrass fields are threatened by physical damage from tourist activities. The current state of the seagrass fields is assessed as “very unfavorable” and algae fields as “moderately unfavorable”.
Coral reefs
The coral reefs around the island have in the past been found to be in good condition, with an average coral cover of 22% in 1999 (Klomp & Kooistra, 2003). The structure of the coral reefs results from the island’s volcanic origins, with most reef communities occurring on large volcanic rocks and boulders that were blown out from The Quill centuries ago (Research group at Scripps Institution of Oceanography UC San Diego, personal communication, June 15, 2017). While no long-term data is available, several recent studies of St. Eustatius’ reefs have shed light on the rapid decline of the island’s coral cover and the shift from coral dominated to algae dominated benthic communities, a shift which is being observed throughout the Wider Caribbean Region. The average coverage at 20 different coral reef locations around the island was found to be just 5% in 2015/2016 (de Graaf, 2015; Piontek, 2016). However, coral reef habitat amounts to only about 19% of the nearshore coastal habitat area of St. Eustatius, where sand, rubble and algal fields dominate (Debrot et al. 2014). Threats to the island’s reefs are many and include erosion from coastal development and overgrazing by free roaming feral cattle, as well as, eutrophication from inadequate wastewater treatment.
Fishery
The island’s most important fishery is the Caribbean spiny lobster (Panulirus argus) with an annual catch of 11 tons per km2/y, which is the highest recorded through its range (de Graaf et al., 2015). In contrast to many other locations in the Caribbean region, St. Eustatius was found to have healthy populations of queen conch (Boman, 2017). While the density of reef fish remains reasonable, there has been an important change in fish composition over the past 100 years with the disappearance of large predators - such as groupers due to overfishing - and a marked increase in herbivore reef fish species. Catches have, however, remained stable over the past 15 years (Graaf et al., 2015), most likely thanks to establishment of non-fishing marine reserves within the St. Eustatius National Marine Park. The long-term effect of the invasive lionfish on the island’s native species is still unclear. St. Eustatius does have a relatively healthy population of reef sharks, most likely due to the fact that they are not targeted by coastal fisheries.
Local and regional stressors
The degraded state of St. Eustatius’ habitats and the decline of some of the species that depend on them is a clear indication that they are under mounting pressure from local, regional and global stressors. Three threats in particular, according to the Wageningen Research report, “largely determine the quality of over 80% of habitats”: (1) overfishing, (2) free-roaming cattle and (3) invasive exotic species.Free-roaming cattle, especially goats, which were introduced centuries ago by colonists, are seen all over the island - all the way to the top of The Quill – and are presently the most serious threat to St. Eustatius’ terrestrial ecosystems. The current density of goats on St. Eustatius is high at 1.07/ha and not sustainable (Debrot et al. 2015). The detrimental impact of these grazers on ecosystems is well known and includes erosion as a result of a reduced vegetation cover and the loss of an important food source for the many animals that live in the island’s forests.
The issue of invasive species is widespread throughout the islands of the Dutch Caribbean, and on St. Eustatius several introduced species have caused major ecological changes, as well as, causing losses in the agricultural and economic sectors. This includes the giant African snail, the lionfish and the green iguana. Some invasions have been incredibly rapid, such as the invasive seagrass H.stipulaceawhich became the dominant species in the seagrass fields of St. Eustatius within two years of its introduction. Others have been impossible to control, the invasive vine species Corallita can reproduce from the smallest of fragments. The recent (2016) invasion of the green iguana is posing a significant threat to the survival of the endemic Lesser Antillean iguana. The green iguana not only competes for food but there is now proof that it is hybridizing with the Antillean iguana.If the green iguana and hybrid animals are not completely eradicated, the survival of the Antillean iguana in the wild on St. Eustatius is not possible.
Climate change
In addition to the many local and regional threats faced by St. Eustatius’ natural environment, the island faces a very uncertain future due to climate change. Even under the most modest of predictions made by IPCC for small island nations in the Caribbean - including a 1.2-1.9 degrees Celsius rise in SST by 2100, a sea level rise of between 0.5/0.6 meters and more intense storm systems (Nurse et al., 2014) - the impact of climate change will be significant and is likely to have major negative consequences on St. Eustatius’ habitats and species and the ecosystem functions they provide. Unfortunately, the current damaged state of habitats means that they will be less resilient to the effects of climate change, especially fragile habitats such as rainforest and coral reefs. Impacts of climate change on the nature and biodiversity of the Dutch Caribbean islands, based on the categories of Nurse (2014), include: loss of coastal habitat (quality), coral bleaching, acidification of surface water, groundwater deterioration, coastal erosion, and loss of terrestrial habitat quality.
Necessary measures
One of the main takeaways from the Wageningen Research report is that not enough is currently being done to reduce the major threats to St. Eustatius’ biodiversity and to slow down the speed of biodiversity loss. This explains why the future perspective of St. Eustatius’ habitats range from “moderately unfavorable” to “very unfavorable”. “The current interventions to threats do not seem enough” states the report “causing the negative trends to continue as expected”. Densities of free-roaming animals are far too high and must be significantly reduced so that the native vegetation can recover and the current rate of erosion slowed down. The Department of Agriculture, Animal Husbandry and Fisheries (LVV) is currently working on a program to reduce the number of free-roaming animals on the island.
If nothing is done to counter the devastating impact of invasive species, “the many unique and endangered species of the islands will become more and more at risk of extinction and the native nature will gradually be replaced by a completely "unnatural" nature” (Debrot et al., 2018). Invasive species whose populations are still small, notably the green iguana and African giant snail, should be completely eradicated. The 2016 green iguana culling program, funded by the Ministry of Economic Affairs, was a positive start. STENAPA and RAVON are still working to catch these green iguanas to prevent the purebred endemic Antilles iguana from disappearing from St. Eustatius within a few decades. Climate change cannot be tackled locally but every possible action should be taken to improve the quality of fragile habitats and thereby increase their resilience to the impacts of climate change. In order to enable the assessment of present and future conservation efforts, monitoring efforts on the island must be greatly expanded so that comparable trend analyzes can be performed.
This article was published in BioNews 23-2019
[1]The Conservation Status is used in the European Union for reporting on the status of species and habitats protected under the European Habitat Directive (see http://ec.europa.eu/environment/nature/legislation/habitatsdirective/ind...).
[2]The Economics of Ecosystems and Biodiversity