A landscape ecological vegetation map of Sint Eustatius (Lesser Antilles)

Abstract:

A semi-detailed landscape-based vegetation map (scale: 1: 37,500) based on field data from 1999 has been available as an update of Stoffers’ 1956 map of the Lesser Antillean island of St. Eustatius, Netherlands Caribbean, but up to now was never finalized or published. In this report we complete the documentation of that map to provide new insights into vegetation change over a period of more than 40 years, and a quantitative reference point for future studies on landscape-level vegetation development for the island.

The principal lower sections of the 21 km2 island of St. Eustatius possess a tropical savannah climate according to the Köppen (1931) classification system, and the documented flora of the island amounts to 505 species. Color aerial photographs (1: 8,000) taken in 1991 and field data from 1999 were used to produce the map. A total of 84 vegetation sample plots were analysed using a stratified random sampling design and TWINSPAN cluster analysis.

Four main and 16 sub-landscape types were distinguished based on geology, geomorphology and different mixes and expressions of the component vegetation types. The five principal landscape types are in descending order of importance: H1, H2, M4, M9 and C, and covered some 67 % of the seminatural habitat of the island. H1 and H2 are the Pisonia-Justicia hills and Pisonia-Bothriochloa hills and are limited to The Mountains area. Analysis of the sampling data resulted in the distinction of 13 (semi)natural vegetation types. The three principal vegetation types were, the Pisonia-Justicia type, Pisonia-Ayenia type and Bothriochloa-Bouteloua type which together accounted for 38 % of total (semi)natural vegetation cover. The following well-developed vegetation types of St. Eustatius represent primary climax communities: Types 1, 2, 3, 5 and 7, all found in and around the Quill in the southwestern part of the island. A comparison of the vegetation types in the present study with those of Stoffers (1956) showed that only one vegetation type closely resembles one in Stoffers’ study. Major changes have taken place in certain types of the natural vegetation of the island in the intervening five decades.

The majority of the central sections of the island around Oranjestad the so-named “Cultuurvlakte”, amounting to approximately 25 % of the surface of the island, have suffered intensive disturbance due to past agriculture, livestock husbandry and invasive species and were not mapped. Only a small remnant portion of the semi-natural lowlands vegetation (L1 and L2) was left in the coastal reaches of Billy’s Gut. Nevertheless, this area is heavily affected by grazing and the invasive vine Antigonon leptopus.

A comparison with the 1950s vegetation map by Stoffers shows that the rarest and most valuable elfin woodland vegetation of the rim of the Quill crater had been largely lost and that the areas he described as “Montane thickets” (Type 2) had declined and been degraded. We speculate that these losses may be most directly attributable to the impact of recent hurricanes and/or grazing by introduced livestock. On the lower slopes of the Quill, several areas mapped by Stoffers as farmland had been abandoned and have evidently regenerated into mixed deciduous and evergreen thorny woodlands.

The vegetation of the Mountains area showed some recovery since the 1950s. There were more evergreen bushes, and less Acacia and Leucaena than Stoffers described. The vegetation Stoffers described for the lowlands had more Acacia than we found but the invasive Antigonon has since dramatically increased as a ubiquitous and often dominant species. The former importance of Opuntia prickly pear cacti in disturbed vegetations has dramatically declined since the 1950s. We ascribe this to the likely effect of the invasive parasitic insect Cactoblastis cactorum. In the 1980s and 1990s many Opuntia cacti were seen affected by this insect (G. Lopes, pers. comm.).

Our field data show that all wilderness areas of St. Eustatius remained heavily affected by grazers. This reduces the resilience of natural vegetations and interferes with natural succession by imparting heavy losses to hardwood seedlings and saplings (see e.g. Melendez-Ackerman et al. 2008), by reducing plant biomass (which increases exposure to wind and sun), and by favoring hardy invasive plant species. In Curaçao, large scale reduction in densities of feral grazers in the Christoffelpark since 1993 has led to rapid recovery of several rare plant species and vegetation types. The problem of feral livestock remains severe. Therefore the number one priority for terrestrial conservation in St. Eustatius will be to reduce feral grazer densities and impacts in key wilderness areas.

Management Recommendations:

Parts of the natural areas of both The Mountains and the Quill are protected by law, but goats and other roaming livestock are omnipresent in all habitats and continue to have evident impacts on the vegetation. Aside from generally reducing the resilience of the vegetation to major disturbance, intensive impact in the herbaceous layers likely affects regeneration of rare hardwood species directly through selective predation and indirectly by overall desiccation and increased exposure to the elements. Goats have a broad diet in the region and species eaten include canopy, mid-canopy and understory species (Melendez-Ackerman et al. 2008). Vegetation degradation further also affects the competitive balance towards (unpalatable) invasive species such as Antigonon leptopus, Pedilanthus tithymaloides, and Leucaena leucocephala, which have expanded massively into natural habitat in the last 50 years. Not only is general vegetation degradation suggested as a problem to endangered breeding seabirds, but goats likely also directly trample nesting burrows of seabirds (Collier & Brown 2009).

Consequently, priorities for nature conservation are to:

• Reduce grazer densities in all areas,
• Protect the most sensitive vegetations using total grazer exclusion, and
• Experiment with methods to spur vegetation recovery, including erosion control, propagation of rare and endangered species and reforestation with indigenous species
• Establish permanent plots in areas with the most sensitive vegetations in order to better understand causal factors of short- and longer term changes. 

This report is part of the Wageningen University BO research program (BO-11-011.05-004) and has been financed by the Ministry of Economic Affairs, Agriculture and Innovation (EL&I) under project number 4308202004.