The analysis of ecological networks: habitat connectivity and population viability on St.Eustatius

This student thesis research is in support of a Tourism Master Plan for Sint Eustatius; this plan is focused on the development of sustainable tourism on St. Eustatius. St. Eustatius is part of the Lesser Antilles and belongs to the Dutch Caribbean islands. The aim of this project was to analyse the connectivity of St. Eustatius’ habitats and the viability of St. Eustatius’ wildlife populations for the current situation and a hypothesized future situation. Increased tourism and other developments could lead to an increase in urban areas and infrastructure. As these developments can be at the expense of nature areas it is of importance that these threats for nature are minimized. Human activities influence biodiversity, where in the past the ranges of species were reduced and several species went extinct. Human activities could lead to a decrease and a further fragmentation and degradation of the remaining nature areas (Moorcroft, 2009). Loss, fragmentation and degradation of habitat are considered as the main threats to biodiversity (Dennis et al., 2013). Small and isolated populations are extra vulnerable for inbreeding depression and the loss of genetic variation, so it reduces the reproductive fitness and the ability to adapt to environmental changes (Simberloff et al., 1992; Traill et al., 2010). Landscape connectivity is described as the ability of a landscape to enable exchange of organisms, material and energy between different areas and is dependent on the structural connectedness of a landscape and the movement characteristics of the considered species or processes (Wu, 2009). Species may utilize specific corridors for their movements. The main purpose of corridors is to enable individuals from various species to migrate to other areas. Migration diminishes isolation and results in a metapopulation structure (Beier, 1992). Species-specific characteristics and landscape connectivity determine whether or not a metapopulation will sustain in an ecological network consisting of physically separated habitat patches. In this report the model LARCH (Landscape ecological Analysis and Rules for Configuration of Habitat) was used to test landscape connectivity and population viability on Sint Eustatius. St. Eustatius covers approximately 21 km2. The southern part of the island consists of a volcano, the Quill National Park; this park hosts several different ecosystems with some endangered species. The northern part of the island is also a National Park area covering four volcano hills, also with some endangered species. The northern and southern parts of the island are separated from each other by the urban centre part of the island. The centre part of the island is divided into different suburbs, mainly consisting of residential and small-scale industrial areas. These areas are located from the northeastern coastline towards the south-western coastline. Fieldwork on St. Eustatius was done between the end of July and the beginning of September 2015 to prepare a habitat map and a barrier map of the island, and to gather information about the island species and about the island policies and possible future changes. All this information was required for the LARCH model: two models were used: LARCH-Scan for the connectivity of habitats and LARCH-Classic for the viability of populations. Six species were modelled for the current situation: the Green-throated carib (Eulampis holosericeus), the Lesser Antillean green iguana (Iguana delicatissima), 4 the Red bellied racer (Alsophis rufiventris), the Caribbean hermit crab (Coenobita clypeatus), the Cloudless sulphur (Phoebis sennae) and the Caribbean duskywing (Ephyriades arcas) representing St. Eustatius’ biodiversity. The Lesser Antillean green iguana and Red bellied racer were modelled for the proposed future scenario. It can be concluded that the two National Parks contain the most suitable habitats for all studied species. The connecting corridor for the studied species is limited to a narrow vegetated strip near the south of the airport and through the residential areas. Besides these residential areas and the airstrip also the NuStar oil terminal, waste heaps and overgrazed grasslands reduce the habitat connectivity. All studied populations form one viable network, except for the Caribbean hermit crab. Four proposed future plans were incorporated in a scenario along with more intense barriers.  Local populations and population viability will decrease with the new proposed plans. Residential development in the Lodi suburb and industrial development in “the Farm” area would reduce the connectivity between the northern and southern National Parks. Based on these modelling results recommendations have been proposed. There are several areas that need special consideration if future developments are proposed in those areas. These are the two National Parks, the remaining corridor area and areas near the coastline. Modelling results show that urban developments in these areas decrease connectivity, isolate local populations and affect population viability. Also development plans in other areas need to be considered carefully regarding its size and location and its impact on the landscape connectivity and animal population viability. In the event of unavoidable negative impact, measures should be considered to compensate for the losses, such as the establishment of culverts or the restoration of areas. Study results show that ground dwelling animals are most affected by the presence of roads, fences and unsuitable habitats, it is therefore recommended that future developments of potential barriers are minimized, with mitigating measures such as open fences or culverts under roads. It is also highly recommended to preserve the remaining habitats of the two species that were modelled and that are considered endangered by the IUCN Red list; the Lesser Antillean green iguana and the Red bellied racer. And it might be worth it to consider the restoration of suitable habitats, enlarging the ecological carrying capacity, to protect these endangered species. At last there are recommendation for a second corridor near the northern coastline of the island that should increase connectivity. This recommendation involves alterations regarding the waste management and the exile of roaming cattle

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