Population Dynamics of Endemic and Non-Endemic Grass and Sedge Species of Guana Island, British Virgin Islands.

The Caribbean Biogeographic Unit, comprising the Caribbean and south Florida, has been identified as the third most important global biodiversity hotspot in the world. This is based on the percentage of endemic plants and remaining primary vegetation. Guana Island, British Virgin Islands, is roughly 343 hectares of tropical forest, mountains, hills, and valleys that contain more flora and fauna than any other island similar in size. However, several non-native grass species (Digitaria spp., Setaria spp., Sporobolus spp., etc.) have been documented to exist on Guana Island. The introduction and competitive nature of several of these non-native species may result in a loss in biodiversity and the extinction of certain endemic grass populations. The previous identification of grass species on the island may not accurately depict what is currently present in the salt flats. Therefore, we conducted a systematic survey of the grasslands of Guana Island in October of 2013 in order to determine the abundance and distribution of endemic and non-endemic grass species. A total of 59 transects measuring 40 to 67 m were run northeast to southwest approximately 3 m apart from one another from the salt pond to White Bay beach. Grass or sedge species were identified and geo-referenced (Trimble GeoExplorer 6000 series gps unit) every 3 m along each transect for a total of 1,087 data points. Data were imported into ArcGIS in order to create spatial distribution maps of grass and sedge species across the salt flats. Data points for each species were counted to determine overall abundance (%). Eight grass/sedge species were identified: broadleaf panicum [Brachiaria adspersa (Trin.) Parodi], Indian bluegrass [Bothriochloa pertusa (L.) A. Camus], thin paspalum (Paspalum setaceum Michx.), goosegrass [Eleusine indica (L.) Gaertn.], common bermudagrass [Cynodon dactylon (L.) Pers.], southern sandbur (Cenchrus echinatus L.), crowfootgrass [Dactyloctenium aegyptium (L.) Beauv.], and tropical fimbry (Fimbristylis cymosa R. Br.). The most abundant species present was Indian bluegrass (76%), while all other species constituted ≤ 10% of the total area, respectively. Indian bluegrass may have been introduced as forage for livestock during the 18^th century when the island was primarily used for sugar cane production. Thin paspalum, although poorly distributed (4% abundance), is endemic to Guana and several surroundings islands within the Caribbean. Coastal sandbur (Cenchrus incertus M. A. Curtis.) is indigenous to the island; however, our surveys only revealed a small population of non-endemic southern sandbur. Land use and anthropogenic activity may have created a population of goosegrass exclusively inhabiting a utility road that traverses the salt flats from east to west. Tropical fimbry, a sedge native to several other islands in the Caribbean, had the second highest abundance (10%). Tolerance to high levels of soil moisture may increase the competitive ability of tropical fimbry present in the salt flats. Most of these plants were observed in small depressions where water runoff collects and/or depth to the water table is less. Information obtained from this survey will be used to understand the conservation significance of several of these species and determine strategies to enhance the growth and survival of endemic plants and the biodiversity of the island flora.