PS 3-46 - Population dynamics and conservation significance of endemic and non-endemic grass and sedge species of Guana Island, British Virgin Islands

Abstract: Background/Question/Methods

Guana Island, British Virgin Islands, is 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 on Guana Island. The introduction and competitive nature of non-native species may result in a loss in biodiversity and the extinction of certain endemic grasses. The previous identification of grass and sedge 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 to determine the abundance and distribution of endemic and non-endemic grass and sedge species. A total of 59 line transects measuring 40 to 67 m (3 m apart) were run from the salt pond to White Bay beach. Grass or sedge species were identified and geo-referenced every 3 m along transects for a total of 1,087 data points. 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.). Data points for each species were counted to determine overall abundance (%).

Results/Conclusion

The most abundant species was Indian bluegrass (76%), while all others constituted ≤ 10% of the total area, respectively. Indian bluegrass may have been introduced as livestock forage during the 18th century when the island produced sugar cane. Thin paspalum, although poorly distributed (4% abundance), is endemic to Guana. Coastal sandbur is indigenous to the island; however, our survey only identified non-endemic southern sandbur. Land use and anthropogenic activity may have created a goosegrass population (5% abundance) exclusively inhabiting a utility road traversing the area. Tropical fimbry, a sedge native to the Caribbean, had the second highest abundance. Tolerance to high soil moisture levels may increase the competitive ability of tropical fimbry present in the salt flats since most of these plants were observed in small depressions where water runoff collected and/or depth to the water table was less. Information obtained from this survey may help 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.