0- Articles -- ----- A IComparmison olF Freslhwater Macroinvertebrate Communlities on Small Caribbean Islands DAVID BASS An ongoing survey of macroinvertebratesinhabiting the relatively unstudiedfreshwaterhabitats on 14 small Caribbeanislands wvas initiatedin 1991. These collections haveyielded almost 200 species; when these species arecombined with collectionspreviously made by other researchers,a total of at least 328freshwater macroinvertebratesare now knownfrom these islands. The dominant taxa on the islands include several species of snails, shrimps, mayflies, dragonflies, damselflies, beetles, and other insects. Many of these species havefairly widespread distributionsacross the islands.Most stream species are associatedwith leafpacks, and mostpond species are associatedwith aquatic macrophytes. As is typical of tropical island systems, the macroinvertebratefaunasof these islands aresparse, most likely because of their oceanic origin, theirsmall size, and thefre- quent disturbances to theirfreshwaterenvironments. Keywords: biodiversityfreshwater,macroinvertebrates, streams, Caribbeanislands M uch still remains unknown about the freshwater facilitated by shredders in temperate streams, must be brought 11 w |invertebrates of the Lesser Antilles and other small about largely by microbes in tropical streams (Ramirez and Caribbean islands. While some groups have been studied, such Pringle 1998, Dudgeon and Wu 1999, Bass 2003, Turner as shrimps and crabs (Chace and Hobbs 1969), dragonflies 2003). Irons and colleagues (1994) determined that the con- and damselflies (Donnelly 1970), and caddisflies (Flint 1968, stant high temperatures of the tropics encouraged leaf litter 1996, Flint and Sykora 1993), many others have yet to be sur- decay through increased and continuous microbial activity in veyed. Where data from previous investigations exist, they are the streams. often limited in scope (FHynes 1971, Harrison and Rankin 1976, Stark 1994) or may be outdated (Hinton 1971, Peters Biodiversity survey 1971, Peck 1981, Bennett and Alam 1985). Furthermore, An ongoing survey of macroinvertebrates inhabiting fresh- additional collections may extend ranges of known species or water habitats on small Caribbean islands was initiated in 1991. uncover new species (Bass and Volkmer-Ribeiro 1998, Bass The islands studied in this investigation include Barbados, 2000,2003). Tobago, Grenada, St. Lucia, Dominica, Montserrat, Nevis, Often differences between the kinds of macroinvertebrates St. Kitts, Saba, Antigua, Cayman Brac, Little Cayman, Grand present in temperate portions of North America and South Cayman, and Guanaja. Objectives of this research included America and those of the tropical Caribbean islands are seen (a) determining the species of macroinvertebrates inhabiting at the species level. Most of the insect orders found in tem- freshwater habitats of small Caribbean islands, (b) noting perate freshwater environments are also present in the trop- microhabitat preferences for species, (c) determining the ics. However, stoneflies are greatly underrepresented on small relative abundance of each species, and (d) comparing the Neotropical islands, with only the genusAnacroneuria being macroinvertebrate fauna known from each island to that of reported from Trinidad and Tobago (Hynes 1971, Stark 1994). other small Caribbean islands. Governmental bodies and Decapod crustaceans, such as prawns, crayfish, and crabs, are nongovernmental organizations on the selected islands were much more diverse and widespread in tropical streams than also interested in obtaining information from a biodiversity in temperate flowing waters (Chace and Hobbs 1969, Covich survey of freshwater macroinvertebrates. 1988). Some of those crustaceans may fill the roles of amphipods and isopods, which are almost absent in tropical DavidBass (e-mail: dbass@ucokedu) isa professor of biology and curatorof streams (Fryer 1977). invertebratesin the Departmentof Biology, University of CentralOklahoma, Tropical stream communities appear to be composed of the Edmond, OK 73034. His research interests includestudying the ecology, evo- same trophic guilds as temperate ones, but they have relatively lution, and biogeography of aquatic invertebrates inhabitingfreshwateren- lower densities of shredders in the headwaters. Therefore, it vironments of small oceanicislands. ©2003 American Institute of Biological has been suggested that the breakdown of leaf litter, which is Sciences. 1094 BioScience * November2003 / Vol. 53 No. 11 Articles The physical environment of the islands Each site has been visited at least once, and many sites were Most of the small islands in the Caribbean basin were formed sampled more frequently as opportunities arose to return to as a result of volcanic activity or sediment uplift (see Craig those sites. Water temperature was recorded at each site. [2003] for more discussion on island formation). On volcanic Several methods of collecting were used to ensure that as islands, the gradient is fairly steep, resulting in many fast- many species as possible were captured. Submerged debris, flowing streams that have bottom substrates composed mostly such as stones, leaves, and wood, was carefully examined in of rocks. Leaf debris is abundant in pools and other places the field, and inhabitants were removed from the substrate where flow is reduced. The islands are primarily forested, with forceps. In addition, a dip net was swept through aquatic and rainfall is abundant, especially at higher elevations. vegetation and the water column to capture macroinverte- Grenada, St. Lucia, Dominica, Montserrat, Nevis, St. Kitts, brates occupying those microhabitats. The microhabitat from Saba, Guanaja, and Tobago are volcanic in origin. However, which each specimen was taken was noted.A drift net was also Tobago is a continental island that was once connected to the used at several sites to collect samples of macroinvertebrates South American continent. There is little relief and less ele- drifting downstream with the current. Specimens were vational gradient on islands formed by sediment uplift. Their preserved in 70% ethanol and returned to the laboratory for forests are less developed and mostly absent, as rainfall is further taxonomic study. not as abundant. This has resulted in few permanent streams In addition to the field collections, the published literature being present, and many ponds have been constructed to was also examined and previous records were included in this provide water supplies. The sediments of these ponds are article. Finally, Sorenson's Index of Similarity (1948) was generally soft, composed mostly of mud and silt. Islands used to calculate faunal similarity between the islands inves- formed by sediment uplift include Barbados, Antigua, Grand tigated and determine whether the aquatic macroinverte- Cayman, Little Cayman, and Cayman Brac. brate populations on each island were composed of the same A total of 205 sites were sampled in various freshwater species or different species. habitats across the Lesser Antilles and other small Caribbean islands (figure 1).AII islands induded in this investigation are A near-doubling of the known species less than 800 square kilometers (km2) in surface area. The Freshwater macroinvertebrates had never been collected on number of sites varied on each island, depending on island several of the islands studied. A total of at least 328 species are size-one site was selected for every 5 to 10 km2 of island area. now known from these 14 islands. Of these, 191 species were Figure1. Map of the Caribbeanbasin, indicatingislands sampled in this investigation. November2003 / VoL 53 No. 1* BioScience 1095 A-rf;,r1,Pc awir .-- v^ rrwrco collected during this investigation, while 186 species had Table 1. Species richnesswithin selected taxa. been previously reported in the literature. As a result of this collecting effort, 147 species were reported for the first time Taxonomic group Number of species from these Caribbean islands. It should be noted that it ispos- Porifera 1 sible several of these taxa may actually represent more than Platyhelminthes I one species. This is due to the poor state of taxonomy in Oligochaeta 5 some groups and the presence of life cycle stages in which Hirudinea 2 diagnostic features have yet to develop. Gastropoda 27 Of the major taxa investigated, the Trichoptera (caddisflies) Pelecypoda I included the most species (table 1).A total of 84 species of cad- Cladocera 1 disflies have been identified from the Caribbean islands in this Ostracoda 19 study. This relatively high number of species is a reflection of Amphipoda 3 intensive collecting in the region by Oliver S. Flint Jr. (Flint Decapoda 19 1968, 1996, Flint and Sykora 1993). It is quite possible there Hydrocarina 1 are additional species belonging to other invertebrate groups Ephemeroptera 14 that will be discovered when collecting efforts equal to those Odonata 39 of the trichopterists have been realized. Many species of Plecoptera I dipterans (two-winged flies), coleopterans (beetles), odonates Hemiptera 33 (dragonflies and damselflies), hemipterans (semiaquatic and Megaloptera 2 aquatic bugs), and gastropods (snails) have already been Trichoptera 84 noted-on these small islands. Decapods (shrimps and crabs), Lepidoptera 2 ostracods (seed shrimps), and ephemeropterans (mayflies) are Coleoptera 40 common as well. Diptera 19 Species richness related to island size and elevation varies at different locations with different storms. The rising