Spider diversity (2007-2009)


Jo-Anne Sewlal has a longstanding interest in spiders (see her TEDx video*), and completed an M.Phil. on the biology of two species inhabiting tree buttress roots, and their commensals (Sewlal, 2005; Kuznetsova et al., 2007; Sewlal and Starr, 2008). She subsequently obtained a scholarship from the University of the West Indies, St. Augustine, for a Ph.D. on the biodiversity of orb-weaving spiders in Trinidad, and completed field work in May 2009. Her thesis was submitted in 2012 and the Ph.D. in Zoology was awarded in 2013. She is currently an Instructor in the Department of Life Sciences.

Dr Jo-Anne N. Sewlal
c/o Department of Life Sciences
The University of the West Indies
St. Augustine
Trinidad and Tobago


The number of spider species currently known stands at about 40,000 (Platnick, 2007), of a total of perhaps 170,000 species (Coddington and Levi, 1991). The worldwide spider fauna is thus still far from completely known, especially in the Neotropics in general, and northern South America and the Caribbean in particular. The spider fauna has not been documented in print at the species level for any Caribbean island, though this has been done at the family level for Trinidad (Sewlal and Cutler, 2003; Cutler, 2005; Sewlal, 2006; Sewlal and Alayón, 2007). Trinidad is separated from South America by only 14 km (the southern passage or Serpent's Mouth), and was part of that continent until the end of the last ice age. Trinidad thus shares a similar climate and biota with Venezuela and the Guianas, but is a fraction the size making it a much more manageable area to sample. Knowledge of the spider fauna of Trinidad is therefore of interest as a guide to the biodiversity in northern South America, and as one of the sources of spiders of the Windward Islands and the rest of the West Indies. There are currently 46 families and 225 species of spiders known in Trinidad, although the latter is expected to be only a third to a half of the true total. A complete biodiversity survey of the spiders of Trinidad would be a very large project. This study therefore focuses on some of the web-building spiders, one of the three ecological guilds of spiders (the others being long-sighted and short-sighted hunters), since their webs make detection easier.

Web-building spiders are defined as those which use their webs primarily to capture prey. They comprise 15 families and currently 67 known species in Trinidad, still too large a subject for a comprehensive survey. A subset of this guild has been selected as more manageable, the three families Araneidae, Nephilidae and Tetragnathidae, which construct orb webs. These spiders lead a rather uniform lifestyle, and regional keys are available for the identification of species newly discovered in Trinidad. However, although identification is possible, very little is known about the natural history of most of the species. Using this group of spiders, the study will determine the contribution of habitat to their diversity, including both natural and man-modified habitats as classified by Beard (1946). One point of general ecological interest is how much the diversity of spider faunas will be influenced by taxonomic diversity of the vegetation. Spiders are at a higher trophic level to most insects, which are the subject of most studies of arthropod diversity, and may be less closely related to vegetation composition than are herbivores; in contrast, structural diversity is predicted to be more important to spiders. Other questions of interest include diversity in the monospecies Mora forest (dominated by Mora excelsa) compared to multispecies forests more typical of tropical areas; the effects of disturbance on diversity in areas of forest of different logging history; and the diversity and thus conservation value of more modified habitats. Overall, the study will produce a comprehensive picture of the biodiversity of orb-weaving spiders in Trinidad in the early 21st Century, as the country embarks on the path to developed country status ("Vision 2020").

Spiders are sampled using time-based survey methods. The optimum has been found to be a sample unit comprising one hour of sweep netting in ground vegetation up to shoulder height, and two hours of visual search for spiders and their webs in accessible locations (but not under leaf litter, logs or stones (Scharff et al., 2003). Each orb-weaver is collected in a numbered snap cap vial in 70% ethanol. Details are recorded of web height, web diameter, inclination, orientation, number of radii, location of web and spider, presence and location of barrier webs and kleptoparasites, and observations on habitat and behaviour including defence and mating. Sweep-netting uses a standard insect sweep net, often by an assisting technician. Spiders are collected in vials of 70% ethanol; or the numbers are recorded where many specimens of the same species are found. This method also yields nocturnal species which may have dismantled their webs and are hidden in daytime retreats or among vegetation, avoiding visual detection. Nocturnal visual search may also be necessary. A further sampling method makes use of predation of spiders by mud-dauber wasps to provision their nests. Nests of three species of Tropoxylon are commonly found and will be examined as a source of orb-weavers. Trap nests of hollow sections of bamboo have been set out (January 2007) in some areas, to attract a fourth species (T. nitidum), as a check of the coverage of the other sampling methods.

Initial studies were undertaken at three sites in two habitats. These showed that cumulative species curves levelled off after five 3-hour periods, which is therefore taken as the standard sampling design for the project. Comparing the three sites showed more similarity between sites in the same habitat, but still a substantial difference in the species composition between two sites in Evergreen Seasonal Forest at Arena (central Trinidad). The project design is therefore for three sites to be sampled for each habitat type, as far as possible covering the geographical extent of that habitat type in Trinidad. More replicates are taken if required to answer particular questions, such as comparison of Mora and more diverse forest types, or man-modified and natural habitats. The effects of disturbance on diversity will build on existing work in the Victoria-Mayaro Forest Reserve in southern Trinidad on other taxa. The need for sampling at night, and separately in the dry and wet seasons, has yet to be determined. Spiders are soft-bodied and cannot be pinned and dried like other arthropods. Taxonomic analysis in the laboratory therefore uses alcohol-preserved specimens, a representative sample of which will be retained in the Land Arthropod Collection of the Department of Life Sciences. Additonal material for comparison is being collected on surveys of other Caribbean islands, which have been funded as separate short-term projects (see www.caribbeanspiders.com).

Jo-Anne Sewlal collecting a spider in Nevis; examining specimens in the laboratory; receiving the Caribbean Academy of Sciences / Third World Academy of Sciences Young Scientist Award 2010 from the Prime Minister of Antigua and Barbuda the Honourable Baldwin Spencer, 17th Caribbean Academy of Sciences General Meeting, St. John's Antigua, November 2010.


Funding

Supervision

Results


References

Beard, J.S. (1946). The Natural Vegetation of Trinidad. Oxford, Claredon Press.

Coddington, J.A., and Levi, H.W. (1991). Systematics and evolution of spiders. Ann. Rev. Ecol. Syst. 22: 565-592.

Cutler, B. (2005). Synotaxidae, another Trinidadian Spider (Araneida) Family. Living World, J. Trinidad Tobago Fld Nat. Club 2005: 49.

Kuznetsova, V., Grozeva, G., Sewlal, J.N. and Nokkala, S. (2007). Karyotype and male reproductive system in Arachnocoris trinitatus Bergroth. First data on the tribe Arachnocorini (Heteroptera: Nabidae: Nabinae). Fol. Biol. (Kraków) 55: 17-25.

Platnick, N.I. (2007). The world spider catalog, version 7.5. American Museum of Natural History, online at http://research.amnh.org/entomology/spiders/catalog/index.html.

Scharff, N., Coddington, J.A., Griswold, C.E., Horminga, G. and Bjorn, P. (2003). When to quit? Estimating spider species richness in a northern European deciduous forest. J. Arachnol. 31: 246-273.

Sewlal, J.N. (2005). Autecology of web-building spiders. M.Phil. thesis, The University of the West Indies, St. Augustine.

Sewlal, J.N. (2006). Identification of spider families of Trinidad and Tobago, West Indies. Living World, J. Trinidad Tobago Fld Nat. Club 2006: 44-50.

Sewlal, J.N. and Alayón, G.G. (2007). Four more Trinidadian Spider (Araneida) families. Living World, J. Trinidad Tobago Fld Nat. Club 2007: 85.

Sewlal, J.N. and Cutler, B. (2003). Annotated List of Spider Families (Araneida) of Trinidad and Tobago. Living World, J. Trinidad Tobago Fld Nat. Club 2003: 9-13.

Sewlal, J.N. and Starr, C.K. (2008). Observations of the insect Arachnocoris trinitatus (Heteroptera: Nabidae) as an inquiline in the webs of the spider Mesabolivar aurantiacus (Araneae: Pholcidae). Carib. J. Sci. 44: 132-135.


* TEDx (an independent event in conjunction with TED Ideas Worth Spreading) which invites people to talk about their life, career and research to inspire others. TEDxUWI "Caribbean Story" was held on 17 March 2012 and included speakers from a variety of backgrounds including music, marketing, architecture and science, who shared their years of experience working and doing research in the region.


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