Spider sampling

Field work was conducted over three distinct seasons, i.e. summer, post monsoon and winter through 2008 and 2009. Pitfall traps were exposed for two weeks on each occasion.

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Pitfall traps were used to target ground-active spiders and their potential prey. At each location, 12 traps (9cm diameter, with 20ml of ethylene glycol) were randomly placed along a transect running through the sada and into the adjacent forest or woodland. Past research has shown that the most reliable way of monitoring invertebrate biodiversity is to sample entire invertebrate assemblages. This can involve large numbers and a great variety of specimens (Andersen, 2008). The limitations of pitfall traps have been discussed by many authors (e.g. Luff, 1975; Topping and Sunderland, 1992; Southwood and Henderson, 2000). Pitfall catches may be influenced by factors such as trap placements, vegetation type, weather conditions, interference by animals and humans. While pitfall traps do not provide an absolute estimate of abundance they have been shown to provide a good approximation of the relative number of species in a range of habitats. Sabu and Shiju (2010), compared the efficacy of pitfall trapping, Winkler and Berlese extraction methods for estimating ground dwelling arthropods in moist-deciduous forests in the Western Ghats and found that highest abundance and frequency of most of the represented taxa indicated pitfall trapping as the ideal method for sampling of ground-dwelling arthropods. Sabu et al. 2011, found that pitfall trapping was most effective for qualitative data for most invertebrate groups.

Pitfall trapping can yield large numbers of singletons (i.e. a single individual representing a species) and a meta-analysis of 71 published studies by Coddington et al. (2009) reported an average of 32% singletons. They suggested that very high percentages of singletons indicate undersampling, but they also recognised that undersampling is virtually inevitable in most tropical regions when dealing with arthropods.

Most previous spiders studies conducted in the Western Ghats have used hand picking as the preferred method to gather spider data (Sudhikumar et al. 2005).

The identification of sampled spiders was done using resources provided by Tikader (1965, 1980, 1982,1987), Koh (1996), Murphy (2000) and Dippenaar (2002).

4.3.4 Sorting and identifying

The contents of the pitfall traps were removed after two weeks in each season and were

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into morphospecies on the basis of characters observed under a dissecting microscope and then classified into broad taxa (Appendix 1). Using morphospecies in place of true species as unit taxa allows thorough comparisons between samples and calculations of biodiversity, in case specimen names are unknown due to the non-availability of identification keys and field guides for many taxa. Only adult specimens were included in the analysis due to uncertainties in classifying juvenile spiders.

4.3.5 Analysis

This data set was used to compare total species richness and abundance between the sada and forest habitat types) and for analysis of assemblage composition and recognition of indicator taxa for both habitats. It also compared the species composition over seasons and noted any

significant differences between year one and two if relevant.

The total abundance of each taxon was tabulated from the data for each season in each year and were sorted to rank abundance and then graphed. An expected result in biologically diverse communities is that a few taxa are present at very high abundance, further taxa exhibit

intermediate abundance but most are relatively rare. This pattern can be indicative of a variety of processes including competition. Very rare occurrences, such as singletons, may be indicative of truly rare species in the sampled habitat or rare vagrants not typical of the habitat. In further analysis, very rare species are typically removed from the dataset in order to extract the main patterns.

Sites were ordinated on the basis of their spider faunas using the ecological analysis package PC- Ord (McCune & Mefford, 1999). Ordination is a multivariate analytical method that arranges sampling units along axes such that similar sites are plotted close together and dissimilar sites are further apart. The result is an objective summary of the relationship between sampling units in a low dimensional species space. The goal is to reveal underlying structure in the data that

represent patterns of species occurrence as determined by environmental variables. The Non- metric Multidimensional Scaling (NMS) used in this study is an ordination method that is well suited to data that are non-normal or are on arbitrary, discontinuous, or otherwise questionable scales. NMS is generally the best ordination method for community data. A Monte Carlo test of significance was included.

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A Multi-Response Permutation Procedures (MRPP) test, which is a non-parametric procedure for testing the hypothesis of no difference between two or more groups of entities, was also

performed. I compared species community composition between the two habitats using the 16 most abundant ant taxa. This reduces the influence of rare or poorly sampled taxa. The input was therefore: apriori groups = 2 (as defined by habitat type); data has 4 locations x 2 habitats; weighting option: C(I) = n(I)/sum(n(I)); distance measure: Euclidean (Pythagorean). With-group dissimilarities are used to calculate a statistic, delta. The probability of a delta this small or smaller is then determined through Monte Carlo permutations. Permutations involve randomly assigning sample observations to groups. The significance test is then the fraction of permuted deltas that are less than the observed delta, with a small sample correction. The effect size independent of sample size is called A (=the chance-corrected within group agreement) and calculated as A = 1 - (observed delta/expected delta). The statistic A is commonly given as a descriptor of within-group homogeneity compared to the random expectation.

An Indicator Species Analysis provides a simple, intuitive solution to the problem of evaluating species associated with groups of sample units. It combines information on the concentration of species abundance in a particular group and the faithfulness of occurrence of a species in a particular group. It produces indicator values for each species in each group as calculated by the method of Dufrene & Legendre (1997). These values are then tested for statistical significance using a Monte Carlo technique.

The fauna was also allocated to functional guilds in order to compare the profile of spiders between habitats. Guilds are defined as group of species that exploit the same class of environmental resources in a similar way. In the case of spiders these resources are a

combination of niche and prey (as mediated by hunting method). There are some limitations with the guild concept in that there is no strictly objective criteria for assigning guild

membership, the limits on membership are not always clearly defined and the causes of guild structure are largely unresolved for most groups of animals. Nevertheless, they are a useful tool for summarizing community organisation and for comparing the profile the local faunas.

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