Journal of Arachnology
Published by: American Arachnological Society
Journal of Arachnology 33(2):269-279. 2005
doi: 10.1636/CT05-43.1
INFLUENCE OF GRAZING BY LARGE MAMMALS ON THE SPIDER COMMUNITY OF A KENYAN SAVANNA BIOME




aDepartment of Invertebrate Zoology, National Museums of Kenya, P.O. Box 40658-00100 GPO, Nairobi, Kenya, cmwarui@yahoo.com;
bDepartment of Zoology and Entomology, Rhodes University, 6140 Grahamstown, South Africa;
cDepartment of Plant Sciences, University of California, Davis, 95616 USA;
dInvertebrate Section, Department of African Zoology, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium
Abstract
Pitfall trap and sweep net samples were taken over a period of fifteen months (2002– 2003) in the Kenya Long-term Exclosure Experiment (KLEE), in which the presence of domestic and wild herbivores have been independently manipulated since 1995. ANOVA and ANCOVA showed that the exclosure treatments significantly affected plant cover, with the presence of cattle significantly reducing the relative vegetation cover and spider diversity. Herbivory by indigenous mega- and meso–herbivores did not have a significant influence on the diversity of the spider fauna, but abundance of three dominant species (Cyclosa insulana Costa (Araneidae), Argiope trifasciata Forskål (Araneidae) and Runcinia flavida Simon (Thomisidae)) decreased in cattle-grazed plots. In contrast, Aelurillus sp. became more prevalent where cattle have been grazing. Multivariate analyses revealed that the spider community responded to grazing pressure by aggregating into three groups that reflected control, cattle grazing and non-cattle grazing clusters. It was probable that the direct effects on vegetation mediated an indirect influence of herbivores on spider diversity. The relative vegetation cover was a positive predictor of spider diversity. Spider communities were found to be an indicator of the activity of mammals and could be used as indicators of land use changes and for bio-monitoring.
Received: 5 April 2005; Revised: 1 September 2005
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Figure 1. Schematic representation of the experimental design of the KLEE study plots at Laikipia, Kenya. Letters in each plot represent the herbivores allowed in: C = cattle, W = meso-herbivores, M = mega-herbivores, O = control (all large mammalian herbivores excluded). N, C and S represent north, central and south blocks respectively. Each plot measures 200 × 200 m. The distance between the furthest placed plots (between north and south block) is approximately 2 km. Adapted from Young et al. (1998)
Figures 2–3.—2. Effects of ‘cattle’ (levels: absent [treatmentsW and MW] vs. present [WC and MWC]) and ‘megaherbivores’ (levels: absent [W and WC] vs. present [MW and MWC]) on relative vegetation cover (mean + SE). Two treatments (O and C) were omitted from the data set so that the analysis was fully crossed. The interaction term was not significant (P = 0.28). 3. Effects of ‘cattle’ (levels: absent [O and W] vs. present [C and WC]) and ‘mesoherbivores’ (levels: absent [O and C] vs. present [W and WC]) on relative vegetation cover (mean + SE). Two treatments (MW and MWC) were omitted from the data set so that the analysis was fully crossed. The interaction term was not significant (P = 0.79)
Figure 4. Effects of ‘cattle’ (levels: absent [O, W and MW] vs. present [C, WC and MWC]) and ‘herbivores’ (levels: herbivores absent [O and C], only mesoherbivores present [W and CW], and both meso- and megaherbivores present [MW and MWC]) on total number of spider species from sweep-netting samples (mean + SE). No treatments were omitted from the data set. The interaction term was not significant (P = 0.81)
Figure 5. Multidimensional scaling (MDS) ordination of the spider community in the sweep-netting samples of spiders collected at Laikipia, Kenya in 2001–2002, with convex hulls superimposed to enclose regions characteristic of control, cattle and non-cattle treatments. In all cases the first letter of any code represents the three study blocks, namely north (N), central (C) and south (S). All other letters represent the animals present, where O = control, C = cattle, W = meso-herbivores, and M = mega-herbivores
Table 1. Results of ANOVA on effects of the factors ‘cattle’ (levels: absent [treatments O, W and MW] vs. present [C, WC and MWC]) and ‘herbivores’ (levels: herbivores absent [O and C], only meso-herbivores present [W and CW], and both meso- and mega-herbivores present [MW and MWC]) on relative vegetation cover. The codes for the treatment abbreviations are (cf. Fig. 1): O = control (no large mammalian herbivores); W = meso-herbivores; M = mega-herbivores and C = cattle. No treatments were omitted from the data set. * = Significant at α = 0.05
Table 2. Correlations between relative vegeta tion cover and four measures of diversity (Shannon-Wiener diversity index [H′], Margalef's richness in dex [d], Pielou's evenness index [J′] and total spider species [S]) for data sets generated at Laikipia, Kenya in 2001–2002 using sweep-netting and pit fall-trapping samples. df = 18. * = Significant at α = 0.05
Table 3. Analysis of covariance (ANCOVA) to establish the effects of the factors ‘meso-herbivores’ (levels: absent [O and C] vs. present [W and WC]) and ‘cattle’ (levels: absent [O and W] vs. present [C and WC]) and two covariates, relative vegetation cover and total monthly rainfall, on the abundance of Cyclosa insulana, Argiope trifasciata, Runcinia flavida and Aelurillus sp recorded at Laikipia in 2001– 2002. The codes for the above abbreviations are such that O = control (no large mammalian herbivores); (W) = meso-herbivores; (M) = mega-herbivores and (C) = cattle. * = Significant at α = 0.05
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