The Effects of Habitat Fragmentation and Livestock-Grazing on Animal Communities in Remnants of Gimlet Eucalyptus salubris Woodland in the Western Australian Wheatbelt. I. Arthropods

Abstract

The effects of habitat fragmentation (remnant biogeographic characteristics) and livestock activity (grazing, trampling) on arthropod communities were examined within 26 remnants of gimlet Eucalyptus salubris woodland in the central wheatbelt of Western Australia. Significant correlations between remnant biogeographic variables were between remnant area and connectivity (+ ve, P < 0001), and between connectivity and distance from the study remnant to the nearest patch of native vegetation (- ve, P < 0.001). Remnant disturbance indices (sheep faecal pellet density, percentage cover of weeds) were significantly correlated with remnant biogeographic characteristics (P < 0.01). Small and poorly connected remnants showed significantly higher intensities of disturbance than larger and better connected remnants. When disturbance indices were used to categorize study remnants into groups with high, moderate or low disturbance, remnants with high intensities of disturbance had significantly less lichen, litter and shrub cover. Highly disturbed remnants were associated with : fewer scorpion species, lower termite and ant diversity (Shannon-Wiener); a lower abundance of scorpions, termites and mygalomorph spiders; more beetle species and higher beetle diversity, and greater abundance of earwigs and beetles. Cockroach, earwig and ant species richness showed no significant response to disturbance. Species richness of termites, and the abundance of lycosid and idiopid (mygalomorph) spiders, isopods, cockroaches and ants, was highest under moderate disturbance. Significant biogeographic covariates were area (abundance of araneaomorph spiders, 'associated subordinate' and 'opportunistic' ants; richness of carabid, scarabid and 'other' beetle species, subordinate and opportunistic ants), connectivity (richness of termites, scarabid beetles) and distance to the nearest native vegetation (richness of 'dominant' ants). When disturbance and biogeographic effects were combined, total termite richness, and the richness of termite functional groups, declined markedly in highly disturbed, small and poorly connected remnants. Termite communities in relatively undisturbed remnants were more similar in species composition to communities in moderately disturbed quadrats than to communities in highly disturbed quadrats. Community similarity values for ants and beetles were similar across the study quadrats with different degrees of disturbance. Arthropod communities were also examined by canonical variate analyses across remnants with different degrees of disturbance, using total abundance and richness, and abundance and richness of predators (scorpions, spiders, carabid beetles, ants) and detritivores/herbivores (termites, isopods, earwigs, cockroaches, weevil and scarab beetles). Effective site separation into the three disturbance categories was found for abundance and richness of all arthropods, and for predators alone. Abundance and richness of detritivores/herbivores separated into two groups of sites: high disturbance sites, and sites with low or moderate disturbance with no separation. In stepwise regression analyses, lichen cover, weed cover and sheep faecal pellet density were the most significant indicators of faunal abundance, richness and diversity. Remnant biogeographic variables explained a low percentage of variation in faunal characteristics. Habitat disturbance was the major influence on the arthropod communities, with remnant biogeographic factors consistently explaining low variations in the abundance or diversity of the fauna. The implications of our findings for the management of remnant vegetation are discussed.