Changes in spider assemblages along grassland and savanna grazing gradients in northern Australia

Spiders are beneficial predators that respond to those land uses that modify their habitat. Cattle grazing is an extensive land use across northern Australian rangelands, yet the impact of grazing on spider habitat is poorly understood. In this study, we evaluated the way in which spider assemblages varied between grassland and savanna habitats in the Victoria River District of the Northern Territory, and also between dry and wet seasons in the savanna habitat. We also investigated changes in spider assemblages, and some common taxa, with distance from cattle watering points, which was used as a surrogate for a gradient in grazing intensity. Spiders were sampled using pitfall traps and sweep nets along two distance from water-grazing gradients. The first gradient was in savanna on calcareous red loam soils at Kidman Springs, and the second was in grassland on cracking black clays at Mount Sanford. Ordinations revealed that spider assemblages differed between grassland and savanna, and between the late-wet and late-dry seasons in savanna. Spider assemblages also markedly changed along the two distance-grazing gradients in response to habitat changes. For example, orb weavers in the family Araneidae increased in abundance as the cover of perennial grasses and litter increased with distance from water; this response was consistent for both grassland and savanna gradients. Patch type was also important to the distribution of spiders. For example, more individuals of Habronestes sp. were caught when the local habitat was patches of perennial grasses rather than patches of annual grasses and litter, bare ground, or tree-shrub canopy. These results suggest that spiders are good indicators of the habitat changes that may occur with grazing in the rangelands of Australia.