Is the reassembly of an arid spider assemblage following fire deterministic?


Present address: Phoenix Environmental Sciences, PO Box 857, Balcatta, WA 6021, Australia (Email:


Fire is a common disturbance in many ecosystems, including arid Australia. Understanding whether fauna respond in a deterministic manner towards a single end-point, or to multiple states, is of crucial importance for conservation management. Why different taxa or assemblages display single or multiple end-points is also important to develop a synthetic theory of succession. To examine the post-fire changes in assemblages of spiders, we established a chronosequence study in spinifex habitat of central Western Australia. Ground-active spiders were pitfall-trapped over nine months in sites representing experimental fires (0 and 0.5 years post-fire) and wildfires (3, 5, 8 and 20 years post-fire). There were significant non-linear changes in species richness, evenness and composition of spiders with increasing post-fire age. For all three measures, the assemblage appeared highly deterministic, converging towards the long unburnt state. Similarity in richness, evenness and species composition to the 20-year-old sites all increased with increasing time since fire (3–8 years). However, experimentally burnt sites did not neatly fit this sequence. We consider two alternative hypotheses to explain this second trajectory: inertia within the system or the rapid migration and recolonization from nearby surrounding unburnt areas. Analyses indicated that half of the 179 species had significant preferences for, or were restricted to, particular post-fire ages. This suggests that adequate pyrodiversity, both in terms of post-fire ages and/or scale and intensity of fires, may be important for the conservation of spiders in this habitat. However, owing to the high number of singletons and low indicator values, the significance of this result for conservation management remains equivocal. Despite this, the high degree of determinism provides hope that managers can develop a good predictive understanding of post-fire successional changes in spider assemblages in arid Australia.