Simplifying assessment of forest management practices for invertebrates: How effective are higher taxon and habitat surrogates for spiders following prescribed burning?

Taxonomic impediments require us to know if simpler approaches such as using higher taxa or habitat surrogates can help assess the needs of invertebrates when managing forests. This study tests whether higher taxon (families for species) and habitat (vegetation structure for species and families) surrogates might assist in determining the effects of prescribed burning on spiders. Spiders were sampled at sites representing a chronosequence of increasing ages since burning (0, 3, 6 and 9 years) by pitfall trapping and vacuuming from understorey vegetation in the South-west Botanical Province of Australia; a global biodiversity hotspot. Reassembly followed a similar pattern for both the richness of species and families. Immediately following burning, mean richness of families and species was low (8.75 and 14.5, respectively). By 3 years, however, there were significant increases in familial and species richness (13.25 and 23.75, respectively). Thereafter, taxon richness remained constant. Multiple regressions suggested habitat variables potentially influencing family and species richness were litter depth, bare ground (%), the density of dead vegetation (0–100 cm high), and the density of live vegetation (40–140 cm). The first three of these variables correlated significantly with time since burning, suggesting these components of habitat structure are useful predictors of taxon richness for spiders following fire. For assemblage composition, the congruence in post-fire response patterns between taxonomic ranks was highly significant, although the amount of variation explained was low (Rho < 0.55). In the first 9 years post-fire, there was a significant shift in the taxonomic composition of spiders at family, but not specific, rank. Family composition at recently burnt sites differed significantly from that at sites burnt 9 years previously. Correlated with assemblage composition were litter depth, bare ground, and the density of live vegetation (0–40 cm above the ground). However, the best correlation obtained by any combination of habitat variables with assemblage composition (Rho < 0.317) was less than that obtained by using family level data. We conclude, therefore, that where resources are limited and there is a need to determine the effects of forest management practices on invertebrates using surrogates, identifying spiders to higher taxa is better than relying on habitat structure.