Rainforest litter invertebrates decimated by high severity burns during Australia's gigafires

Abstract

Climate change is increasing the frequency of extreme fires. In 2019–2020, extreme fires burned 97 000 km2 of native vegetation in south-eastern Australia, affecting many areas of rainforest, which has historically burned less frequently. One year post-fires, we surveyed litter macroinvertebrates in 52 temperate rainforest sites. Sites had experienced increasing levels of fire severity (unburnt, medium severity and high severity). We asked how fire severity affected: (1) litter macroinvertebrate habitats; (2) the abundance of litter macroinvertebrate taxa per unit area; and (3) abundance relative to litter habitat (volumetric density). We also estimated the loss of litter macroinvertebrates across rainforests in the study region. High severity burns supported only a fifth of the litter volume and canopy cover as unburnt sites, lower soil moisture and higher herb cover. Medium burns were intermediate. Macroinvertebrate abundance declined with burn severity: high severity burns supported only 26% of the abundance in unburnt sites; medium severity burns supported 80% of that in unburnt sites. Patterns were similar for all taxa, with millipedes declining most. High severity fires resulted in up to 1.90 million fewer macroinvertebrates per hectare; 0.53 million fewer per hectare of medium burn rainforest. Across the study region, we estimate that 60 billion fewer litter macroinvertebrates persisted in temperate rainforests alone. Volumetric densities of many litter macroinvertebrate taxa in high severity burns were marginally higher than in unburnt sites, suggesting nutrients may be more available post-fire, or that persisting individuals become concentrated in the leaf litter. For less desiccation-tolerant groups (e.g., amphipods), density declines with increasing severity may reflect the combined impact of low soil moisture and reduced litter cover. Many taxa persisted following high severity fires, but declines were substantial, and taxa differed in their vulnerability. Longer-term monitoring is required to understand the recovery trajectory and impacts on ecological function.