Effects of fire on grass-layer savanna macroinvertebrates as key food resources for insectivorous vertebrates in northern Australia

摘要

This paper documents the effects of fire on grass-layer invertebrates in tropical savannas of the Kimberley region of north-western Australia, in the context of resource availability for consumers. Inappropriate fire regimes have been identified as a factor threatening a number of vertebrate groups, including small mammals, across northern Australia, and a possible mechanism might be through the effects of individual fires or fire regimes on food availability. We test for a fire effect on grass-layer invertebrate resources, which may affect insectivorous savanna vertebrates. Wet season sweep-net invertebrate samples were taken in two tropical savanna habitats, with contrasting laterite and sandstone substrates, in 2008, 2009 and 2010. Sites were stratified by post-fire interval to analyse invertebrate successional change after fire. In addition, experimental burns were implemented in 2010 to investigate immediate post-fire invertebrate responses. Total invertebrate numbers declined by 80–90% immediately (1?week) following fire, reflecting the loss of grass-layer habitat. Of the commonly sampled invertebrate groups, Araneae, Coleoptera, Hempitera, Lepidoptera, Formicidae and Diptera were all reduced in numbers immediately post-fire, whereas Orthoptera showed no immediate post-fire decline. Invertebrate numbers were rapidly restored to pre-fire levels by the first wet season after fire, and no detectible change was observed in numbers or composition from 1 to 3–4?years post-fire (the longest post-fire interval available). This suggests that the effects of individual fires on grass-layer invertebrates are very short-lived. Such short-lived post-fire responses among grass-layer invertebrates, plus evidence that most ground-layer invertebrate groups are fire-resilient, suggest that food resource limitation is not a tenable explanation for fire-related declines among insectivorous savanna vertebrates. However, wet season burning could result in significant invertebrate resource depletion in highly flammable habitats (Triodia and Sorghum spp. savannas) during important vertebrate breeding/recruitment periods, if burning is extensive and if grass-layer invertebrates do not recover within a few weeks of fire.