The effects of long-term burning regimes on savanna spider assemblages

Abstract

Fire is an important disturbance in many biomes throughout the world and plays a major role in savannas, where it affects the composition and structure of vegetation, which in turn structures faunal assemblages. Higher intensity and more frequent fires tend to simplify habitat structure, leading to a decrease in faunal abundance and diversity. Fire is often used as a tool in conservation areas, but the effects of fire, in particular long-term burning regimes, on fauna is largely unknown, especially in the case of invertebrates. This is problematic given the pivotal roles that invertebrates play in ecosystem functioning. The study aimed to address this paucity of studies on the effect of long-term burning regimes on invertebrates by making use of the longterm fire experiment in Kruger National Park, South Africa, initiated in 1954 and using spiders, which are important predators that play critical roles in ecosystem functioning and are a highly diverse group, known to be sensitive to changes in vegetation structure. Specifically, the study aimed to investigate whether spider taxonomic diversity, composition and functional traits were affected by four burning regimes and how this response varied across three different savanna types along a rainfall gradient. This was explored in the context of changes in vegetation structure. It was hypothesised that spider assemblages would be influenced more by high intensity, frequent fires and that the response to the burning regimes would be greatest at the wettest savanna site, where burning has a greater effect on vegetation structure. The burning regimes used were annual and triennial, high intensity winter burns, respectively, and triennial, low intensity summer burns, which were all compared to unburnt plots, enabling comparisons of the effects of fire intensity, frequency and the presence of fire versus its absence. Burning regimes were replicated three times in three savanna types and spider sampling took place in autumn, spring and summer using a variety of sampling techniques in order to encompass seasonal variation in spider assemblages and to sample spiders in all components of the savanna systems. Spiders were found to be highly resistant and resilient to changes in habitat structure related to the burning regimes examined, with no major differences in abundance, species richness, assemblage composition and distribution of spiders in the various functional trait groupings investigated across burning regimes. However, there was a response by spiders between annual, high intensity burns and unburnt plots at the wettest savanna site in terms of abundance suggesting the possibility of a greater response by spiders to burning in wetter savannas as hypothesised. Spider abundance and species richness decreased along the rainfall gradient and assemblages were shown to change across the study areas as predicted. This study is one of the most comprehensive studies on savanna spiders (and the effects of long-term burning regimes on them) in terms of spatial and temporal extent and sampling intensity but more studies are required to further investigate the questions asked in light of the enormous turn-over in species that is encountered when sampling spiders.