Trial by fire: Social spider colony demographics in periodically burned grassland

Abstract

Nature conservation authorities in southern Africa burn the grassland in nature reserves to reduce the incursion of woody plants and to encourage the growth of new grass for grazing. Studies have shown significant negative effects of burning on the abundance, species diversity and community composition of smaller vertebrates and invertebrates. Social arthropods are likely to suffer from repeated burning, as destruction of the colony means a permanent loss of the entire reproductive unit. Nests of social spiders of the genus Stegodyphus (Eresidae) are a prominent feature of African savannas and their size and visibility make them potentially good indicators of ecological consequences of fire. The colonies are sedentary and their nests may persist for several years, although individuals have an annual life cycle. We explored the mortality patterns and the effects of burning on survival of colonies of two species of social spider, Stegodyphus mimosarum and S. dumicola that live sympatrically. We compared colony mortality in burned and unburned areas to determine if colony mortality was due to burning or, alternatively, to natural senescence. Analysis of five years of nest mortality data together with information on the burning regime in Spioenkop Nature Reserve, KwaZulu Natal province (South Africa), indicates that colony extinction in S. mimosarum is independent of the burning regime, while burning is a significant cause of colony mortality in S. dumicola. The different responses of the two species are likely a result of different colony dynamics and nesting sites.

Key words: Stegodyphus, Eresidae, extinction, fire ecology.