Tree basal area and dead wood as surrogate indicators of saproxylic insect faunal integrity: a case study from the Australian lowland tropics

Abstract

Saproxylic (dead wood associated) insects are well-known in Europe for their associations with the mature timber habitat (old trees and dead wood)—features which have a bearing on their response to forest management and which have encouraged their use as indicators of ecological continuity. In the tropics and elsewhere, their relationships with the mature timber habitat have yet to be characterised, preventing their consideration in determining the sustainability of forest management. Furthermore, the practical difficulties of adequately sampling a tropical saproxylic insect fauna may well preclude their consideration as indicators in their own right, in which case surrogate structural indicators will need to be sought. To investigate this, the saproxylic beetle fauna of lowland rainforest was studied in a series of sites in the Daintree region of northeast Queensland, Australia. Study sites were chosen to represent a range of intensities of past management from old-growth forest, through selectively logged forest to re-growth forest that had arisen following previous clearance. Beetle abundance, incidence, species richness and assemblage composition were considered in relation to various mature timber habitat attributes, comprising living tree basal area, coarse woody debris and standing dead trees. Volume of coarse woody debris proved the strongest positive correlate of species richness, while the basal area of larger-diameter trees proved a more robust indicator of abundance, incidence and assemblage composition, and was also correlated with species richness. Given its ease of measurement compared to recording dead wood or saproxylic beetles, the basal area of larger-diameter trees is considered an appropriate surrogate measure of saproxylic beetle faunal integrity in tropical rainforest. Given the wider ecological role of old trees in forest ecosystems, the development of locally-defined indicators based on larger-diameter trees for monitoring forest management is strongly encouraged.