Influence of some environmental factors on indigenous earthworms in the northern jarrah forest of Western Australia

Abstract

Indigenous species of earthworms (Family Megascolecidae) are widely distributed throughout the northern jarrah forest of Western Australia. They occur not at random, but in an aggregated pattern well described by a negative binomial. Earthworms are only active in the topsoil (to depth of 5.5 cm) when soil moisture (gravimetric) exceeds 4%. This occurs from May until November. The number of species occurring in jarrah forest at any place, and the frequency of occurrence, biomass and density of earthworms, vary with average annual rainfall. In the zone of highest rainfall six or seven species occur together, frequency of occurrence is 40-60%, and total biomass and total density do not exceed 8 g m-2 and 40 m-2 respectively, and generally average much less. In the lowest rainfall zone only one or two species occur in any one locality, with total density much less than 10 m-2. Multiple regression analyses between four earthworm variables and 20 site and soil characteristics were highly successful in that up to three predictor variables yielded coefficients of determination exceeding 0.70. The most useful predictor variables were rainfall zone, forest basal area, soil pH and proportion of silt and clay in the topsoil (to depth of 15 cm). Recurrent low intensity (spring) fires and a single moderate intensity fire (summer) have minor direct and indirect effects on earthworms. Logging also has scant effect on their frequency of occurrence. The land use most adverse to earthworms is the clearing of forest for mining of bauxite or gravel. The effect on earthworms of clearing of forest and subsequent reforestation with pines varies: in one case there was an adverse effect, whereas in two others there was no difference.