Riparian gravel banks of upland and lowland rivers in Victoria (south-east Australia): arthropod community structure and life-history patterns along a longitudinal gradient

Abstract

Riparian sand and gravel banks are inhabited by a fauna that is well adapted to varying river water levels and frequent inundation of the banks. Arthropods found in these habitats were studied from November 1998 to January 1999 in the upper and lower floodplains of the main rivers and tributaries in ten major catchments in the Victorian Alps. Wolf spiders (Lycosidae, 68%) and ground beetles (Carabidae, 7.8%) were the most abundant arthropods, with densities averaging 14.6 ± 1.8 (s.e.) and 2.3 ± 0.4 individuals m–2 respectively. Species composition and wolf spider densities changed substantially between upland and lowland rivers. These differences correspond with changes in altitude, shading, and gravel and gravel bank size. Comparison of our results with similar studies conducted in temperate Northern Hemisphere floodplains showed significant differences. Carabidae, not Lycosidae, are the dominant arthropod group in Northern Hemisphere floodplains. Wolf spider densities are higher in upper than lower reaches of rivers in the Victorian Alps, but do not change along rivers in the Northern Hemisphere. In contrast, carabid beetles showed similar densities between upland and lowland floodplains in Victoria, but increase in density along rivers in the Northern Hemisphere. A second, monthly sampling program on gravel banks along the Avon River (Gippsland) over a one-year period in 1996 and 1997 provided information on the life histories of eight common gravel-bank arthropods: Venatrix lapidosa, V. arenaris, two undescribed Artoria species (‘A’, ‘B’) (Lycosidae), Eudalia macleayi, Elaphropus ovensensis, Perileptus constricticeps and an unidentified Loxandrus species (‘B’) (Carabidae). Artoria sp. A and sp. B are diplochronous. Despite its smaller size, Artoria sp. B matured one month after Artoria sp. A. Lower temperatures at upland streams and rivers, the typical habitat for Artoria sp. B, may delay its development in comparison with Artoria sp. A, which is generally found further downstream. The life histories of both lycosid spiders and carabid beetles, characterised by prolonged reproductive period and short larval development, appear to have some adaptive value in regard to the disturbance-prone environment.