Macroinvertebrate community structure in the streams of the southern forest of Western Australia: The influence of seasonality longitudinal gradients

The macroinvertebrate fauna of Carey Brook, a 28 km tributary of the Donnelly River (and typical of the streams found in the southern forest region of Western Australia) was sampled at twelve sites between November 1988 and August 1989. The distribution of taxa among invertebrate families was similar to that of jarrah forest streams further north although more dipteran and fewer coleopteran taxa were collected in Carey Brook. Most environmental variables and flow descriptors followed longitudinal gradients, however, seasonality appeared to have the most influence on community structure. The abundance and distribution of most functional feeding groups supported the predictions of the River Continuum Concept (RCC). Categorisation of invertebrates into flow exposure groups and physical descriptors of water movement supported stream hydraulics theory and indicated that elements of both this theory and the RCC could be usefully combined to explain longitudinal changes in macroinvertebrate distributions. The immediate effects of forestry activities on community structure were examined in the headwaters of Carey Brook. The fauna at four sites on an upland stream which ran through a logging coup were compared with the fauna at four nearby undisturbed sites, before and after clearfelling in 1989 and 1990. Taxonomic richness and invertebrate abundance did not appear to be affected greatly by clearfelling, however, the composition of the macroinvertebrate fauna in the affected stream changed in comparison to the undisturbed sites after logging commenced but returned to pre-logging composition after winter and spring rains had ceased. The changes in the composition of the fauna in the disturbed stream were associated with increases in suspended solids. Changes in the macroinvertebrate communities eight years after logging were examined in two sets of paired catchments near Carey Brook. Both paired catchments contained an undisturbed stream and another where clearfelling had been taken to the stream edges. One of the paired catchments also contained a third stream where a 100 m wide riparian buffer zone had been retained during clearfelling. Differences in taxonomic richness and abundance of invertebrates between undisturbed and clearfelled streams were obscured by differences between sites within each stream. However, differences in community composition were observed in both catchments and these were associated with differences in conductivity, the amount of benthic organic material and total nitrogen. The 100 m wide buffer appeared to be effective in ameliorating long term disturbance due to clearfelling. The nature of the invertebrate community structure of the fauna in the streams of the southern forest region of Western Australia are discussed in relation to the longitudinal and seasonal distribution of taxa and the results of both immediate and long term effects of forestry activity on the macroinvertebrate fauna. The fauna appears to be individualistically arranged with both equilibrial and non-equilibrial attributes. Management implications of this study for future forestry activities are also discussed, particularly the importance of protecting the fauna and integrity of first order streams and others from forestry activities with riparian buffer strips