Abstract

The effects of forest harvesting on wet eucalypt forest stream macroinvertebrates were examined in Tasmania, Australia. Sites were selected in second to fourth order streams across a gradient of catchment forest harvesting history and in paired reference catchments. Historical data for every forest parcel (coupe) were obtained for each catchment, detailing forest operational type, area and year of operation, length of roads and number of stream road crossings.

We observed substantial differences in macroinvertebrate community composition and abundance across the 14 site pairs. We observed higher abundances of orthoclad midges, Leptophlebiid mayflies and Elmid and Scirtid beetles, and reduced abundances of worms and Notonemourid stoneflies at forestry test sites, than at their paired reference sites. A consistent shift in community composition was observed for 12 of the 14 site pairs. This was accompanied by a shift in functional feeding group representation away from shredders at test compared to reference sites. Community compositional similarity was substantially higher between test than reference sites, with a reduction in community diversity among streams due to a history of upstream forestry.

Community dissimilarity between paired test and reference sites was negatively correlated with the average parcel age and time since initial harvesting. We observed declines in abundance with increasing average coupe age of the mayfly family Leptophlebiidae and midge sub-family Chironominae at test sites relative to reference sites. Time since initial disturbance and the number of road crossings jointly accounted for 52% of the variance in paired-site Bray-Curtis dissimilarity values, with both variables negatively correlated with dissimilarity.

We observed a shift related to catchment forestry disturbance in macroinvertebrate community composition away from taxa typical of south-east Australian ‘depositional’ headwater stream environments to greater representation of taxa characteristic of higher power, more ‘erosional’ stream reaches. This support the hypothesis of a transition from predominantly degradational responses to forest harvesting operations (in the geomorphological sense) in headwater streams toward transient ‘aggradational’ effects further downstream with concomitant biological responses.