The immediate impacts of timber harvesting on terrestrial invertebrates inhabiting medium rainfall jarrah forest in south-west Western Australia

Karin Henriette Strehlow

The ground-dwelling invertebrate fauna of medium rainfall jarrah forest was surveyed in three forest blocks (Kingston, Warrup and Winnejup) approximately 25 km north-east of Manjimup in the south-west of Western Australia. Invertebrates were collected by means of pitfall traps on 18 sampling occasions over a period of 22 months (May 1994-March 1996). Approximately 500 000 invertebrates were collected from thirty-five Classes, sub Classes or Orders. The Blattodea, Orthoptera and Araneae were further identified to morphospecies and males and females were distinguished. The forest litter invertebrate community was dominated by Collembola (springtails) (60.8% of total abundance), the Formicidae (ants) (16.8%), Diptera (flies) (6.1%) Coleoptera (beetles) (4.7%) Apocrita (wasps), (2.8%), Araneae (spiders) (2.6%) Acarina (mites) (2.0%) Hemiptera (true bugs) (1.4%) Blattodea (cockroaches) (0.4%) and Orthoptera (crickets and grasshoppers) (0.4%). These accounted for 98% of the total abundance. Of the three taxa identified to morphospecies, spiders were the most diverse, with 31 families and 108 morphospecies present, followed by the grasshoppers and crickets with 3 subfamilies and 67 morphospecies and the cockroaches with 4 families and 32 morphospecies. Six arachnid families (the Actinopidae, Idiopidae, Micropholcommatidae, Nemesiidae, Orsolobidae and Toxopidae) with Gondwanan affinities were present. Marked seasonal trends were present, with overall invertebrate activity (excluding Collembola) highest during spring and lowest during winter. The main factors affecting activity were temperature and rainfall. Changes in the numerical dominance of the taxa occurred between seasons and between years. Marked spatial variability was also evident and appeared to be associated with changes in microclimatic conditions and small-scale spatial variations in habitat. The responses of these invertebrate communities to timber harvesting practices (clearing without post-harvest burn) were examined as part of a larger integrated study (the Kingston Project) conducted by the Western Australian Department of Conservation and Land Management (CALM). A modified BACI (Before/After Control/Impact) design was used. Twenty sites, 8 Internal Controls, 4 External Controls and 8 Impact sites were sampled simultaneously before and after logging. Sampling was undertaken on 8 occasions prior to logging and 10 following logging. Logging occurred between March and April 1995. No significant impacts on total invertebrate abundance and richness were observed after logging. Moreover, only five taxa, Blattodea, Coleoptera, Diptera, Hemiptera and Orthoptera, showed significant changes in abundance, with Blattodea being the most severely affected. Impacts detected by ordination were also of short duration, with post-logging communities resembling those of undisturbed sites after 10 months. Retrospective power analysis revealed that the experimental design used had sufficient power (0.8+) to detect a 30-40% change in total invertebrate abundance. Moreover, the design had power to detect changes of 10-50% for most taxa. Identification of three taxa to morphospecies revealed significant effects of logging, not only on the taxa found to be sensitive to this disturbance (Blattodea and Orthoptera), but also in Araneae, which had shown no impacts at the level of Order. However, for spiders at least, identification to the level of family was preferable to morphospecies as the impacts of logging were masked by environmental noise at those resolutions. The impact of logging on these assemblages was also only of a short duration. Logged sites followed the same seasonal patterns as control sites in the ordination plots, indicating that seasonal and inter-annual climatic changes appear to be more important determinants of community structure and function than a logging event.

The immediate impacts of timber harvesting on terrestrial invertebrates inhabiting medium rainfall jarrah forest in south-west Western Australia

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