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Grazing and tree 'clearing' alter grass-associated invertebrate assemblages in an Australian tropical grassy woodland
journal contributionposted on 05.03.2019, 00:00 by Wayne HoustonWayne Houston, Alistair MelzerAlistair Melzer
To evaluate the response of invertebrates to 'clearing' and grazing pressure impacts, a previously grazed but uncleared grassy woodland in central Queensland was manipulated to provide four grazing pressures (destocked, low, moderate and high) and two tree treatments (with trees, i.e. untreated, and 'cleared', i.e. trees and saplings poisoned with herbicides), with two replicates of each, making 16 plots in total. Monitoring was carried out in 1998, approximately four years post-establishment of the treatments. Two types of samples were taken: Pitfall for ground-active fauna and suction for grass-associated fauna. Overall, 23 orders of invertebrates were sampled by pitfalls and 22 by suction. Significant effects of grazing on invertebrate assemblages were detected by both methods, but no effects were detected from 'clearing'. There was a gradation in the invertebrate assemblages from low to high grazing pressure, the invertebrate assemblages in the paddocks with the highest grazing differing most from those in the destocked and low-grazing-pressure paddocks. Notwithstanding the lack of effect of 'clearing' at the assemblage level, ground-active invertebrates and some grass-associated invertebrates increased in abundance following 'clearing', possibly reflecting an increase in the quality of the resource base. However, ground-active invertebrates and grass-associated invertebrates showed contrasting responses to grazing pressure, the former increasing, possibly reflecting changes in trapability due to the more open vegetation structure at higher grazing pressures. The abundance of grass-associated invertebrates declined by 50-80% with increased grazing-although with complex changes in assemblage structure. Despite those declines, the basic trophic pyramid remained, and, along with that, the potential for recovery of invertebrate assemblages and associated ecosystem services with reduction in grazing intensity. With 80% of Queensland grazed, the reduction in invertebrate abundance has implications for the viability of insectivores, particularly mobile fauna such as birds, at a landscape scale. It is recommended that the utility of using suction samples as a basis for assessing ecosystem functional health be investigated and that grazing pressure be reduced to increase invertebrate assemblages of rangeland pastures and to improve sustainability. © 2018 Australian Rangeland Society.