Impacts of wildlife grazing on pastures in the Midlands, Tasmania

Management of Tasmania‚ÄövÑvºs native and introduced wildlife on private land is a contentious issue for landowners, animal welfare groups and the Tasmanian State Government. In 2005 the use of the poison 1080 (sodium monofluoroacetate) to kill wildlife was banned from use on public lands and the State Government has planned to cease all use by 2015. Many farmers believe that the impact of grazing by native wildlife on pastures is significant and results in a considerable financial impost. However, only limited research has been undertaken to quantify this wildlife grazing impact. Grazing and browsing wildlife include Forester kangaroo (Macropus giganteus tasmaniensis), Bennett‚ÄövÑvºs wallaby (Macropus rufogriseus rufogriseus), Tasmanian pademelon (Thylogale billardierii), brushtail possum (Trichosurus vulpecula) and fallow deer (Dama dama). Results of a grazing impact study in the Midlands region of Tasmania found that in the year 2009 alone, average pasture loss for the area 0-800 m from the native vegetation edge was 1,730 kg dry matter (DM)/ha. These losses of pasture decreased with increasing distance from native vegetation and varied between 0-100% depending on season and distance from native vegetation. Periodic harvests of pasture plots and collection of wildlife faecal pellets indicated shifts in grazing behaviour with reference to seasonal pasture feed availability. Pasture losses and faecal collections were lowest during spring 2009, while pasture losses were greatest during winter 2008, matching highest and lowest pasture growth rates over the experimental period. Production of perennial and annual grasses was greater in protected plots (areas protected by grazing exclusion cages) than exposed plots (not protected by cages), while the amount of subterranean clover (Trifolium subterraneum) increased in 2009 in exposed plots possibly due to reduced competition from grasses. Composition of annual grasses was greater in enclosed plots in close proximity to the native vegetation and the amount of bare ground was greater in exposed plots. Exclusion of grazing for 2 years had no significant (P>0.05) effect on soil health parameters such as: ammonium nitrogen, nitrate nitrogen and organic carbon levels, pH, electrical conductivity, and root biomass. Microbial analysis also indicated no significant (P>0.05) effect on bacterial biomass, fungal biomass, total active microbial biomass, and fungal/bacterial ratio. These results indicated that either 2 years may not have been a long enough trial period to detect changes in soil health, or that the size of exclosure treatments may have been too small to prevent buffering influence from outside the exclosure. A study investigating the influence of grazing damage during pasture establishment found that wildlife grazing had a significant (P<0.05) effect on production of all 4 pasture types sown. Pasture types containing phalaris (Phalaris aquatica) produced the highest DM and had greater ground cover than pasture types based mainly on ryegrass (Lolium perenne) and cocksfoot (Dactylis glomerata). Pasture biomass losses under some conditions were found to be as high as 100% within 25m and 68% within 800 m of native vegetation. However, feed availability was found to be a large determinant in the distance and direction wildlife will travel to graze. Continued exposure to wildlife grazing resulted in a higher proportion of bare ground and reduced production of annual and perennial grasses. Control of wildlife grazing during pasture establishment may be necessary to reach optimum production and protect pasture species susceptible to grazing at the seedling stage. Continued grazing of pastures by wildlife is likely to amplify the effects of drought. The results of this thesis provide important information to land owners and that can better equip them to manage wildlife not only at a property scale, but also a catchment scale.