Habitat preferences and environmental impacts of feral pigs (Sus scrofa) in lowland tropical rainforests of north-eastern Australia

Amanda Elledge (2011). Habitat preferences and environmental impacts of feral pigs (Sus scrofa) in lowland tropical rainforests of north-eastern Australia PhD Thesis, School of Geography, Planning & Env Management, The University of Queensland.

       
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Author Amanda Elledge
Thesis Title Habitat preferences and environmental impacts of feral pigs (Sus scrofa) in lowland tropical rainforests of north-eastern Australia
School, Centre or Institute School of Geography, Planning & Env Management
Institution The University of Queensland
Publication date 2011-02
Thesis type PhD Thesis
Supervisor Clive McAlpine
Iain Gordon
Peter Murray
Total pages 168
Total colour pages 5
Total black and white pages 163
Subjects 05 Environmental Sciences
Abstract/Summary Tropical rainforests are renowned for their high biodiversity and are reported to support approximately 50 per cent of the world’s species. However, tropical rainforests are being rapidly degraded due to habitat loss, climate change and the invasion of alien species. Feral pigs (Sus scrofa) are one of the main invasive species threatening rainforests across the globe, as they directly impact native fauna and flora through predation and herbivory, and they can also indirectly impact nutrient concentrations, water infiltration, and plant regeneration via their rooting foraging behaviour. This thesis aims to further develop our understanding of feral pig ecology in lowland tropical rainforests using the Wet Tropics World Heritage Area (WTWHA) of north-eastern Australia as a case study, and provides recommendations for the improved management of feral pigs in the region. Although past studies have assessed spatial and temporal patterns in the occurrence and ecological impacts of feral pig rooting in rainforests, these studies are limited particularly in Australian rainforests. Furthermore, the relative effects of feral pig rooting and season on environmental variables have not been compared. My thesis addresses this imbalance through advancing our understanding of the importance of microhabitat variables in explaining the selection of patches for rooting by feral pigs, and assessing the recovery of rainforest with the short- and long-term exclusion of feral pigs. I reviewed 35 studies that measured soil and plant responses to feral pig rooting. The treatments used in these studies ranged from a comparison of undisturbed and pig disturbed areas to the recovery of habitat with the exclusion or removal of feral pigs. Overall, feral pigs reduced plant species richness and cover but recovery can occur if rooting pressure on the ecosystem is reduced. In contrast, the response of seedling density to experimental treatments was highly variable among studies as a result of the different classes of plant sizes measured, habitat type and season. Generalisations on the effect of feral pigs on soil fertility and soil invertebrate populations were limited by the high temporal and spatial variability among studies. Firstly, I modelled the importance of spatial, temporal and plot scale habitat variables to explain foraging patch choices by feral pigs for rooting. This was achieved by surveying 120, 1 x 1 m plots bimonthly over a year and then using generalised linear modelling and model-averaging to explain the relative importance of the measured ecosystem variables. I found that soil texture, rock cover, soil compaction and sand texture were the most important explanatory variables in the occurrence of pig rooting. Increases in soil compaction and distance to roads had a negative influence. Although no seasonal differences were observed in the number of plots with pig rooting present, the majority of pig rootings (65.7%) occurred during the dry season. Secondly, I evaluated the recovery of rainforest with the exclusion of feral pigs and assessed seasonal effects on the measured environmental variables. This was achieved by comparing plots where fencing had been used to exclude pigs for 2 and 14 years with unfenced controls at continual risk to pig damage. Surveys occurred in both the wet and dry seasons, and recovery was quantified using a range of soil and leaf litter properties, earthworm biomass and seedling establishment. Generalised linear models were used to explain the response of these variables to treatment and season. The only response variables to have a detectable treatment effect were a greater level of litter moisture, seedling densities (11 to 100 cm high) and electrical conductivity in the long-term fenced plots. However, biological trends were observed for earthworm biomass and seedling densities 11 to 100 cm high with greater values in fenced plots. Although treatment effects were not detected for the majority of environmental variables after both the 2 and 14 year exclusion of feral pigs, it should not be assumed that pig impacts are not cumulative. For example, no difference in seedling densities less than 10 cm high was detected between plot types, but there were more seedlings in the long- than short-term study. In contrast, many of the response variables in this study were different between the wet and dry seasons. The relatively strong influence of season on these variables can make it difficult to detect changes due to pig disturbance. The lack of treatment effects detected in this study is most likely a reflection of the low extent and intensity of feral pig rooting in the study area and should not be interpreted to mean pigs have no impact whatsoever. This is particularly important in conservation areas where spatially restricted and endemic species may be highly sensitive to even a small amount of pig disturbance. In such circumstances, temporal and plot scale habitat factors can be used to predict the future occurrence of pig rooting in order to prioritise areas for protection and/or strategically implement pig control activities.
Keyword Disturbance, exclusion, feral pig, foraging, habitat preferences, impact, recovery, regeneration, rooting, Sus scrofa
Additional Notes Print in colour - 1, 28, 33, 85, 109 Print on A3 paper - 42-47 Print landscape - 42-47, 90-91, 96-97, 114-115, 118-119

 
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Created: Fri, 07 Oct 2011, 15:46:43 EST by Miss Amanda Elledge on behalf of Library - Information Access Service