Do feral pigs increase soil erosion? A monsoonal northern Australia case study

Animals are recognised biological agents that can turn over large amounts of soil, influence soil structure and composition and may allow soil to more easily erode. Feeding activities of feral pigs (Sus scrofa) are known to produce considerable soil disturbance. Here, pig disturbance, together with soil erosion and deposition patterns, are quantified along two hillslope transects in northern Australia over 10 years. Annual disturbance by pigs was similar to 1% of surface area. The erosion rate was low with a range of 0.008-0.13 mm year(-1). Both transects had similar hillslope profiles, vegetation patterns and similar appearance. There was a greater number of pig disturbances, area and mass of material disturbed by pigs for one transect compared to the other moving downslope. However, for the second transect, the number of disturbances significantly decreased moving downslope while the mass of material exhumed remained relatively constant along the hillslope. For the first transect, this suggests that the greater number of pig digs moving downslope reduces hillslope connectivity and therefore reduces erosion. That is, the greater the amount of disturbance by pigs, the less erosion. The pig digs produce a pit and a mound with the material diffusing locally and the pit capturing material from upslope reducing hillslope connectivity. These pits have been observed to last many years. A control on pig disturbance was likely to be rock content. The average surface rock content of the two transects was significantly different (9% and 20%, respectively) and with the higher rock content reducing pig disturbance and erosion for the second transect. The bio geomorphic influence that pigs represent in the landscape for biogeochemical cycling requires ongoing investigation.