Surface disturbance and erosion by pigs: a medium term assessment for the monsoonal tropics.
Introduced pigs (Sus scrofa) are recognised as having significant environmental impacts. Here, we quantify the effect of feral pigs in a catchment (undisturbed by Europeans) in the monsoonal tropics of northern Australia. Field data collected over a 5-year period showed that the areal extent of pig disturbance ranged from 0.3 to 3.3% of the survey area (average 1.2%, σ=0.9%). Mass of exhumed material was considerable and ranged from 4.3 to 36.0 Mg ha-1 y-1 (average 10.9 Mg ha-1 y-1). The excavations produce surface roughness which acts as sediment traps. Over the 5-year study period, there was no evidence to suggest that pigs produce any rill or gully erosion. There does not appear to be any relationship between rainfall amount and area disturbed or volume of material exhumed. However, a significant positive relationship was observed between number of disturbances and rainfall. The location of any disturbance appears to be random and has no relationship with topography or geomorphic attributes such as slope, upslope contributing area or wetness indices derived from a high-resolution digital elevation model of the site. While pigs are disturbingly relatively large volumes of soil, there is no clear evidence to support any increase in local erosion and soil structural change may be occurring slowly and only be observable over the long term.