Feral goats (Capra hircus L.) in the Macleay River gorge system, north-eastern New South Wales, Australia. I. Impacts on soil erosion

Abstract

The impact of feral goats (Capra hircus) on the rate of erosion in steep gorge country was estimated. The erosion (sediment flux) in a manipulated treatment area before and after the removal of goats was compared with erosion in two adjacent unmanipulated areas: one with goats at high density (~20 goats km–2) and one with very few goats (~0.2 goats km–2). Erosion was measured with 36 2-m-wide catch fences, collecting debris (soil and rock) moving down 40° slopes over 10 collection periods spanning 31 months. In the central manipulated area, goats were initially at high density but were completely removed during the third collection period. Over the 10 collection periods, erosion was consistently greater in the area with many goats than in the area with few goats (mean five times greater, range 2.4–11.8). This difference was significant for 6 of the 10 collection periods. Before goats were removed from the manipulated treatment, the erosion in this area was not significantly different from that in the area with many goats, but was significantly greater than the area with few goats. After goat removal erosion in the manipulated area decreased relative to each of the other treatments. By the final collection period erosion in the manipulated (goats removed) area was significantly less than in the area with many goats, but not significantly different from the area with few goats. Initial reduction in erosion following goat removal was rapid, followed by a continued slower decline over the next two years coincident with a relative increase in ground-cover vegetation. It was thought that both direct physical disturbance by the goats and secondary effects due to goat impacts on the substrate and ground-cover vegetation contributed to the increase in erosion associated with the presence of goats.