The effects of feral goats and sheep on the shrub populations in a semi-arid woodland.

Abstract

Shrubs are increasing in density in extensive areas of semi-arid woodland in Queenstand and New South Wales, reducing their carrying capacity for stock and increasing the difficulty of sheep management. A case study is reported in which an area exclosed from grazing increased from 6000 to 9000 shrubslha in 3 years. Grazing by sheep or goats both reduced the establishment ofAcacia aneura (mulga) and Dodonaea 11iscosa (broad-leaf hopbush) seedlings. Cassia spp. (punty and silver cassia), Eremophila sturtii (turpentine), E. rnitchellii (budda) and Geijera parviflora (wilga) were not reduced. Sheep did not affect mature shrubs to any extent. Extremely heavy goat pressure destroyed several problem shrubs, including Acacia arzeura and Dodorzaea viscosa, and reduced Cassia spp. Mechanical shrub-clearing is often followed by sprouting from the roots of some species. Moderate goat pressure failed to control such sprouts; heavy goat pressure eliminated sprouts from E. rnitchellii and G. parviflora but E. sturtii was not eaten, probably because of its higher oilcontent, and increased to 166% of its preclearing density in two years. Mechanical clearing increased shrub seedling establishment four-fold. Future increases in shrub density may be expected in semi-arid woodland communities and goats do not offer an answer to the problem because they are selective between species and because managerially significant reductions in shrub populations can only be achieved at extremely high stocking pressures. The expense of fencing for such a treatment, the damage to the herb layer and the lack of a post treatment management, that would prevent the replacement of palatable shrubs by unpalatable ones, makes it an unsuitable technique for extensively grazed properties.