Grazing and drought reduce cyanobacterial soil crusts in an Australian Acacia woodland

Cyanobacteria-dominant soil crusts are important components of unvegetated interspaces in Australia's semi-arid rangelands. We studied the changes in crust cover and floristics at increasing distances up to 500 m from three stock watering points in western Queensland's Mulga bioregion between 2002 and 2005. Sampling times corresponded with pre-drought (2002), drought (2003), early post-drought (2004) and late post-drought (2005) periods. Cyanobacterial cover increased markedly with increasing distance from water, with the largest changes occurring within 200 m of water. The change in cover with distance from water was most pronounced before the drought compared with during or after drought. Crust composition also changed in relation to distance from water, and the abundance of three cyanobacteria (Scytonema sp., Stigonema ocellatum, Porphyrosiphon notarissi) was greater away from water. The diversity of crust species increased significantly with distance from water, and was greatest in the late post-drought period. Although cyanobacterial crusts were present close to water (<100 m) before the drought, cover close to water had not recovered to pre-drought levels 2 years after drought. Our results indicate that excessive damage by stock trampling has an immediate impact on crusts and reduces their capacity to recover after drought. Changes in the severity or intensity of drought may therefore accelerate degradation processes and compromise the ability of crusts to recover after grazing.