Rabbits and livestock grazing alter the structure and composition of mid-storey plants in a wooded dryland

Abstract

Woody plants are critical for supporting wildlife and maintaining ecosystem structure and function in drylands. Grazing has the potential to alter woody plant community structure by affecting the size and survival of shrubs and saplings. We assessed the role of recent and historic grazing by livestock, and recent grazing by kangaroos and rabbits, on woody plant structure and composition. We measured the density and species occurrence of saplings and shrubs within three plant communities across 0.5 M km2 of eastern Australia and used generalised linear mixed models to examine how grazing, soil texture and the basal area of mature trees altered the structure and composition of mid-storey woody plants. Due to co-evolution, we expected a stronger negative effect of grazing on shrub and sapling density and species occurrence by introduced herbivores than native herbivores and that shorter individuals would be more susceptible to recent grazing activity. We found that the density and occurrence of shrubs and saplings generally responded more negatively to introduced herbivores irrespective of their height. Sapling density was strongly reduced by recent rabbit and livestock grazing but increased in density at sites with greater kangaroo and historic livestock grazing. Recent livestock grazing also reduced the occurrence of sapling species. There were no strong effects of any grazing on shrub density, but rabbit grazing strongly reduced shrub species occurrence, with few strong responses among individual species. Our data provide strong evidence that recent grazing by livestock and rabbits can reduce sapling density and alter woody plant occurrence. This has the potential to induce long-term changes in ecosystem structure by limiting shrub and sapling establishment which could lead to aging and contracting woody plant populations.