The impact of rabbits and cattle on an arid calcareous shrubby grassland in central Australia

Abstract

Multivariate methods of classification and ordination were used to examine changes in plant composition over a three year period on a calcareous shrubby grassland grazed by rabbits and cattle in central Australia. In a site-time ordination of the data for all years, the season and amount of rainfall was the most important influence, followed by several soil factors and rabbit density. In a site ordination of each year, rabbit density was a minor influence for the pretreatment data but increased in importance over the three years until it was highly correlated with vector one in the final year. This was a result of the exclosure treatments diverging from the controls with extremely high rabbit populations at two of the eighteen plots. When site-time trajectories were plotted through species space, most sites tended to move with season over the three year period towards a common domain. The two heavily grazed sites exhibited a different trajectory; this was due to the behaviour of seven plant species under grazing. In multiple regression analysis, rabbit density, cattle presence and yearly rainfall explained 66% of the variation in total standing biomass. In general, total cover was not affected by grazing treatment, although short term utilisation effects were usually visible in the late dry season. Given its present degraded state, this arid vegetation type is seen as reasonably resilient, requiring extremely heavy rabbit grazing to prevent the plant community responding to the dominating influence of season.