Plant species responses along a grazing disturbance gradient in Australian grassland

Floristic data from paired roadside-paddock analyses from grassland in central Queensland, Australia, were ordinated. The mean direction of the vectors between these pairs was almost perfectly aligned with the indirect gradient represented by the first axis of Non-metric Multi-dimensional Scaling. It confirms anecdotal evidence of a trend from infrequently grazed roadsides to constantly grazed paddocks. The increasing abundance of annual herbs and grasses along this putative gradient is consistent with documented trends from elsewhere in the world. The response patterns of individual species along the disturbance gradient is consistent with ecological theory predicting unimodal peaks in abundance along physical environmental gradients. The ancestral perennial dominants of the grasslands, Dichanthium sericeum and D. queenslandicum, exhibited a declining response to grazing disturbance. Even the generally unpalatable perennial grass Aristida leptopoda declined considerably in the upper segments of the grazing disturbance gradient. A suite of herbaceous trailing legumes had peaks in their abundance near the middle of the grazing disturbance gradient, trends that can be readily explained by the combination of their palatability and intolerance to competition from tall perennial grasses. Several species including the noxious exotic herb Parthenium hysterophorus showed increasing abundance along the grazing disturbance gradient. The methodology may have application as a rapid method of assessing disturbance impacts elsewhere, and is most suited where a management differential between paired plots can be reliably generalized and where the physical environment is relatively monotonous.