Plant species responses along a grazing disturbance gradient in Australian grassland

Abstract

Abstract. 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.

Predicting plant species’ responses to grazing

“…A number of studies have shown unimodal species' responses to grazing intensity (Hacker 1984(Hacker , 1987Fensham, Holman & Cox 1999). Noy-Meir, Gutman & Kaplan (1989) found that, in a number of cases, inconsistent grazing responses were congruent with unimodal responses common to a variety of ecological gradients.…”

“…Some studies distinguished monotonic from non-linear unimodal responses to grazing intensity (e.g. Hacker 1987;Fensham, Holman & Cox 1999). Certain studies also allowed the relative intensity at which abundance was maximal (the peak) to be determined.…”

“…We determined responses for several studies subjectively from trends through time or across grazing intensities (e.g. Biddiscombe 1953;Friedel 1981;Fensham, Holman & Cox 1999). While this subjectivity was undesirable, the great majority of rangeland grazing studies are not fully replicated, long-running, powerful experiments allowing clear significance tests of responses.…”

“…Species often show abundance responses along a gradient of grazing intensity that are not monotonically increasing or decreasing, for example first increasing, peaking at some intermediate level and then decreasing at higher grazing intensities (Dyksterhuis 1949;Hacker 1987;Noy-Meir, Gutman & Kaplan 1989;Fensham, Holman & Cox 1999). For such species, comparisons between low and moderate grazing would show an opposite response to comparisons between moderate and high grazing.…”

Global assessment of the non‐equilibrium concept in rangelands

“…Degradation is common in the presence of key resources and water sources, which are considered a precondition for grazing under semiarid or arid condition (Fensham et al 1999, Bestelmeyer et al 2006. A typical example of a stable key resource is the forage available in wet meadows located within rangelands (Buttolph and Coppock 2004); in addition, local degradation typically occurs around water sources (Hanan et al 1991), settlements (Sullivan 1999), nomad tents (Evans and Geerken 2004), and stock posts (Zhao et al 2007).…”