Managed livestock grazing is compatible with the maintenance of plant diversity in semidesert grasslands

Abstract

Even when no baseline data are available, the impacts of 150 years of livestock grazing on natural grasslands can be assessed using a combined approach of grazing manipulation and regional-scale assessment of the flora. Here, we demonstrate the efficacy of this method across 18 sites in the semidesert Mitchell grasslands of northeastern Australia. Fifteen-year-old exclosures (ungrazed and macropod grazed) revealed that the dominant perennial grasses in the genus Astrebla do not respond negatively to grazing disturbance typical of commercial pastoralism. Neutral, positive, intermediate, and negative responses to grazing disturbance were recorded amongst plant species with no single life-form group associated with any response type. Only one exotic species, Cenchrus ciliaris, was recorded at low frequency. The strongest negative response was from a native annual grass, Chionachne hubbardiana, an example of a species that is highly sensitive to grazing disturbance. Herbarium records revealed only scant evidence that species with a negative response to grazing have declined through the period of commercial pastoralism. A regional analysis identified 14 from a total of 433 plant species in the regional flora that may be rare and potentially threatened by grazing disturbance. However, a targeted survey precluded grazing as a cause of decline for seven of these based on low palatability and positive responses to grazing and other disturbance. Our findings suggest that livestock grazing of semidesert grasslands with a short evolutionary history of ungulate grazing has altered plant composition, but has not caused declines in the dominant perennial grasses or in species richness as predicted by the preceding literature. The biggest impact of commercial pastoralism is the spread of woody leguminous trees that can transform grassland to thorny shrubland. The conservation of plant biodiversity is largely compatible with commercial pastoralism provided these woody weeds are controlled, but reserves strategically positioned within water remote areas are necessary to protect grazing-sensitive species. This study demonstrates that a combination of experimental studies and regional surveys can be used to understand anthropogenic impacts on natural ecosystems where reference habitat is not available.

Ecosystem structure, function and composition in rangelands are negatively affected by livestock grazing

“…The economies of many people, particularly from rural areas, depend on the provision of goods and services such as milk, meat, wool, and hide derived from livestock. The economic importance of grazing and the reports of positive or neutral effects of grazing on species richness in specific studies (e.g., Socher et al 2013, Fensham et al 2014, Kimuyu et al 2014 has led some to suggest that introducing or reintroducing commercial herds of livestock to intermittently grazed or ungrazed areas (e.g., alpine high country of Australia; Williams et al 2006) might have benefits for biodiversity and ecosystem functioning (Lunt et al 2007, Williamson et al 2014). …”

“…Grazinginduced habitat modification alters species composition by reducing the diversity of plants and terrestrial invertebrates, small mammals, birds, reptiles, and soil crusts (e.g., Williams et al 2008, van Klink et al 2014. Grazing also alters community structure by influencing, for example, the return interval of wildfires and the accumulation of flammable fuel (Kimuyu et al 2014) or plant community composition (Lunt et al 2012, Fensham et al 2014. These direct structural and compositional shifts have often large, indirect effects on ecosystem functions.…”

“…Firstly, grazing by domestic livestock occurs over more than half of its land mass (Fensham et al 2014). Secondly, it provides a unique opportunity to assess the role of grazing in a system less likely to be confounded by evolutionary changes in plant communities in response to livestock grazing (Dorrough et al 2012).…”

Unassisted invasions: understanding and responding to Australia’s high-impact environmental grass weeds

“…Notable examples include native perennial grasslands covering black soil plains across vast areas of northern Australia (Fensham et al 2014; R Silcock pers. comm.…”

Invasive Plant Species and Novel Rangeland Systems

“…Point C represents a maximum level of socioeconomic value that could be gained from a rehabilitated ecosystem where biodiversity and socioeconomic goals are both key. Grazing can increase the diversity of species present within ecosystems, particularly if the dominant species are palatable (Fensham et al 2014;Lunt et al 2007) and depending on the climatic region and evolutionary history of grazing (Milchunas et al 1988). In contrast to the model suggested by Lamb et al (2005) for rainforest communities, there is a higher gain in socioeconomic value with every unit of increase in biodiversity value in pasture communities.…”