SGS Biodiversity Theme: impact of plant biodiversity on the productivity and stability of grazing systems across southern Australia

Abstract

The relationships between productivity and plant species diversity were assessed using data from the Sustainable Grazing Systems (SGS) 10 national experiment sites. Each site applied up to 7 different management treatments. Sites varied in the amount of data available for analysis. Plant species diversity was assessed in terms of both the total and native species present at each site. More than 200 plant taxa were recorded over the period of the experiment, about one-third of which were native. In the majority of cases, the native species present within a treatment remained there throughout the experiments, even when fertilised and oversown with introduced species. The number of native species increased by 1 or 2 over the 3–4 years of each experiment where grasslands were less intensively used (average herbage mass >2 t DM/ha), but decreased in more heavily grazed treatments. Native grasses made much greater contributions to herbage mass than other native species. Of the more than 70 native plant species found, the most numerous were broadleaf species, which tended to be more variable under management treatments. As total species richness increased, there was a tendency for pasture productivity to be less, for the mean standing herbage mass to be less and for seasonal growth to be less stable. This depended upon experiments and tended to apply at the sites with higher annual pasture growth rates. All treatments had >10 plant species within the small (about 1.5 ha) paddocks used and larger paddocks often had many more. It was apparent from the high plant species richness at each site (about 25–100 species) that many more niches exist in these paddocks than the number of species usually sown in a pasture. Opportunities to redesign pasture mixtures to exploit more of the resources available obviously exist. Understanding of the relationships among management practices, productivity and plant species diversity is limited, but will become important as we seek more sustainable grazing systems. The studies analysed here suggested that where the herbage mass was maintained between 2 and 4 t DM/ha then species were maintained and productivity was optimised.