Implications of grazing management systems incorporating planned rest for biodiversity conservation and landscape function in rangelands
Livestock grazing is recognised as a major driver of biodiversity decline and land degradation in rangelands around the globe. Protected areas alone cannot conserve global biodiversity, and therefore off-reserve conservation is necessary to achieve biodiversity conservation outside reserves and improve connectivity between reserves. Grazing management strategies that promote both ecological and production outcomes have the potential to conserve biodiversity and maintain or improve landscape function in agricultural landscapes. However, there is a lack of understanding of the response of biodiversity and landscape function to different grazing management systems in arid and semi-arid rangelands. This thesis explored the effects of commercial grazing practices that incorporate frequent periods of rest from grazing on biodiversity and landscape function, and determined the potential for using these alternative grazing practices to achieve broad-scale conservation outcomes.
A systematic review and meta-analyses of scientific literature comparing grazing management incorporating periods of planned rest (strategic-rest grazing, SRG) with continuously grazed (CG) and ungrazed (UG) systems was undertaken to determine the effect of SRG on ecological and animal production variables. Where significant differences occurred, the trend analysis of ecological and animal production responses to grazing management predominantly favoured SRG over CG, except for animal weight gain, and favoured SRG over UG systems for plant, mammal and bird richness and diversity, but not invertebrate richness and diversity, biomass and ground cover. Most studies that compared plant species composition reported differences in response to grazing management. While we did not find any differences overall between grazing contrasts, meta-analyses of plant richness, diversity, animal weight gain and animal production per unit area indicated that management incorporating longer periods of rest compared to periods of grazing have the potential to improve animal weight gain and production per unit area, but reduce plant richness. The type of SRG system was also important, with multi-paddock SRG systems having lower plant richness relative to CG systems, and SRG systems based on seasonal or deferred grazing having greater diversity than CG systems. Most of the literature comparing SRG with CG or UG did not consider the response of ecological and animal production response variables simultaneously. Greater collaboration between ecological and animal production scientists is recommended to better understand the ecological and socio-economic trade-offs associated with different grazing management strategies.
Understorey floristic species composition and plant biodiversity measures were compared between commercial properties managed under alternative grazing management (incorporating frequent and long periods of rest), traditional (continuous) grazing management, and adjacent ungrazed areas managed for conservation across a broad region of the semi-arid rangelands in western NSW. Significant variation in understorey floristic composition was driven by soil type (clay and sand), season, preceding rainfall and geographic location. These variables were the major drivers of floristic composition. The effect of grazing treatment on floristic composition at the regional scale was comparatively small and not significant. However, infrequent species were more likely to be recorded in conservation areas. Measures of floristic biodiversity varied with the scale of observation, season of sampling and soil type. In comparison to traditional grazing management, alternative grazing management generally resulted in greater understorey floristic species richness and diversity, depending on the season and scale of sampling. Few differences were found in plant species richness, diversity or functional diversity between alternatively grazed properties and adjacent areas ungrazed by commercial livestock and managed for biodiversity conservation. This suggests that alternative grazing management may be compatible with biodiversity conservation on commercial livestock properties in western NSW rangelands, but potentially at the expense of rare species.
Ground cover, soil properties and landscape function were also compared between alternative grazing management, traditional grazing management and conservation management in semi-arid NSW. Alternative grazing management had greater total ground cover in comparison to traditional grazing management systems. However, both alternative and traditional grazing management treatments had significantly less ground cover than adjacent areas managed for conservation. Alternative grazing management properties did not differ significantly to areas managed for conservation in terms of landscape function, but many indices of landscape function (stability, nutrient cycling, landscape organisation index, patch area and average interpatch length) were significantly reduced under traditional grazing management compared to conservation. This suggests that alternative grazing management was more beneficial for landscape function than traditional grazing management.
Significant differences were observed in floristic biodiversity measures, ground cover, soil properties and landscape function between clay and sandy soils in the study region. Clay soils had greater soil organic carbon and organic nitrogen, and lower bulk density than sandy sites. Soil stability, nutrient cycling and landscape organisation indices were also greater on clay than sand soils, and average interpatch length was shorter on clay soils. There was no difference in total ground cover between sand and clay soils, although clay soils had greater vegetative cover than sand soils, while sandy soils had greater cryptogam cover. Floristic biodiversity measures (species richness, evenness, diversity, turnover) were significantly greater on sandy than clay soils at larger plot and site scales, but there was no difference in species richness at the finest scale of sampling (1 m² quadrats). Despite the common perception that clay soils are more resilient to disturbance than sand communities, we found no difference between sand and clay soils in floristic biodiversity measures, ground cover, landscape function, soil organic carbon, soil organic nitrogen, or bulk density in response to grazing management. This indicates that alternative grazing management may provide a sustainable option for conservation of biodiversity and landscape function across both sandy and clay soils in western NSW semi-arid rangelands.
Floristic composition, biodiversity measures and ground cover were also compared at a local scale between an ungrazed public nature reserve and an adjacent rotationally grazed commercial property in Acacia aneura woodland in semi-arid NSW. Significant differences in understorey floristic composition were observed between the two grazing treatments, including a greater frequency of palatable species in the nature reserve and more unpalatable species on the rotationally grazed property. There were no significant differences in understorey floristic species richness, diversity, functional diversity measures or ground cover between the nature reserve and rotationally grazed property. However, these measures increased with distance from water on the rotationally grazed property, highlighting the negative effects of increasing grazing intensity. These results suggest that at a whole-paddock scale (beyond the sacrifice zone of high grazing intensity surrounding water points), rotational grazing management, along with careful management of grazing intensity and stocking rates, has the potential to sustain biodiversity and ground cover and may offer an alternative to grazing exclusion to achieve broad-scale conservation objectives in semi-arid rangelands. However, management would still need to address the impacts on floristic composition.
In conclusion, I found improved understorey plant species richness, diversity, ground cover and landscape function under alternative grazing management compared to traditional grazing management, and few differences in these measures between alternatively grazed and ungrazed areas managed for conservation. These results provide support for utilisation of alternative grazing management practices to improve biodiversity conservation and landscape function outside of the public reserve system in semi-arid rangelands. Results also show incorporation of planned periods of rest in grazing management regimes has the potential to achieve dual ecological and animal production outcomes in grazing landscapes throughout the world. Further research is necessary to understand the circumstances in which commercial grazing is compatible with the conservation of biodiversity, landscape function and animal productivity, and to identify best grazing management practices for biodiversity conservation purposes.