New research on impacts of grazing by native species

New research led by the University of NSW and supported by Bush Heritage Australia has shown that overgrazing by overabundant native herbivores is degrading fragile ecosystems to the detriment of other native species that rely on this same habitat to survive.  

The study, which was published in the scientific journal Global Ecology and Conservation in December 2020, demonstrates the extensive damage done by overabundant kangaroos to land managed for conservation.  

The research found that intensive grazing due to high kangaroo populations causes: 

• Degraded soil quality. Overgrazing causes a loss in key nutrients like carbon and phosphorous. It also causes more compacted soil resulting in less water being absorbed in rainfall events. 
• Reduced and degraded vegetation. Grass cover was reduced as was the amount of plant species. 

Ecologists, including myself, drew these conclusions using data gathered from herbivore exclosures – fenced areas designed to keep certain species out – from four nature reserves: our own Boolcoomatta Station Reserve in south eastern SA as well as NSW’s Mungo National Park, Yathong Nature Reserve and Oolambeyan National Park, all managed by the NSW National Parks and Wildlife Service.  

These 25m x 25m zones excluded either kangaroos, rabbits or both, in addition to an unfenced control area. We compared the health of the soil and vegetation inside the exclosures with the areas outside. We also looked for signs of land degradation specific to each species (rabbits or kangaroos) and monitored animal populations, with kangaroos the most populous herbivore across all four reserves. 

A closer look at Boolcoomatta 

Results of the study are particularly important given the recent few years of severe drought in semi-arid Australia. These conditions were felt intensely on Boolcoomatta where rainfall totals of 89.2mm and 56.3mm were recorded in 2018 and 2019 respectively, in comparison to the mean annual rainfall of 190mm.  

In addition, the estimated population densities of kangaroos, specifically Red Kangaroos, present on Boolocomatta over these two years reached unsustainable levels of up to greater than 30 kangaroos per km². 

As a result, the reserve's biodiversity suffered. There was very little perennial grass which had a negative effect on a range of other species that rely on vegetation cover and seed to survive such as granivorous birds, small mammals and reptiles. 

One of the key native species found on Boolcoomatta likely to be impacted by lack of cover and food resources due to overgrazing is the critically endangered Plains Wanderer, which has been recorded on the reserve on several occasions. The most recent sighting of these rare birds was in May 2019, but subsequent monitoring on the reserve has failed to detect any since then.

In addition, I and other Bush Heritage staff are assisting with broader working groups on improving our understanding of kangaroo grazing impacts and better ways to manage overabundant macropods in the South Australian rangelands.