Overgrazing could have significant impacts on the dead vegetation that plays an essential role in Australia’s arid ecosystems

In what appears to be science turning on its head, researchers have been looking at the value of dead plant material in the arid zone. Normally they’d be looking at preserving living matter in that harsh environment.

But new research by scientists at the University of New South Wales Sydney has found that overgrazing by herbivores, like kangaroos and goats, can disrupt the desert food webs between dead plant material, termites, and the animals that rely on them as their main food source.

From field work in the arid region of South Australia, the researchers have found that excessive grazing can reduce the amount of dead vegetation cover, with cascading effects on the small vertebrates – like lizards, desert frogs, and dunnarts.

“We found that less dead biomass due to overgrazing herbivores can lead to a reduction in termites,” says Baptiste Wijas, a PhD student at UNSW Sydney and lead author of the study.

“Fewer termites, the principal decomposers in these environments, could ultimately result in a reduction in the number of lizards and small mammals in arid ecosystems, as many of these small vertebrates feed on termites.”

These latest findings, which have been published in Ecosystems, could have significant implications for the conservation and management of arid ecosystems in Australia.

Read more: Saving Australia’s native mammals from invasive predators.

Unlike green food webs, which begin with the consumption of living, photosynthesising vegetation by herbivores, so-called ‘brown food webs’ are based on the consumption of dead or decaying vegetation by detritivores – organisms such as termites or earthworms that consume decomposing plant and animal parts.

“A lot of research in arid ecosystems has focused on the green food webs that follow ‘boom periods’ prompted by large rainfall events,” says Mike Letnic, Professor in the School of Biological, Earth & Environmental Sciences at UNSW Sydney and senior author of the study.

“These ‘boom periods’ see spectacular growth and blooming of desert plants and increases in the populations of many animal species that feed on the growth, such as herbivores and rodents. Drier periods are a lot less exciting and consequently have attracted less attention.”

A small lizard with green and white stripes in arid south australia
Ctenotus in Boolcoomatta. Credit: Baptiste Wijas

The team did the research on Boolcoomatta Station Reserve in South Australia, a conservation reserve managed by Bush Heritage Australia. The Adnyamathanha and Wiljakali peoples are the Traditional Owners of Boolcoomatta.

The area is predominantly used for sheep grazing and, as it’s inside the dingo fence in SA, the dingo is functionally extinct there. As a result, these top predators have almost no influence on the populations of free-ranging prey species, like kangaroos and feral goats.

“Kangaroos occur in large numbers across much of arid Australia because populations of their principal predator, the dingo, have been suppressed. The creation of artificial water points to supply water to livestock and inadvertently to kangaroos have also helped kangaroos to survive through dry periods,” says Letnic.

Read more: Feral cats a major threat to the critically endangered Kangaroo Island dunnart.

The researchers compared the cover of living and dead vegetation, and the abundance of termites and of their predators, inside exclosures from which kangaroos were excluded and in plots where they were not.

They found that there was more cover of living and dead vegetation inside the exclosures where kangaroos were excluded because they eat the plants before they can mature, dry out and become food for detritivores.

“Our findings are one of the first to show in arid ecosystems that where herbivores were excluded, there was greater biomass of dead grass. In turn there were more termites and predators of termites inside the exclosures,” says Wijas.

These small vertebrates play an important role in desert food webs by being prey for larger animals, such as larger marsupials, birds of prey, snakes and goannas. 

“The research has important implications for the conservation of biodiversity in arid Australia because it sheds new light on how grazing can affect the functioning of arid ecosystems,” says Letnic.

“Conservation managers can use this information to make informed decisions on how to manage herbivore populations and look for early signs of habitat disruption that is critical for conserving other species.”

Dr Katherine Tuft, Chief Executive of Arid Recovery – an independent, not-for-profit wildlife reserve in South Australia’s arid north – told Cosmos: “The brown food web of dead plants, termites and their prey revealed in this study is a surprisingly important part of arid ecosystems.

“Importantly, it demonstrates how much deserts lose in productivity when irruptive herbivores like kangaroos aren’t regulated by predation pressure.”

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