The ‘brown food web’: dead vegetation plays essential role in desert ecosystems
It is well understood that overgrazing by herbivores such as kangaroos can dramatically alter ecosystems, but the impact that overgrazing has on the cover of dead vegetation – and cascading effects on small vertebrates such as lizards, desert frogs and dunnards – has not been extensively studied. not.
Now scientists at UNSW Sydney have shown that overgrazing can disrupt the desert food webs that exist between dead plant material, termites and animals that rely on termites as their main food source. This latest discovery has important implications for biodiversity conservation in arid Australia.
Researchers from the School of Biological, Earth and Environmental Sciences conducted fieldwork in the arid region of South Australia and published their findings in the journal Ecosystems.
Lead author PhD student Baptiste Wijas and Professor Mike Letnic observed brown food webs, which are based on the consumption of dead or decaying vegetation by detritivores, such as termites or earthworms. Mr Wijas and his team found that overgrazing by kangaroos in arid ecosystems can negatively disrupt the brown food web – in a nutshell, kangaroos eat the plants before they can mature, dehydrate and become food for detritivores.
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“We found that less dead biomass due to overgrazing by herbivores can lead to a reduction in termites,” says Mr Wijas. “Fewer termites, the main decomposers in these environments, could eventually lead to a reduction in the number of lizards and small mammals in arid ecosystems, as many of these small vertebrates feed on termites.”
Green and brown food webs
Most food web research has focused on “green food webs” which, unlike brown food webs, begin with the consumption of living, photosynthesizing vegetation by herbivores.
“Much research in arid ecosystems has focused on the green food webs that follow ‘blooming periods’ caused by large rainfall events,” says prof. Latin. “These ‘bloom periods’ see spectacular growth and flowering 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 much less exciting and consequently attracted less attention.”
The researchers focused their work on Boolcoomatta Station Reserve, a conservation reserve managed by the non-governmental organization Bush Heritage Australia. The Adnyamathanha and Wiljakali peoples are the traditional owners of Boolcoomatta.
Boolcoomatta Station Reserve is within the dingo fence in South Australia, an area where the dominant land use is sheep grazing and where the apex predator, the dingo, is “functionally extinct”. This means that dingo numbers have decreased to such an extent that they have a negligible effect on free-ranging prey species, such as kangaroos and wild goats.
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“Kangaroos occur in large numbers across much of arid Australia because populations of their main predator, the dingo, have been suppressed. The creation of artificial water points to provide water to livestock and accidentally to kangaroos has also helped kangaroos to by surviving dry periods,” says Prof. Letnic.
The team compared the cover of living and dead vegetation, the abundance of harmful termites, and their predators within the exclusions, which excluded kangaroos, with nearby control plots.
The researchers found that there was more cover of living and dead vegetation within the exclusions, where kangaroos were absent. As a result, there were more termites and small vertebrate predators of termites, such as lizards and dunnarts, within the exclusions.
“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 within the exclusions,” says Mr Wijas.
Inside the enclosures, where kangaroos were kept out, the team found increasing numbers of lizards and dunnarts. Photo: Baptiste Wijas.
These small vertebrates are an important component of biodiversity in desert environments and play a significant role in desert food webs by being prey for larger animals, such as larger marsupials, birds of prey, snakes and goannas.
Implications for biodiversity conservation in arid Australia
The latest study contrasts with the results of research conducted in different, more fertile environments, which found that the presence of herbivorous animals can increase decomposer populations. And while these findings are new, there are more questions that need to be answered.
The team recommends that to continue to explore the impact of grazing in deserts, future studies should manipulate termites sufficiently to examine the relationship between termite abundance and the diversity and abundance of small vertebrates in more controlled conditions.
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Prof. Letnic says the latest findings could have significant implications for the conservation and management of arid ecosystems in Australia.
“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.
“Conservation managers can use this information to make informed decisions about how to manage herbivore populations and watch for early signs of habitat disruption that are critical for the conservation of other species.”
