- Grazing mammals and plant-eating insects contribute to maintaining the dominance of a key native grass species
- Three-year study looked at native Kangaroo grass (indicative of healthy grassland) and invasive African lovegrass in grassy woodland
- Findings contribute to the management and restoration of Australia’s threatened grassy ecosystems.
The three-year study, conducted in lowland grassy woodlands of New South Wales’ Bega Valley, found that removing all herbivores, including insects, caused a shift in plant dominance.
QUT PhD researcher Nadia Chinn (pictured), who led this study jointly with Dr Gabrielle Lebbink (now with DETSI), from QUT’s School of Biology and Environmental Science, said that through repeated defoliation, animal and insect herbivores helped the dominant native Kangaroo grass (Themeda triandra) maintain its vigour, and limit opportunities for the spread of invasive species.
“Lowland grassy woodlands are listed as endangered in NSW and critically endangered under federal legislation,” Ms Chinn said.
“The global expansion of agriculture and other human activity has seen the intentional and accidental introduction of non-native pasture plants, which may spread quickly, outcompete native species and alter ecosystem diversity and function.
“Livestock grazing can further contribute to the success of some invasive pasture grasses, often at the expense of native plant species and particularly in areas with a shorter evolutionary history of non-native grazing animals, like Australia.”
The project’s lead researcher from QUT, Professor Jennifer Firn with Swiss collaborator, PD Dr Anita C. Risch, from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, designed and established the large field experiment in 2019 using a series of fences that excluded different types of herbivores.
“The outermost enclosure kept out livestock, an inner fence excluded native mammals such as kangaroos and wallabies, and the innermost boxes excluded invertebrates like plant feeding insects such as aphids and grasshoppers,” Ms Chinn said.
“Few studies have tested the combined impacts of the exclusion of these herbivore groups in the same experiment on vegetation, the only other study of this type having been conducted in the subalpine grasslands of the Swiss National Park.
“The 12 sites we chose were either dominated by the native Kangaroo grass or co-dominated by Kangaroo grass and the invasive non-native grass African lovegrass (Eragrostis curvula).
“Kangaroo grass is a key native species in healthy grassy woodlands across Australia, whereas African lovegrass is a significant invasive weed that poses major challenges for landholders and conservation managers."
Ms Chinn said that after three years of herbivore exclusion, the researchers measured the plant growth and leaf characteristics of the most abundant grasses, as well as the amount of other vegetation, such as smaller grasses and wildflowers, that had accumulated in the experimental enclosures.
“We found that excluding all aboveground herbivores, including invertebrates, had much stronger effects on plant growth than excluding livestock, and native mammals alone,” she said.
“Kangaroo grass saw a significant decrease with the exclusion of all above-ground herbivores, whereas, African lovegrass was largely unaffected, except where livestock were excluded.
“Under livestock exclusion alone, African lovegrass increased only at sites where it was previously co-dominant with Kangaroo grass.
“These herbivores appear to sustain Kangaroo grass’s dominance by promoting a cycle of defoliation and compensatory regrowth, which reduces self-shading and stimulates the production of new, healthy leaves.
“Our study underscores the collective role of all herbivores in sustaining native plant dominance and their ecological functions in grassy ecosystems, which includes resisting invasion by non-native plants.”
Professor Firn said these results challenged the assumption that herbivore size mattered and suggested that even the smallest herbivores were critical to the management and restoration of Australia’s threatened grassy ecosystems.
“More studies to understand the role invertebrate herbivores play in maintaining native species plant diversity are needed.”
The research team comprised Ms Chinn, Dr Gabrielle Lebbink (now at DETSI), Professor Jennifer Firn, and Professor James McGree from QUT; and PD Dr Anita Christina Risch, from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Switzerland. The project was supported by an Australian Research Council Discovery Grant
The study, Total herbivore exclusion alters plant species dominance and trait expression in endangered grassy woodlands, was published in the journal Ecology.
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