A QUT-led study has determined that grazing mammals and plant-eating insects collectively play a significant role in preserving the health of Australia's endangered grassy woodlands. This three-year study, conducted in the lowland grassy woodlands of New South Wales' Bega Valley, investigated the impact of herbivores on native Kangaroo grass (Themeda triandra) and invasive African lovegrass (Eragrostus curvula).
Researchers found that removing all herbivores, including insects, led to a significant shift in plant dominance.
Animal and insect herbivores contribute to native Kangaroo grass maintaining its vigor and limit the spread of invasive species through repeated defoliation. These lowland grassy woodlands are classified as endangered in NSW and critically endangered under federal legislation.
The Study's Approach
The experimental setup involved a series of fences designed to exclude different types of herbivores:
- An outermost enclosure for livestock.
- An inner fence for native mammals (kangaroos, wallabies).
- Innermost boxes for invertebrates (aphids, grasshoppers).
The study sites were selected based on their existing vegetation, being either dominated by Kangaroo grass or co-dominated by Kangaroo grass and African lovegrass.
Key Findings After Three Years
The study revealed that excluding all above-ground herbivores, including invertebrates, had more substantial effects on plant growth than excluding only livestock and native mammals.
- Kangaroo grass experienced a significant decline when all above-ground herbivores were excluded.
- African lovegrass was largely unaffected in most exclusion scenarios. However, it showed an increase only at sites where it was previously co-dominant with Kangaroo grass and where livestock were excluded.
The Mechanism of Preservation
These herbivores appear to sustain Kangaroo grass's dominance by promoting a crucial cycle of defoliation and compensatory regrowth. This process reduces self-shading and stimulates the production of new leaves, contributing to the native grass's vigor. The study emphasizes the collective role of all herbivores in maintaining native plant dominance and their essential ecological functions in grassy ecosystems, including resistance to invasion by non-native plants.
Challenging Assumptions and Broader Significance
These findings challenge the assumption that herbivore size is the primary factor in ecosystem health.
Instead, the research suggests that even the smallest herbivores are crucial for the effective management and restoration of Australia's threatened grassy ecosystems. The research team included members from QUT and the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.