Changing the smell around threatened plants may help protect them.
Across New South Wales, many threatened plant species are at risk of extinction simply because too many are being eaten - by both invasive herbivores, like goats and deer, and native herbivores, like wallabies. We try to protect these plants using fencing, repellents or other methods that focus on controlling herbivores. But each approach has limitations. They can be expensive, harmful to the animals or ineffective. We are yet to find a long-term, effective way to stop herbivores eating threatened plants.
Now, researchers at the University of Sydney are leading a trial of a new method - using and altering odours to confuse herbivores and reduce plant browsing. They are testing this promising 'odour misinformation' method, working with the Department of Climate Change, Energy, the Environment and Water's Saving our Species program and NSW National Parks and Wildlife Service. The team is testing the method on some of our most valuable plant species across the state, funded by the NSW Environmental Trust.
Smelling a new solution
Swamp wallabies and other herbivores rely heavily on their sense of smell when deciding what to eat. They use not just taste but the smell of plants from further away to choose which 'plant neighbourhoods' to forage in. Studies have shown that:
- herbivores tend to avoid areas that smell of low quality (less tasty) plants (Pietrzykowski et al 2003)
- high-quality (tasty) plants are less likely to be eaten when surrounded by these low-quality neighbours, something known as associational refuge (Stutz et al 2015).
It's some of these high-quality plant species that are under threat from herbivores. But relying on low-quality, neighbouring plants to help protect high-quality plants has its drawbacks. The neighbouring plants can compete for resources, like water and nutrients. That's where odour misinformation has a unique advantage. We can replace real low-quality neighbours with just their smell. From afar, herbivores smell information that tells them the area is mainly low quality, and they stay away, leaving the threatened plants untouched.
A bonus is that we don't need to create the exact smell of the low-quality plants - which can include hundreds of different chemical compounds. We use a new method to work out the few, specific, smelly compounds that herbivores use to find or avoid those plants. This makes our job much easier.
The sniff test - focus species and field trials
This method is now being trialled to protect 4 threatened plant species in New South Wales. Each species has unique challenges, including the threat of being eaten by herbivores:
- Haloragodendron lucasii (Endangered) is found at only 4 known sites in northern Sydney and Ku-ring-gai Chase National Park.
- Caley's Grevillea (Grevillea caleyi - Critically Endangered) survives in fragmented populations in Sydney's northern suburbs, including Ku-ring-gai Chase and Garigal national parks.
- Bossiaea fragrans (Critically Endangered) exists only within an area near Abercrombie River, south of the Bathurst region.
- Granite zieria (Zieria obcordata - Endangered) is known only from small populations near Bathurst and Wellington in central NSW.
Up until now, it has been hard to protect each species from herbivores. Odour misinformation may offer a non-invasive and cost-effective method that works when other methods are costly or unsuitable.
Where we're at
So far, the trial has gathered camera evidence confirming that these 4 plant species are being eaten by either native or invasive herbivores. We're now in the experimental phase, testing our odour misinformation methods at sites for all 4 species. Motion-activated cameras are helping us assess whether herbivores visit and browse less.
These trials will continue through most of 2025 to test how well the method works in the long term. Early signs are promising, and we're excited to see whether tricking herbivore noses could be the next big step in plant conservation.
