Odours of mass deception: an inventive new way of protecting native species

By James McGurk

Dr Catherine Price visited Durham Tuesday 5th December 2017 to present her research into combatting invasive species in New Zealand.

Invasive species are a massive economic and environmental problem globally. For instance, in 2005 their impacts cost the US $120 billion and were the principal threat to 42% of endangered and threatened species.

New Zealand is an invasion hotspot where 2788 species are threatened and half all vertebrate species have been lost since humans arrived. As with countless islands worldwide, rats, cats and other mammals are the main threat for many species. Many species are susceptible because they’ve never evolved defence mechanisms against predatory mammals, despite coping with native, non-mammal predators.

Currently, there are two realistic options to protect native species from mammals: kill the mammals, or fence them out. Killing is expensive. It’s hard to kill every target in a large area, and methods such as poison can harm other, non-problem species; additionally, animal rights activists often protest against the killing of invasive species, despite the harm they can cause. Meanwhile, fences obstruct other species and can be broken or tunnelled under.

Dr Price’s research aims to protect native species without killing.

The invasive mammals use their strong senses of smell to hunt, and are not deterred by camouflage like native predators. Dr Price aims to exploit mammals’ sensitive noses to reduce their foraging success on native species.

Ideally, she’d make them choose to hunt other invasive species instead. For instance, many mammalian predators were introduced to control invasive rats or rabbits. But, given the choice to hunt prey that had co-evolved defences, or defenceless native fauna, the predators choose the latter. Price’s research asks if we could interfere with this prey selection process, to make rats and suchlike the more appealing prey.

The answer – quite possibly. In fact, stoats feed mostly on rodents when rodents are abundant, and turn to New Zealand’s native fauna when rodents are scarce. Prey switching already occurs, and Price believes that it could be possible to reduce the threshold rodent density at which stoats turn to native species.

Her initial experimental results seem promising. Mice and stoats found less food when a prey smell was spread throughout their enclosures – camouflaging the prey’s scents – compared to scents only being placed at food patches within the enclosure. However, over five nights, animals learned to hunt by alternative cues, restoring their foraging success.

Encouraged, Dr Price tried an alternative technique. Her team spread quail odours throughout the enclosures for a week. Only in the second week were quail eggs were scattered in the enclosures too. By this point, rats were habituated to the odour being meaningless. They ate far fewer quail eggs than rats in enclosures without the pre-treatment.

The next step was testing whether this treatment would work in the natural environment. Dr Price’s team worked with the double-banded plover, which has highly-camouflaged ground nests predated by introduced stoats, cats, rats, hedgehogs and ferrets.

Unfortunately, there was no way of getting enough plover odour to cover the whole colonies. Instead, the team used a general bird odour, made by delightful, highly technical processes including cutting out glands from dead chickens and squeezing them into a vat. Apparently it was only the second most disgusting thing she’d done in the name of science – no-one asked what came first.

The odour was spread around the colony sites every three days before the breeding season. The data had just come in when Dr Price gave her talk, but it seemed her method gave a 20-60% improvement in hatching success compared to control sites.

Preliminary analyses suggested that all predators save hedgehogs had reduced foraging success, implying they were habituated to nest smells and couldn’t search by smell. However, hedgehogs awoke from hibernation too close to the breeding season to become habituated. This made them the most successful predators.

Despite the problems with hedgehogs, camouflaging the plover’s smell boosted its breeding success. Further study will be needed to check the method won’t harm native predators, ensure it doesn’t interfere with the prey species’ behaviour, and refine it. But it’s a promising technique to mitigate the impacts of invasive mammalian predators worldwide.

Dr Price’s study is also one of the first to explore habituation in the wild – despite being almost universal to all organisms, habituation has mostly been studied in laboratories only. Her work also implies that invasive mammals hunt by general scent cues, not species-specific ones, which was not previously known.

The New Zealand government has set the ambitious target of eradicating all invasive vertebrate predators by 2050, so Dr Price’s research may only be used in the short term there. Nonetheless, it will be highly relevant elsewhere in tackling the widespread economic and environmental problems posed by invasive species.

Photograph: Nick Read via Wikimedia Commons

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