When animals invade new places they either sink, swim, or just get along. Unfortunately, when an invader does well in its new land, it can have devastating consequences for the native animals and plants. Suddenly there is a new competitor or predator in the midst and the natives don’t have the skills, spines or teeth to compete or defend themselves. Evolution works over many generations and there’s rarely time to adapt. But people can help.
Understanding how animals interact may help us tip the ‘balance’ in favour of native Australian animals. Here’s one example from the ongoing battle against the invasive cane toad (Bufo marinus – a native of South America). Can meat ants and cat food slow them down?
Meat ants (Iridomyrmex reburrus) are Australian and are voracious scavengers. They’re known for cleaning the meat off rotting carcasses – hence their name. But they also happen to be quite good at eating the first adult stage of cane toads (the metamorph).
The cane toad secretes bufotoxins which can deter its predators from eating it, potentially poisoning them if they do consume one (don’t lick these toads!). That’s one of the reasons cane toads have been able to spread across the top end of Australia. But its spread has caused major ecological problems because there are lots of these toads, they poison their predators and eat just about anything they can fit in their mouth!
The meat ant seems to be quite immune from the toxin and will happily tuck into a baby toad. Professor Rick Shine, Georgia Ward-Fear, Matt Greenlees and Greg Brown from Sydney University thought there might be more to it.
“Meat ants are abundant around tropical water bodies, and we often see them eating small toads, so we suspected that there might be some kind of mismatch between the invader and its newly invaded range, for example something about the toads’ behaviour that makes them vulnerable to a predator that poses little danger to native frogs.” says Professor Shine
They went to work in the lab and the field to discover the intriguing differences between native frogs and the cane toads.
Native frogs seem to make all the right moves to avoid the meat ants.
- Native frogs spotted the meat ants coming and hopped away, cane toads didn’t seem to notice them and stayed put!
- Native frogs were more active during the night which helped avoid the ants, cane toads were active during the day when the ants are foraging.
- Metamorph cane toads were not good at jumping. Once the meat ants found the toads there was no escape.
- Cane toad metamorphs often ‘played dead’ (crypsis) when the ants attacked…in the hope the ants would just go away?
It seems that the cane toad metamorphs are at a disadvantage compared to Australia’s native frogs. If only there were enough meat ants, they might put a dent in the cane toad population without harming Australia’s native frogs.
That’s where the cat food comes in. By placing cat food baits around water holes containing cane toad tadpoles, the team managed to increase the abundance of meat ants. The ants did well on the additional food source. When the tadpoles turned into the cane toad metamorph and emerged from the ponds the hungry meat ants were waiting.
Not many toads escaped the meat ants. The ants found 98% of the cane toad metamorphs, 84% of them were attacked within 2 minutes. Over 50% of the of the attacks killed the toads before they could escape and 88% of the toads that did manage to escape died within 24 hours. The meat ants were very effective.
Professor Shine and his team acknowledge that its not the panacea for Australia’s cane toad problem. But it certainly looks like a method that works with nature to take advantage of a weakness in the cane toads life cycle, without endangering native frog species.
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