Scaring animals is good for nature

New research published in the journal Nature Communications suggests that when it comes to controlling wild populations of animals we've, in part, got it wrong. Normally, we cull deer or elk because their large numbers have significant impacts on the environment; but it turns out that simply fearing predators may be as effective as predation itself at keeping species numbers in check. Graihagh Jackson has the story...

Graihagh -   This is the sound of a racoon having a very happy time foraging on a beach in Canada.  They're so cute and cuddly with those stripey tales and bandit masks but actually, they're a bit of a nuisance to those who have to live side by side with them as my Aunty Elaine found out when she moved across the pond.

Elaine - We decided to landscape the back garden so we had all this fresh sod delivered and laid - it looked lovely.  Anyway we left our garden that night only to come down in the morning to find all the turf had literally been rolled back like a carpet; beautifully done, complete straight lines, no damage to any of the grass and we'd no idea who or what could have done it.  We never suspected the racoons so we rolled it all back into place and stamped it down and the next morning we woke to the same scene, and so began the battle, and it actually went on for weeks - it turns out they can roll the lawn up indefinitely.  We finally won with a mixture of cayenne pepper and water sprayed on the grass and, apparently, racoons hate it.  Actually, it took a lot of diligence on our part but gradually they stopped or maybe they actually just found a fresh garden to terrorise - who knows.

Graihagh - Garden wrecking aside, these racoons have succeeded in being such a nuisance, in part, because of us humans...

Justin - So my name is Justin Suraci and I am a PhD student at the University of Victoria.  Humans have succeeded in wiping out large carnivores in habitats across the globe and one of the major consequences is outbreaks of the things that large carnivores eat.  So, things like deer and elk, or smaller predators like coyotes and racoons and so these species can devastate the plants and animals that they consume when the large carnivores are no longer around to keep them in check.

Graihagh - And so normally people's solution to this is just to trap and hunt them but your research is showing that that isn't enough?

Justin - Yes exactly.  So we've attempted to control these outbreaks through hunting and trapping programmes; but what we're doing there is we're killing some of their prey but what we can't replicate is this fear that a large carnivore would instill in all of it's prey, essentially all of the time, which can lead to reductions in prey foraging behaviour and, essentially, play a major role in keeping the populations in check.

Graihagh - How did you find out or come to the conclusion that fear is really important here?

Justin - So we came to that conclusion through an experiment that we did in the Gulf Islands of British Columbia in Canada.  So this is a coastal archipelago in which all the native large carnivores have all been killed off - it's essentially racoon paradise. So there's nothing to keep these smaller racoon predators in check; they're essentially devastating the small songbirds and the intertidal crabs and fish and we had the hunch this had something to do with the fact they no longer had anything to fear.

Graihagh - So you had the hunch - how did you test it?

Justin - We effectively reintroduced just the fear of large carnivores. So on multiple islands across multiple years we would choose shoreline sites and we would play either the sounds of large carnivores - so in this case barking dogs - or non-scary control sounds.  And the way we would do this is we would hang speakers from a tree and we monitored the behaviour of the racoons and the abundance of the marine prey.  And we found that when we reintroduced the fear of large carnivores, this has such a strong effect on racoon behaviour that you actually see an increase in the abundance of the crabs and the fish that raccoons eat.

Graihagh - How much of a difference though are we talking about here?

Justin - This 66% drop in raccoon foraging led to pretty substantial increases in the abundance of their prey.  Some of their major prey types are small intertidal fish, which we saw an 81% increase in the abundance of these fish and also large intertidal crabs - a species called the red rock crab - and we saw a 61% increase.

Graihagh - It's interesting to hear that fear alone had such a big impact.  Are you then advocating that we should be installing - I don't know, barking trees all over the world over?  Is there evidence to suggest that this would work in other species besides raccoons?

Justin - Yes, there is evidence to suggest that other species would be similarly sensitive to just the sounds of their predators however, it's unlikely to be an effective long term solution.  Like you said, if you've got a tree that's been barking at you for three months, you're going to eventually learn that that tree is not actually a threat.  So what we would suggest this study shows like the main conservation implication is that we actually need to promote the conservation, and potentially the reintroduction, of large carnivores or even allowing large carnivores to naturally recolonise.