Ecotourism may threaten predators and boost rodents

The mere sound of the human voice in the wild will send large and medium-sized carnivores into hiding and encourage rodents to prosper, according to new research.

Scientists from the University of California, Santa Cruz, US, wanted to find out what impact humans have on wildlife from our presence alone – that is, without hunting or interfering with the environment.

Their findings, published in the journal Ecology Letters, show that our presence can create a “landscape of fear,” with potentially disastrous effects on predators, while opening up the environment for rats and mice to forage more brazenly and widely. 

Lead author Justin Suraci says the team set up a grid of 25 speakers in two remote sites of the Santa Cruz Mountains that are home to a range of predators including pumas, bobcats, opossums and skunks. Both are in areas closed to the public, but wildlife would still have had some exposure to humans through activities such as illegal shooting.

The sound of humans talking was played intermittently through the speakers, as well as frog calls and periods of silence, and the behaviour of wildlife was monitored through a range of techniques, including GPS collars and food stations.

Pumas, the researchers found, responded to the sound of human voices by significantly reducing their activity and keeping their distance. Tracking showed they slowed their movement by 34% when voices were playing.

“When the frog recordings played, they would move right through the landscape,” says Suraci. “But when they heard human voices, they went out of their way to avoid the grid.” {%recommended 8917%}

Medium-sized predators changed their behaviour in significant ways, too: bobcats became much more nocturnal; skunks reduced their overall activity by 40%; and opossums reduced their foraging activity by a stunning 66%. 

“Bobcats pretty much gave up on daytime activity, shifting almost entirely to the night, when they presumably feel safer,” says Suraci.

Over time, these behavioural changes could have dire consequences for pumas and the other predators if their food intake drops, he says. 

In contrast, deer mice increased their range by 45%, and the intensity of foraging by mice and wood rats increased by 17%. 

The researchers interpret this result as showing that the rodents were aware that the risk of running into their natural predators was reduced, and taking advantage of the opportunity to forage more freely.

“It turns out, the mere perceived presence of humans triggers a disruption of natural predator-prey interactions – and rodents really benefit,” says Suraci.

The researchers say this is the first large-scale experiment that documents how fear of the human “super predator” cascades through the food web from top predators to the smallest prey, for whom human presence can act as a kind of shield by keeping the top carnivores at bay. 

“If similar human shield effects for small mammals are common where human activity is high, this could ultimately lead to increased small mammal abundance in wildlife areas frequented by people, a potentially undesirable consequence of ecotourism,” the researchers note.