For innumerable generations across western North America, cougars and mule deer have been locked in the struggle of predator versus prey. But humanity and its ubiquitous artificial light may now be scrambling this relationship.
The Intermountain West region of the US is sparsely populated, but contains many cities growing at an explosive rate. This means the region鈥檚 inky black night skies are becoming increasingly illuminated, and this extends beyond the vicinity of just roads and houses, says Mark Ditmer, a wildlife ecologist at Colorado State University.
Most studies on the impact of artificial light on animal behaviour have been conducted in laboratory settings, and targeted small animals like insects and birds. Little is known about how larger animals respond to brighter nights.
Advertisement
Ditmer and his team obtained detailed data on sources of artificial light in Arizona, California, Colorado, Idaho, Nevada, New Mexico, Oregon and Utah from the NASA-NOAA Suomi polar-orbiting satellite. They then compared this with GPS location data for hundreds of cougars (Puma concolor) and mule deer (Odocoileus hemionus) in the region, and the locations of more than 1500 sites where cougars killed mule deer.
Taken together, the team found that mule deer in floodlit areas are drawn closer to the light source 鈥� often a home surrounded by tasty, green vegetation. Cougars must target these deer by hunting from the shadows in the darkest parts of the landscape. This contrasts with how cougars normally hunt at night. They typically choose to hunt in spots that are slightly brighter than their surroundings 鈥� although these spots are still dim.
The findings also suggest that if there is a lot of artificial light, deer sometimes change their behaviour again. They begin to congregate in darker pockets instead, leaving the cougars only well-lit areas full of humans from which to hunt. Cougars may avoid these places entirely, allowing the deer to live another day.
Night-time artificial light is profoundly changing how these two species interact with each other, with potential ecological reverberations, says Ditmer. 鈥淎ltering predator-prey interactions could ecologically impact everything from the plant community down to soil,鈥� he says.
Colleen St. Clair, a wildlife biologist at the University of Alberta, Canada, thinks the study suggests that artificial light is contributing to the global phenomenon of burgeoning populations of urban deer, which use the high illumination as cover from predators.
The research 鈥渦nravels some very complex effects of anthropogenic light on predator-prey relationships that have probably been building for decades without anyone noticing鈥�, she says.
In the future, Ditmer wants to investigate how illuminated roads influence wildlife behaviour and to see if light wavelengths beyond humans鈥� visible spectrum have similar effects.
Ecography
