Light pollution alters predator-prey interactions between cougars and mule deer in western US

A new study provides strong evidence that exposure to light pollution alters predator-prey dynamics between mule deer and cougars across the intermountain West, a rapidly growing region where nighttime skyglow is an increasing environmental disturbance.

The University of Michigan-led study, published online Oct. 18 in the journal Ecography, is the first to assess the impacts of light pollution on predator-prey interactions at a regional scale. It combines satellite-derived estimates of artificial nighttime lights with GPS location data from hundreds of radio-collared mule deer and cougars across the intermountain West.

The study found that:

• Mule deer living in light-polluted areas are drawn to artificial nighttime lighting, which is associated with green vegetation around homes.
• Cougars, also known as mountain lions and pumas, are able to successfully hunt within light-polluted areas by selecting the darkest spots on the landscape to make their kill.
• While mule deer that live in dark wildland locations are most active around dawn and dusk, those living around artificial night light forage throughout the day and are more active at night than wildland deer--especially during the summer.

The animal data used in the study were collected by state and federal wildlife agencies across the region. Collation of those records by the study authors yielded what is believed to be the largest dataset on interactions between cougars and mule deer, two of the most ecologically and economically important large-mammal species in the West.

"Our findings illuminate some of the ways that changes in land use are creating a brighter world that impacts the biology and ecology of highly mobile mammalian species, including an apex carnivore," said study lead author Mark Ditmer, formerly a postdoctoral researcher at the University of Michigan School for Environment and Sustainability, now at Colorado State University.

The intermountain West spans nearly 400,000 square miles and is an ideal place to assess how varying light-pollution exposures influence the behavior of mule deer and cougars and their predator-prey dynamics. Both species are widely distributed throughout the region--the mule deer is the cougar's primary prey species--and the region presents a wide range of nighttime lighting conditions.

The intermountain West is home to some of the darkest night skies in the continental United States, as well as some of the fastest-growing metropolitan areas, including Las Vegas and Salt Lake City. Between the dark wildlands and the brightly illuminated cities is the wildland-urban interface, the rapidly expanding zone where homes and associated structures are built within forests and other types of undeveloped wildland vegetation.

For their study, the researchers obtained detailed estimates of nighttime lighting sources from the NASA-NOAA Suomi polar-orbiting satellite. They collected GPS location data for 117 cougars and 486 mule deer from four states: Utah, Arizona, Nevada and California. In addition, wildlife agencies provided locations of 1,562 sites where cougars successfully killed mule deer.

"This paper represents a massive undertaking, and to our knowledge this dataset is the largest ever compiled for these two species," said study senior author Neil Carter, a conservation ecologist at the U-M School for Environment and Sustainability.

Deer in the arid West are attracted to the greenery in the backyards and parks of the wildland-urban interface. Predators follow them there, despite increased nighttime light levels that they would normally shun. Going into the study, the researchers suspected that light pollution within the wildland-urban interface could alter cougar-mule deer interactions in one of two ways.

Perhaps artificial nighttime light would create a shield that protects deer from predators and allows them to forage freely. Alternatively, cougars might exploit elevated deer densities within the wildland-urban interface, feasting on easy prey inside what scientists call an ecological trap.

Data from the study provides support for both the predator shield and ecological trap hypotheses, according to the researchers. At certain times and locations within the wildland-urban interface, there is simply too much artificial light and/or human activity for cougars, creating a protective shield for deer.

An ecological trap occurs when an animal is misled, or trapped, into settling for apparently attractive but in fact low-quality habitat. In this particular case, mule deer are drawn to the greenery of the wildland-urban interface and may mistakenly perceive that the enhanced nighttime lighting creates a predator-free zone.

But the cougars are able to successfully hunt within the wildland-urban interface by carefully selecting the darkest spots on the landscape to make their kill, according to the study. In contrast, cougars living in dark wildland locations hunt in places where nighttime light levels are slightly higher than the surroundings, the researchers found.

"The intermountain West is the fastest-growing region of the U.S., and we anticipate that night light levels will dramatically increase in magnitude and across space," said U-M's Carter. "These elevated levels of night light are likely to fundamentally alter a predator-prey system of ecological and management significance--both species are hunted extensively in this region and are economically and culturally important."

Carnivores living near people feast on human food, threatening ecosystems

 Ecologists at the University of Wisconsin-Madison have found that carnivores living near people can get more than half of their diets from human food sources, a major lifestyle disruption that could put North America's carnivore-dominated ecosystems at risk. The researchers studied the diets of seven predator species across the Great Lakes region of the U.S. They gathered bone and fur samples for chemical analysis from areas as remote as national parks to major metropolitan regions like Albany, New York. They found that the closer carnivores lived to cities and farms, the more human food they ate.

While evolution has shaped these species to compete for different resources, their newfound reliance on a common food source could put them in conflict with one another. That conflict could be reordering the relationship between different carnivores and between predators and prey, with an unknown but likely detrimental impact on ecosystems that evolved under significant influence of strong predators.

Jon Pauli, a UW-Madison professor of forest and wildlife ecology, and his former graduate student Phil Manlick, published their findings this week in the Proceedings of the National Academy of Sciences. The study is the most comprehensive look yet at how most of the region's major carnivores -- like gray wolves, coyotes, and bobcats -- have changed their diets in response to people.

How much human food they ate varied considerably by location. On average, more than 25 percent of the carnivores' diets came from human sources in the most human-altered habitats.

It also varied by species. For instance, committed carnivores like bobcats ate a relatively small amount of human food. "But what you see is that the sort of generalist species that you might expect -- coyotes, foxes, fishers, martens -- in human-dominated landscapes, they're getting upwards of 50 percent of their diet from human foods," says Manlick, the lead author of the study who is now a postdoctoral researcher at the University of New Mexico. "That's a relatively shocking number, I think."

Pauli and Manlick found that relying on human food sources increased how much carnivores overlapped one another in their competition for food. Compared to when these predators vie for distinct prey, this increased competition could lead to more conflicts between animals. Their reliance on human food could also make the carnivores vulnerable to human attacks near towns, or even change how and when they hunt traditional prey, with potentially harmful ecological consequences.

The researchers studied the diets of almost 700 carnivores, including red and gray foxes, fishers, and American martens. They gathered bone and fur samples from Minnesota, Wisconsin, New York and the Upper Peninsula of Michigan with the help of state and federal researchers and citizen-science trappers. The researchers compared the carnivores' diets to the extent of human development in the region, which varied from essentially pristine wilderness to urban sprawl.

Thanks to quirks in how plants incorporate carbon as they grow, a sample of bone or fur is enough to get a snapshot of an animal's diet. Different weights, or isotopes, of carbon are common in different plants -- and in the animals who ultimately eat them.

"Isotopes are relatively intuitive: You are what you eat," says Manlick. "If you look at humans, we look like corn."

Human foods, heavy in corn and sugar, lend them distinctive carbon signatures. In contrast, the diets of prey species in the wild confer their own carbon signatures. The ratio of these two isotope fingerprints in a predator's bone can tell scientists what proportion of their diet came from human sources, either directly or from their prey that ate human food first.

The geographic extent of the study and the large number of species the ecologists examined demonstrate that the trend of human food subsidies in carnivore diets is not limited to a single location or species. The ultimate outcome of such widespread disruptions remains unclear.

"When you change the landscape so dramatically in terms of one of the most important attributes of a species -- their food -- that has unknown consequences for the overall community structure," says Pauli. "And so I think the onus is now on us as ecologists and conservation biologists to begin to understand these novel ecosystems and begin to predict who are the winners and who are the losers."