In ecology, as in comedy, timing is everything.
Hours, minutes or even seconds can make the difference for an animal between stumbling upon a predator and avoiding one, between finding a bush full of berries and discovering branches that have already been nibbled. Mere moments can determine whether a raccoon comes face to face with a bobcat at night, whether a flock of cocky turkeys finds its field already busy with cranes, and whether a deer disappears into the trees before a coyote appears on the scene.
Animal fortunes, and the health of entire ecosystems, can hinge on these ephemeral encounters—or not, fortunate encounters. “An animal has to be in the right place, at the right time, to avoid predators, find food, and reproduce successfully,” said Neil Gilbert, a postdoctoral researcher at Michigan State University.
In this way, the interactions between animals in a given ecosystem resemble a stage production, he said, adding, “For a production to be successful, every actor has to be on stage, in the right place, and they have to act and deliver their lines at the right time.”
Now, a new study reveals how humans may be inadvertently rewriting these environmental scripts, changing how characters interact and fueling more interspecies encounters.
To conduct the study, Gilbert and his colleagues analyzed images captured by Snapshot Wisconsin, a citizen science project run by the Wisconsin Department of Natural Resources. Since 2016, volunteers have deployed more than 2,000 wildlife cameras across the state, capturing tens of millions of images of Wisconsin’s fields, farms, and forests—and the animals that frequent it most.
Scientists reported at PNAS last month that wild animals of different species were more likely to live nested lives — appearing at local camera sites in faster succession — in human-altered landscapes, such as ranches, than in undisturbed locations, such as national forests.
The findings suggest that human disturbances can bring animals closer together, increasing the odds of them running into each other. “There’s less room for the elbows,” Gilbert said.
Although more research is needed, this interspecies pressure could have implications such as making it more difficult for prey to avoid predators, intensifying competition for resources or increasing the risk of disease transmission between species, the researchers say.
“Compression of species niches is likely to lead to new interactions between species with unknown outcomes,” Benjamin Zuckerberg, an ecologist at the University of Wisconsin-Madison and author of the study, said in an email.
Strangers on the plain
Snapshot Wisconsin was created by the Wisconsin Department of Natural Resources in an effort to collect continuous, statewide data—at all hours of the day and during all seasons of the year—on local wildlife populations. It relies on an army of volunteer camera hosts to install, monitor, and maintain its wildlife cameras, on both public and private lands throughout the state.
Cameras, triggered by motion and body heat, captured an array of animals going about their daily lives: bald eagles scavenging in the snow, bears climbing trees, newborn fawns, and a flock of otters gambling on a grassy fairway. “It’s a very large number of otters,” said Jennifer Stenglen, a quantitative research scientist with the Wisconsin Department of Natural Resources and author of the new study.
(The department posts many of the photos to Zooniverse, an online citizen science platform where volunteers from around the world can help identify the creatures in each shot.)
For the new study, the researchers analyzed nearly 800,000 photos of animals taken over four years. To assess a species’ “co-occurrence,” they calculated the amount of time that elapsed between the moments when members of 74 species—turkeys and deer, or wolves and skunks—appeared at a given camera location.
If coyotes and skunks routinely appear at the same place within an hour or a day of each other, they are more likely to have overlapping habitats and routines—and to encounter each other in the real world—than if days or weeks pass between appearances, the scientists reasoned.
The time periods between detections vary greatly. Sometimes, cameras captured the odd pairs of animals in the same frame; Other times, days or weeks may pass between their appearances.
But overall, across all pairs of animals, the trend was clear: In relatively pristine habitats, like national forests, about six days elapsed, on average, between detections. In the most human-altered habitats, this interval decreased to an average of four days.
Over a three-month period, the researchers estimated that highly hostile pairs—that is, a pair in which one species is likely to kill the other, such as bobcats and rabbits or foxes and squirrels—would encounter each other seven more times more often. Landscapes are highly turbulent compared to less turbulent landscapes. (Even when animals aren’t face-to-face, simply hearing or smelling a predator can have “dramatic effects” on the behavior of a prey species, Gilbert noted.
“It’s going to be great to see who will be the winner and who will be the loser in this human-pushed niche space,” Zuckerberg said.
“For example, will prey and lower competitors need to adapt new defenses or behaviors?” question. Can they even do that?
The scientists also found that much of the effect was driven by differences in relative abundance; Species like raccoons and squirrels tend to be more numerous in human-caused landscapes — where dumpsters are overflowing and fields thick with grain — than in wild landscapes.
But these differences did not fully account for the findings, suggesting that some species may also alter their behavior in human-altered habitats, becoming active at different times of the day or ranging on a smaller scale. (Gilbert noted that animals with less room to run around would be more likely to collide, like gas molecules in a shrinking container.)
However, many questions remain, including whether the findings generalize to other species and ecosystems and what, specifically, happens when these creatures meet, even when the encounters are captured on camera.
How did bobcats chase coyotes? Who won the skunk vs raccoon showdown? And why does this deer look as if it’s about to kick an opossum in the face? (“Like, what did that poor opossum do?” Gilbert wondered.)
More broadly, do species like deer and raccoons interact with each other when they meet on a dark path? Or are they just fleeting, like sentient ships in the night? “It’s hard to completely disengage,” Zuckerberg said.
But the study shows the potential for using wildlife cameras to examine aspects of an animal’s behavior that might otherwise be hard to notice, Stengeln said.
“We didn’t sit in the field and watch the animals interact,” she said. “But there is a lot of power in being able to use this tracking camera data to understand how animals behave. It just, to me, opens up a door of possibilities.”