Hours, minutes or even seconds can make the difference for an animal between stumbling upon a predator and avoiding one, between finding a bush loaded with berries and discovering branches that have already been gnawed bare. 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 occupied by cranes, whether a deer disappears into the trees before a coyote appears on the scene.
An animal’s fortunes, and the health of entire ecosystems, can hinge on these ephemeral encounters — or lucky non-encounters. “An animal must be at the right place, at the right time, to avoid predators, find food, reproduce successfully,” said Neil Gilbert, a postdoctoral researcher at Michigan State University.
In that way, the interactions between the animals in a given ecosystem are like a theatrical production, he said, adding, “For the production to be a success, each actor has to be onstage, in the right place, and they must act and deliver their lines at the right time.”
Now, a new study reveals how humans might unwittingly rewrite these ecological scripts, altering how the 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 fauna that frequent them.
Wild animals of different species were more likely to lead overlapping lives — appearing at local camera sites in quicker succession — in human-altered landscapes, like farms, than in more undisturbed locations, such as national forests, scientists reported in PNAS last month.
The finding suggests that human disturbance can squeeze animals closer together, increasing the odds that they bump into each other. “There’s a little less elbow room,” Gilbert said.
Although more research is needed, that interspecies squeeze could have effects such as making it harder for prey to evade predators, intensifying competition for resources or increasing the risk of interspecies disease transmission, the researchers say.
“The compression of species niches will likely lead to new interactions among species with unknown consequences,” Benjamin Zuckerberg, an ecologist at the University of Wisconsin-Madison and an author of the study, said in an email.
Strangers on a Plain
The Wisconsin Department of Natural Resources created Snapshot Wisconsin in an effort to collect continuous, statewide data — at all hours of the day and during all seasons of the year — on local wild animal populations. It relies on an army of volunteer camera hosts to install, monitor and maintain wildlife cameras, on both public and private land across the state.
The cameras, which are triggered by motion and body heat, have captured a menagerie of animals going about their everyday lives: bald eagles scavenging in the snow, bear cubs climbing trees, a newborn fawn, a bevy of otters gamboling down a grassy trail. “It’s just so many otters,” said Jennifer Stenglein, a quantitative research scientist at the Wisconsin Department of Natural Resources and an author of the new study.
(The department posts many of the photos on 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 captured over the course of four years. To assess species “co-occurrence,” they calculated how much time elapsed between the moments when members of 74 species pairs — turkeys and deer, for instance, or coyotes and skunks — appeared at a given camera site.
If coyotes and skunks are routinely showing up in the same place within an hour or a day of one another, they are more likely to have habitats and routines that overlap — and to encounter one another in the real world — than if days or weeks pass between appearances, the scientists reasoned.
The time intervals between detections varied enormously. Sometimes the cameras captured the odd animal couples in the same frame; other times, days or weeks might pass between their appearances.
But overall, across all animal pairs, the trend was clear: In relatively pristine habitats, such as national forests, roughly six days elapsed, on average, between detections. In the most human-altered habitats, that interval dropped to an average of four days.
Over a three-month period, the researchers estimated, highly antagonistic pairs — that is, duos in which one species was likely to kill the other, such as bobcats and rabbits or foxes and squirrels — would encounter each other seven additional times in the most highly disturbed landscapes compared with the least disturbed ones.
(Even when the animals do not come face-to-face, simply hearing or smelling a predator can have “dramatic effects” on the behaviors of prey species, Gilbert noted.)
“It will be fascinating to see who will be the winners and who will be the losers in this human-compressed niche space,” Zuckerberg said.
“For example, will prey and lesser competitors need to adapt new defenses or behaviors?” he wondered. Can they even do so?
The scientists also found that much of the effect appeared to be driven by differences in relative abundance; species such as raccoons and squirrels tended to be more numerous in human-disturbed landscapes — where dumpsters overflow and fields are thick with grain — than in wilder ones.
But these differences did not entirely account for the findings, suggesting that some species might also change their behavior in human-altered habitats, becoming active at different times of day or ranging less widely. (Animals with less space to roam would be more likely to collide, like gas particles in a shrinking vessel, Gilbert noted.)
Still, many questions remain, including whether the findings generalize to other species and ecosystems and what, precisely, is happening when these creatures meet, even when the encounters are caught on camera.
How did the bobcat chase off the coyote? Who won the skunk-raccoon faceoff? And why does that deer look as if it’s about to kick a snarling opossum in the face? (“Like, what did this poor opossum do?” Gilbert wondered.)
More broadly, are species like deer and raccoons actually engaging with one another when they meet on a dark trail? Or are they simply passing by, like sentient ships in the night? “It is difficult to fully tease apart,” Zuckerberg said.
But the study illustrates the potential for using wildlife cameras to probe aspects of animal behavior that might otherwise be difficult to observe, Stenglein said.
“We didn’t sit in the field and watch animals interact,” she said. “But there’s so much power in being able to use this trail camera data to understand how animals are behaving. It just, to me, opens up a floodgate of possibilities.”