Here, looking out from the Anthropocene, we tend to regard nature as something other than ourselves. Wild places and wildlife, it seems, are commodities to be preserved and boxed off in national parks or in other protected areas where people do not live; while we reside in cities or towns, places that we think of as definitely not “natural.” This way of looking at nature probably started in the late 19th century, when a new, utilitarian cast of mind took hold; when America’s wild lands became “conservancies,” and wildlife stopped being fellow creatures with whom we share the Earth and instead turned into “resources” to be managed.
There were some, though, who early on warned us that this reasoning was faulty. For at least a hundred years, environmentalists have tried to impress upon us that every being and every process on Earth is connected. For example, author, naturalist and environmental philosopher John Muir famously wrote, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.”
Now, science is showing us how right that concept is. In North America, says a new scientific study, mammals live most successfully alongside people. And from a second report, we know that people thrive best when there is a biodiversity of wildlife nearby.
North American mammals benefit by our presence
In a study, published in the journal Global Change Biology in April 2021, a team of researchers led by scientists at the University of California, Santa Cruz, analyzed data from 3,212 camera traps to show how human disturbance (defined as any disruption caused by people, such as deforestation, mining, pollution or urbanization) could be shifting the makeup of mammal communities across North America. Previously, the team had studied how wildlife in California’s Santa Cruz Mountains responded to human disturbance. They found that some species, such as bobcats and cougars, are less likely to be active in areas where humans are present, while deer and wood rats became bolder and more active. But generalizing findings such as these across larger geographic areas is difficult because human-wildlife interactions are often regionally unique.
So, to get a continent-wide sense for which species of mammals might be best equipped to live alongside humans, the team combined their local camera-trap data with that of researchers throughout Canada, Mexico and the United States. This allowed them to track 24 species across 61 regionally diverse areas to see what larger trends emerged. The team was especially interested in understanding how mammals respond to different types of human disturbance and whether these responses were related to animal traits, such as body size, diet and the number of young they have.
What they found was that 33 percent of mammal species responded negatively to humans—meaning that they were less likely to occur in places with higher disturbance and were less active when present—while 58 percent of species were positively associated with human disturbance.
Wildlife profits by living within the human footprint
The researchers then broke down their results by two different types of human disturbance. One was the footprint of human development: the things that people build, such as agricultural fields, houses and roads. The second was the mere presence of people, including activities such as recreation and hunting, since fear of humans can change an animal’s behavior and use of space.
In comparing continent-wide data from camera-trap locations with varying levels of human development, researchers found that grizzly bears, lynx, wolverines and wolves were generally less likely to be found in more developed areas and were less active when they did visit.
Meanwhile, raccoons and white-tailed deer were more likely to hang out in more developed areas and were more active in these spaces. Coyotes and striped skunks weren’t more likely to be found in developed landscapes, but they did tend to be more active in these areas.
Some of the species that frequent more developed areas may truly benefit from living in these places, although that’s not necessarily the case. For example, while raccoons can thrive in developed areas by finding food in our garbage cans and avoiding predators, higher levels of cougar activity in these same places could mean something very different. It could be that a camera trap may have been set up in the one pathway that a cougar can use when it’s navigating its way through an otherwise heavily developed landscape. In other words, some animals in the study may be increasingly active or present on cameras near human development simply because there are so little remaining natural habitats.
Still, there were certain traits that emerged across species as clear advantages for making a living within the footprint of human development. Overall, mammals that were smaller, faster-reproducing and having generalist diets were the most positively associated with development. Researchers expected that they might find similar results in comparing camera-trap data by levels of human presence; but in fact, both positive and negative responses to human presence were observed for species across the spectrum of body sizes and diets.
Elk were less likely to stick around in places frequented by humans; and moose and mountain goats were less active in these habitats. On the other hand, bighorn sheep and black bears were more likely to be found in areas frequented by humans, while gray foxes were more active.
Outdoor recreation means more wildlife pathways—up to a point
One trend that may be influencing these findings is the growth of outdoor recreation, which increases levels of human presence in otherwise remote and wild landscapes. The study’s results may indicate that most mammals are willing to tolerate some level of human recreation in order to remain in high-quality habitats, and they could instead be increasing their nocturnal activity to avoid humans. Some animals may even take advantage of hiking trails and fire roads as easier passages.
But the study also clearly demonstrated that there is a limit to how much human impact animals can withstand. Even among species that were either more active or more likely to be present around humans or in developed areas, those effects peaked at low to intermediate levels of human disturbance, then began to decline beyond those thresholds. Red foxes were the only animals in the study that seemed to continue to be more active or present at medium to high levels of human disturbance.
Ultimately, most species have both something to lose and something to gain from being around humans. Understanding the cutoff where costs outweigh the benefits for each species will be important in the future for maintaining suitable habitats that support diversity. And that may prove to be the University of California, Santa Cruz, scientists’ most important contribution. The thresholds that they’ve started to mark can help us get a sense of how much available habitat is really out there for recolonizing or reintroducing species, hopefully allowing us to more effectively coexist with wildlife in human-dominated landscapes.
At the same time that the University of California, Santa Cruz, study came out, a paper published in Proceedings of the National Academy of Sciences of the United States of America by National Science Foundation scientists concluded that living with a biodiversity of wildlife is good for human health.
There is a persistent myth that wild areas with high levels of biodiversity are hot spots for disease. The thinking goes that the more wild animals there are out there, the more the likelihood of dangerous pathogens existing. But biodiversity isn’t a threat to us; it’s actually protecting us from the species most likely to make us sick.
Zoonotic diseases such as COVID-19, Ebola and severe acute respiratory syndrome are caused by pathogens that are shared between humans and other vertebrate animals. But animal species differ in their ability to pass along microorganisms that make humans sick. In landscapes with more animal diversity, these risky reservoirs become less abundant, so biodiversity has a protective effect.
Animals with short lives, such as rats, tend to be more efficient at transmitting pathogens. Creatures that live fast, die young and have early sexual maturity with lots of offspring tend to invest less in their adaptive immune responses. They are often better at transmitting diseases, compared to longer-lived animals—such as wolves—with stronger adaptive immunity. And when biodiversity is lost from ecological communities, it’s the long-lived, larger-bodied species that tend to disappear first, while smaller-bodied species with short lives tend to proliferate. Living with larger species of wildlife, then, is much better for our health.
Another author, naturalist and environmental philosopher, Aldo Leopold, once wrote, “There are some who can live without wild things, and some who cannot.”
I wholeheartedly agree with the second part of that statement; but now, I think, we can say to the first, most likely not.
Here’s to finding your true places and natural habitats,