Don’t let the rats fool you. Although the pizza-pilfering vagabonds — and a variety of other creatures — thrive in cities, for many wild animals urban environments are unappealing homes, covered in concrete and carved up by car traffic. As buildings go up and roads are laid down, some species seem to vanish from the landscape, and animal communities often become less diverse, scientists have found.
But not all cities are created equal. Urbanization appears to take a greater toll on wild mammals in hotter, less vegetated locales than in cooler, greener ones, according to a new study, which was published in Nature Ecology & Evolution on Monday. The findings suggest that climate change could exacerbate the effects of urbanization on wild animals.
“As our climate warms, the heat of our cities is something that is going to continue to be a challenge to both us and wildlife,” said Jeffrey Haight, a postdoctoral scholar at Arizona State University and an author of the new study.
The researchers analyzed photos snapped by wildlife cameras at 725 sites across 20 North American cities. The cities, which included Chicago, Phoenix, and Tacoma, Wash., were participants in the Urban Wildlife Information Network, an ongoing effort to collect data on urban biodiversity. In each city, the cameras were deployed in an assortment of locations; some camera sites, like those near airports or freeways, were highly urban, while others, like parks and trails, were less developed.
The scientists studied the photos taken during the summer. They detected a total of 37 native mammal species, including raccoons, squirrels, rabbits, foxes, cougars and deer.
In general, the researchers found, wild mammals were more common and more diverse at less urbanized sites, reinforcing findings from other studies. But wildlife seemed to cope better with urbanization in cities that were cool or lush — homes to plenty of healthy, green plant life — than in those that were warmer or more barren.
For instance, as camera sites became more urban, mammal diversity dropped off more sharply in warm Los Angeles than it did in cooler Salt Lake City. And although Sanford, Fla. and Phoenix, Ariz. are both similarly warm, Sanford has much more greenery than Phoenix. Urban areas of Sanford supported more diverse mammal communities than equally urban areas of Phoenix, the scientists found.
The researchers cannot yet say what underpins these patterns, but cities are known to trap heat, making them warmer than less developed areas nearby. In cities that are already in warm climates, this urban heat island effect could “just be making it harder and harder to live,” Dr. Haight speculated. In cooler locales, the relative warmth of cities might also be a boon to animals looking for a temperate home.
When it comes to vegetation, the greenery itself could provide welcome food and habitat for urban animals. But green cities also tend to be wetter cities, which could mean other resources, like water, are easier to come by, Dr. Haight said.
Larger-bodied animals, such as cougars and elk, were also more negatively affected by urbanization than smaller ones, the researchers found. That may be because larger animals require more space to roam. “Although there is plenty of habitat within cities, it’s often pretty broken up,” Dr. Haight said. Humans might also be less tolerant of large animals that wander into cities, he added.
Urban mammals are not as well studied as urban plants or birds, and compiling data on 37 species across 20 cities was “a massive feat,” said Christine Rega-Brodsky, an expert on urban ecology at Pittsburg State University in Pittsburg, Kan., who was not involved in the research. “Our world is rapidly urbanizing and experiencing a global extinction crisis, so we urgently need to understand how human actions impact our native wildlife and overall biodiversity,” she said in an email.
The study had limitations. Cameras are not equally good at detecting all species, and the scientists only analyzed photos from North American cities in the summer; different patterns might emerge in other places or seasons.
But the research highlights the way in which human-driven changes to the environment can have compounding effects, Dr. Rega-Brodsky said. It also points toward potential solutions, suggesting that perhaps hot, barren cities can help safeguard their animal residents by providing greenery, water and places where wildlife can escape the heat.
“Every city in the world has particular features that make it ecologically different from the next and require different strategies to conserve its biodiversity,” Dr. Rega-Brodsky said.