Don’t let the rats fool you. Although pizza-pilfering vagrants – and various other creatures – thrive in cities, for many wild animals, urban environments are unattractive homes, shrouded in concrete and carved by car traffic. As buildings go up and roads are laid, some species seem to disappear from the landscape, and animal communities often become less diverse, scientists have found.
But not all cities are created equal. Urbanization appears to have a greater impact on wild mammals in warmer, less vegetated areas than in cooler, greener, according to a new study, published in Nature Ecology & Evolution on Monday. The findings suggest that climate change may exacerbate the effects of urbanization on wildlife.
“As our climate warms, the heat of our cities is something that will continue to challenge us and wildlife,” said Jeffrey Haight, a postdoctoral scholar at Arizona State University and an author of the new study. .
Researchers analyzed images taken by wildlife cameras at 725 sites in 20 North American cities. The cities, which include Chicago, Phoenix, and Tacoma, Wash., are participants in the Urban Wildlife Information Network, an ongoing effort to collect data on urban biodiversity. In each city, cameras are deployed in an assortment of locations; some camera sites, such as those near airports or freeways, are highly urban, while others, such as parks and trails, are less developed.
Scientists studied the photos taken last summer. They found a total of 37 native mammal species, including raccoons, squirrels, rabbits, foxes, cougars and deer.
Overall, the researchers found, wild mammals were more common and more diverse in less urbanized sites, reinforcing findings from other studies. But wildlife seems to cope better with urbanization in cities that are cool or lush – homes to lots of healthy, green plant life – than in warmer or more barren ones.
For example, when camera sites became more urban, mammal diversity declined more sharply in hot Los Angeles than in cooler Salt Lake City. And although the Sanford, Fla. and Phoenix, Ariz. are both hot, Sanford has more plants than Phoenix. The urban areas of Sanford supported more diverse mammal communities than the same urban areas of Phoenix, the scientists found.
Researchers can’t yet say what underlies these patterns, but cities are known to trap heat, making them warmer than less developed areas nearby. In cities in warm climates, this urban heat island effect could “make it harder and harder to live,” speculated Dr. Haight. In colder areas, the relative warmth of cities can also be a boon to animals looking for a temperate home.
When it comes to plants, the garden itself can provide welcome food and shelter for urban animals. But green cities tend to be wetter cities, which may mean other resources, such as water, are more readily available, Dr. Haight.
Large-bodied animals, such as cougars and elk, have been more negatively affected by urbanization than smaller ones, the researchers found. That may be because larger animals need more space to roam. “Although there is a lot of housing within the cities, it is often destroyed,” said Dr. Haight. People may also be less tolerant of large animals roaming cities, he added.
Urban mammals have not been studied as much as urban plants or birds, and compiling data on 37 species in 20 cities was “an enormous undertaking,” said Christine Rega-Brodsky, an urban expert. 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 affect our native wildlife and overall biodiversity,” he said in an email.
The study has limitations. The cameras aren’t equally good at detecting all species, and the scientists only analyzed images from North American cities in the summer; different patterns may appear in other places or periods.
But the research highlights the way in which human-driven changes in the environment can have compounding effects, said Dr. Rega-Brodsky. It also points to potential solutions, suggesting that perhaps hot, barren cities can help protect their animal residents by providing plants, water and places where wildlife can escape the heat.
“Each city in the world has specific features that make it ecologically different from the next and require different strategies to preserve its biodiversity,” said Dr. Rega-Brodsky.