A photo of a monarch butterfly perched on a pink flower.
Gregory Bull/AP

The milkweed needed to stabilize the country’s monarch-butterfly population thrives in metropolitan areas—especially on residential land.

The number of Bengal tigers is dwindling. Orangutans and some African elephant populations are also at risk. Monarch butterflies are dropping out of the air, and may end up on the endangered list by 2020, too. Blame the encroachment of human footprints and human-driven development for their deaths—indeed, blame humans for much of the deforestation, overfishing, and climate change that are shrinking the variety of the natural world.

But a growing body of research suggests that human-dense cities and flourishing wildlife aren’t incompatible, after all. It’s in urban areas that animals like fishers and coyotes and bullfinches and peregrines are finding new life, and on patches of city terrain that birds and dragonflies and butterflies are perching as they complete their migratory paths. Partly because they tend to be in coastal and riparian areas with high biodiversity, cities are becoming crucial havens for many animal species as once-open lands are transformed by agriculture and development.

A pair of new articles by researchers at the Keller Science Action Center and the U.S. Fish and Wildlife Service, published in the open-access journal Frontiers in Ecology and Evolution, suggests that cities could actually be integral in “help[ing] curb a potential ‘sixth mass extinction’”—but only if they act quickly, and smartly, to become sites of conservation.

The researchers suggest cities start by focusing on preserving monarch butterflies, whose numbers have fallen precipitously in the last two decades as the milkweed plant—where butterflies must lay their eggs—disappears. Because of their distinctive appearance and popularity, monarchs are the “ideal ambassadors” for the conservation movement, the researchers argue, although they’re part of a larger group of pollinators that include flower beetles, hoverflies, and mosquitoes, too.

“Beetles are incredible pollinators, but a campaign to ‘plant for beetles’ probably wouldn’t go anywhere,” Derby Lewis, a senior conservation ecologist at Chicago’s Field Museum and a co-author of both papers, said in a statement. “By helping Monarch butterflies, we’re helping other pollinators, which are on the decline.”

Together, pollinators are responsible for helping 80 percent of the world’s flowering plants, like avocados and apples, reproduce, and in turn, allowing many of the world’s animals to eat.

To help stabilize the monarch population, we need to plant more milkweed—North America needs as many as 1.8 billion additional stems. How much of that could go in metropolitan areas?

To answer that, the researchers used satellite and spatial-resolution data to identify where milkweed already thrives in four metropolitan regions: St. Paul-Minneapolis, Chicago, Kansas City, and Austin, all of which are along the monarchs’ migratory route.

Baseline average milkweed densities by US census blocks for all four metropolitan areas. Darker areas indicate a higher milkweed density than lighter areas. (Keller Science Action Center)

They also analyzed how these cities’ “low-quality green space,” like grass lawns, could be transformed “into high-quality homes for these butterflies.” Turns out it’s residential single-family and multi-family land uses, right-of-way areas, and vacant lots that have the most potential for planting. The maps below take the dense streets of Chicago and strip them to their plantable space.

The first map shows the intensity of development; the second, a more detailed analysis of land use; and the third, an isolated view of plantable space, prime for milkweed. (Sources: Multi-Resolution Land Characteristics Consortium (U.S.), “National Land Cover Dataset (NLCD)” and University of Vermont Spatial Analysis Lab, “Chicago Urban Tree Canopy.”)

Strikingly, in three of the four metro areas, the plantable land that was both most readily available and best fit for growing milkweed was residential in nature, meaning it lies partly in people’s backyards. (Agricultural land, while technically most free for planting, is largely sullied with milkweed-killing pesticides, and already heavily farmed.)

Taking those four cities as regional ideals, the researchers predict that if all the land that’s fit for milkweed planting in the entire urbanized northern and northeastern U.S. was used, it could support 15 percent of the stems needed to stabilize the Eastern monarch population. And if the effort was expanded to include everywhere east of the Rocky Mountains, that number could account for 30 percent.

Cities have grown pollinator gardens, pollinator boulevards, and pollinator parks, but individuals have been slower to commit their own yards to the cause. ”Certainly, having the term ‘weed’ in the name of milkweed—which isn’t a weed—has not helped the reputation of these superstar native plants,” Lewis said. Changing that perception, and encouraging more green infrastructure projects (on rooftops and in parks and beyond), is a good place to start.

Urbanized areas make up only 3 percent of the country’s land mass but are home to 80 percent of its people. Those humans could reshape their habitats to better welcome pollinators, too.

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