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Tuesday, April 16, 2024

You Can Turn Your Backyard Into a Biodiversity Hot Spot

People have long stoked an urban-versus-rural rivalry, with vastly different cultures and surroundings. But a burgeoning movement—with accompanying field of science—is eroding this divide, bringing more of the country into the city. It’s called rurbanization, and it promises to provide more locally grown food, beautify the built environment, and even reduce temperatures during heat waves. 

It’s also reversing the longstanding assumption that growing food is straight-up bad for biodiversity because clearing land for agriculture necessitates removing native plants and animals. Ecologist Shalene Jha of the University of Texas, Austin says this idea was based on observations of rural agriculture, where growing industrialized swaths of corn or wheat can be catastrophic for existing ecosystems. But that doesn’t hold for the urban farms, gardens, and even smaller green spaces.

In a recent paper in the journal Ecology Letters, Jha and her colleagues showed that urban gardens can actually boost biodiversity—particularly if residents prioritize planting native species, which attract native insects like bees. “The gardener actually has a lot of power in this scenario,” says Jha. “It doesn't matter how large or small the garden is. It's the practice of cultivating the landscape—and the decisions they make about the vegetation and the ground cover—that ultimately decide the plant and animal biodiversity there.”

Jha’s team characterized the biodiversity of 28 California urban gardens over the course of five years. Far from the mono-cropped monotony of a wheat field, they found rich ecosystems humming with activity that, in turn, increased species diversity. The researchers found predators like birds and ladybugs, which prey on crop-munching insects and thus help increase yields, and an abundance of pollinators like bees, which also benefit from crop diversity and increase plant productivity. That means urban gardens aren’t just producing food for people, but for other species as well. “They’re actually supporting incredibly high levels of plant and animal biodiversity,” Jha says. 

This biodiversity is largely due to a strategic trade-off. One of the challenges of urban gardening is that it requires intensive manual labor: You can’t drive a combine through a city at harvest time. But that limitation turns out to be an ecological blessing. Because everything is done by hand, urban farmers can grow all sorts of plants right next to each other, packed in tightly to increase yields. 

In another study published this month in the journal Agronomy for Sustainable Development, a separate team of researchers looked at 72 urban agriculture sites in France, Germany, Poland, the United Kingdom, and the United States. “We see pretty diverse growing spaces that often are growing a huge variety of crops, as well as non-food products,” says study author Jason Hawes, an environmental sustainability researcher at the University of Michigan. On average, the sites grew 20 different crops. “Lots of folks were also just growing flowers for fun in their visual gardens, and the community gardens have flowers planted to make the space more pleasant,” he says. “These sorts of things do contribute to local biodiversity.”

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That biodiversity hardens urban ecosystems against pests that would have a field day with a monocrop. “You all of a sudden have a mimic of what naturally occurs in terms of insect predation. Because if you build it, they will come,” says Colorado State University horticulturalist Jennifer Bousselot, who studies the practice of growing crops on rooftops and wasn’t involved in either of the new papers. Aphids may descend on your garden, for instance, but so too will the ladybugs that keep their population in check.

An additional benefit is that gardeners can stagger the growth of flowering plants throughout the year—known as succession planting—essentially extending the food supply for local pollinators. While native plants are best at attracting native animals, she says, pollinators can make do with something like an imported tomato plant. “I am a big fan and advocate for native plants, but I’m an even bigger advocate for planting a plant with purpose,” says Bousselot. “And believe it or not, most pollinators can adapt.” 

Bees in particular love a good urban garden, especially if it’s loaded with ornamental flowers. Previous research has shown that bee diversity can actually be higher in cities than in surrounding rural areas. It’s counterintuitive, but the flower diversity in an urban garden can be greater than that of a corn or wheat field. 

And while brown patches of dirt may not look nice to you, bees literally dig them. Unlike honey bees that live in swarming hives, the vast majority of species are actually solitary, and many burrow into dirt for shelter. Having open ground in a garden provides them habitat. Bees also hate open spaces because that’s where birds, dragonflies, and other predators patrol. “It’s like putting a giant target on their back saying ‘Come and eat me,’” says biologist Gerardo Camilo, who studies urban bees at Saint Louis University but wasn’t involved in the papers. “They like messiness.” If you leave your yard looking a little shabby, bees will have places to hide as they bounce from flower to flower.

“There’s a take-home message here for everyday gardeners: With relatively minimum effort, you can make a big change,” says Camilo. “You don’t need to be consciously improving the environment. You can just concentrate on your one little thing—which is the growth of some food—and do it in the right way, and you can have significant impacts.” 

But there are still limits to what an individual gardener can do. A bee has to get to your backyard in the first place. That means cities need chains of green spaces—with some open dirt and messy vegetation—so the insects can travel safely. “The neighborhood that surrounds your garden must be inviting to bees, to afford them essentially a road to travel to work,” says Camilo.

That kind of solution requires group action and city planning, but it comes with a whole host of knock-on benefits. Greenery absorbs stormwater, for instance, mitigating urban flooding. Plants also “sweat” water vapor, dramatically lowering temperatures, which get much higher in cities than in rural areas because of the preponderance of concrete. Green spaces are great for mental health. Food waste can go straight into urban gardens as compost, reducing dependence on synthetic fertilizers, which are terrible for the planet. And producing food closer to where it’s consumed could reduce the emissions associated with shipping produce.

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There’s also mounting evidence that growing crops in urban areas can be far more productive than on rural farms. Yields for certain vegetables, like cucumbers, can be up to four times greater. That’s due in large part to urban farmers tending their crops by hand, which requires a lot of work but also creates jobs. 

Deploying more gardens in cities, though, comes with challenges. Real estate is expensive, so it won’t be cheap to set aside land. And urban agriculture needs water, which is always at a premium. Ideally, it could come from rain catchment systems, but cities might also be able to build infrastructure to funnel stormwater into green spaces, where it can soak into the ground. “I think we saw in the recent flooding in California that there is a huge amount of public support for finding new ways to deal with runoff that don’t just involve letting it run out to sea,” says Hawes.

There’s real urgency behind doing whatever it takes to bring more agriculture into urban areas: More than half of humanity now lives in towns and cities, a figure expected to rise to 5 billion people by the end of this decade. “Urban gardens are producing 15 to 20 percent of our food supply globally, and that number is just increasing,” says Jha. “And so their value—not only in terms of food production, but also in terms of the plants and animals they support—is increasingly important.”

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