It sounds like good news for city dwellers: Urban areas harbor more plant species than rural areas. But it's really not.
There may be more species in urban areas, but they come from fewer plant lineages than their country cousins. Thus, they have a narrower, more homogenized genetic base and so are less likely to thrive under climate change and other environmental perturbations. Also, they harbor traits that favor their spread into new habitats and don't support pollinators like bees and butterflies.
Those are some of the conclusions of a new study, led by University of Minnesota ecologist Jeannine Cavender-Bares and first author Sonja Knapp, a postdoctoral fellow from the Helmholtz Centre for Environmental Research in Germany. Published in the journal Ecology, the report documents how the makeup of plant communities changed as one moved from the city of Minneapolis to the University's Cedar Creek Ecosystem Science Reserve an hour to the north.
They found that assemblages of plants in cities had lower phylogenetic diversity, meaning they comprised fewer branches of the plant kingdom. But the difference may level out as "city-slicker" plants move into the country, bringing less-than-desirable traits with them.
"There's concern that losing diversity in urban areas affects the ability of plant communities to adapt and continue to provide ecosystem services in a changing environment," says Cavender-Bares, an associate professor of ecology, evolution and behavior. "Those services include water retention, shade, water purification, carbon sequestration, and soil stabilization and quality."
The study builds on work by the Twin Cities Household Ecosystem Project (TCHEP), showing that the actions of householders influence urban pollution.
"This new study demonstrates that household choices about landscaping have major impacts on the diversity of urban ecosystems as well," says co-author and CBS ecology professor Sarah Hobbie, who is a TCHEP project leader.
More species, less diversity
Why are there more species in urban areas?
"I think it's because people like different components in their yards," says Cavender-Bares. "We like wooded areas, shrubs, perennial beds, annual beds, lawns and vegetable gardens. Even the biggest yards have the same components as the smallest."
There are two human "filters" that lower the diversity of urban environments, she says. First, the "urban heat island" effect is well documented; it selects for plants that can tolerate higher temperatures.
"You also can get strong winds down city blocks, which affects dispersal of seeds and pollination," says Cavender-Bares.
Second, humans choose which species to cultivate, based on ease of cultivation, history, appearance and so forth. But species from that pool can escape cultivation and jump to new areas. Such plants represent just a select few lineages that are both chosen by humans and capable of escape.
Thus, urban yards act like stepping stones for "spontaneous" yard species like dandelions and buckthorn, which invade easily and are quickly spread. As cities expand, more of these species may turn up in the landscape at large.
Threats from self-pollinators and exotics
The researchers point out that the urban environment favors self-pollinating species. The isolation of yards makes it hard for bees, butterflies, and moths to visit all the flowers, and so the do-it-yourself strategy is favored. But as self-pollinators spread, the pollinators' food supply and ease of visiting all the nectar-bearing flowers shrinks. And so may their numbers.
Also, exotic species tend to live in yards rather than in rural areas, in large part because people choose them. In the study, 59 percent of all spontaneous yard species were exotic, while just 16 percent of species at Cedar Creek were exotic to Minnesota.
But we're not helpless
If humans are creating the problem, we can also mitigate it, Cavender-Bares says.
"There are nurseries that specialize in native plants," she says. "Many are shade-tolerant. Also, re-establishing prairies is gaining popularity.
"The big thing is that we have control; we can do something about this just by being aware. We [should] plant native species and think about what we value in planting. Is it diversity? Resemblance to natural areas? Or only showy flowers? If we have an esthetic that values form and function, we can manage our urban environment to increase the services it provides."
— Deane Morrison
“We should plant native species and think about what we value in planting. Is it diversity? Resem- blance to natural areas? Or only showy flowers? If we have an esthetic that values form and function, we can manage our urban environment to increase the services it provides."
— Jeannine Cavender-Bares