University of Minnesota

Live fast, die young

New study shows urban plant communities have traits that make it harder for them to adapt to change than their countryside counterparts.

April 17, 2012

Cities harbor more plant species than rural areas. However, plant species of urban areas are more closely related to each other and often share similar functions. As a result, urban ecosystems are likely to be more sensitive to environmental impacts than rural ecosystems, suggests a field study led by Jeannine Cavender-Bares, associate professor in the College of Biological Sciences and resident fellow of the Institute on the Environment at the University of Minnesota, and Sonja Knapp, a postdoctoral fellow from the Helmholtz Centre for Environmental Research in Germany. The new study was published March 19 as a preprint in the journal Ecology and was highlighted in the latest issue of Nature.

Knapp, Cavender-Bares, and colleagues from the University of Minnesota, the Max-Planck Institute for Biogeochemistry in Germany, the University of California and Whittier College in California compared plant diversity in private yards of the Twin Cities metropolitan area with plant diversity at the nearby Cedar Creek Ecosystem Science Reserve, part of the Long-Term Ecological Research network supported by the U.S. National Science Foundation. Using a newly established global plant trait database (TRY), they found that typical spontaneous backyard plant species are short-lived and fast-growing, produce small seeds, use humans rather than insect pollinators to spread, and are adapted to high temperatures. Many of these species are exotic horticultural species that now spread on their own. Rural plants, on the other hand, include fewer exotics and have a higher diversity of survival strategies.

The scientists expressed concern about invasive species being dispersed beyond yard boundaries and suggested that cultivating more native plant species could have positive effects on the evolutionary diversity and potential of urban vegetation. They also pointed out that the promotion of self-pollinated species by the urban environment might result in cascading effects on pollinators: "If self-pollinating species are supported by urbanization and consequently increase their frequency in the regional species pool, fewer pollinators such as bees or butterflies will be supported,“ said Sonja Knapp, the first author of the study.

“As cities expand, understanding how urbanization and urban gardening impact biodiversity and ecosystem services becomes increasingly important,” said Cavender-Bares. “The loss of diversity in the evolutionary lineages of plants that live in cities might, on the long term, make it more difficult for urban ecosystems to adapt to changes. These results suggest that urbanites should consider gardening and harboring a higher number of native species.”

The Minnesota study builds on previous work through the Twin Cities Household Ecosystem Project (TCHEP), showing that household choices are important influences over urban pollution. “This new study demonstrates that household choices about landscaping have major impacts on the diversity of urban ecosystems, as well,” said Sarah Hobbie, a collaborator on the biodiversity study and one of the TCHEP project leaders.

The study confirms results obtained by Knapp and colleagues in a 2008 study analyzing 14 million records of plant occurrences in a German database. Those researchers concluded that in the face of changing environmental conditions, conservation should focus not only on the protection of a high number of species, but also on more subtle aspects of diversity, such as how closely or distantly related those species are to one another. As urbanization reshapes our planet, they said, we’ll need better conservation strategies for urban biodiversity.

“As cities continue to grow, it becomes increasingly critical that we understand the impacts of backyard management on the overall strength of plant communities,” Cavender-Bares said. “Homogenization of urban yards through loss of native species and elimination of whole lineages from the plant ‘tree of life’ in cities could reduce the resiliency of urban areas to perturbation.”

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