Little is known about what drives rare biodiversity in cities. Rare plant species in urban gardens may be the answer. The plants in turn attract rare bee and bird species, according to a Dartmouth College-led study looking at urban gardens in northern California.

The U.S. National Science Foundation-supported results, published in Ecological Applications, show that women, older gardeners, and those who live near the gardens tend to curate more rare plants. “There appears to be a cascading effect of people planting uncommon species on the accumulation of other uncommon bee and bird species,” says lead author Theresa Ong.

More than 50% of the plants observed in the urban gardens were rare. “People are planting a great variety of uncommon plants,” says Ong. “What we found is that what is rare in an urban garden can be quite common elsewhere and is not necessarily how we would define rarity in less managed systems. In less managed systems, rare species are often those at greatest risk of extinction.”

In urban gardens, rare organisms may indeed be at higher risk of extinction, but they could also be simply less adapted to the urban environment, or in the case of plants, less popular to grow. “Rare status could be a sign that urban gardens are acting as an important conservation habitat for rare species in cities but could also foreshadow what’s to come for the future of these species,” says Ong.

Leaf cutting bees (Megachile apicalis) and thistle (Cirsium sp.). Credit: Malcolm Bowey

The research was based on fieldwork in 18 community gardens in the counties of Santa Clara, Santa Cruz and Monterey, California. The urban gardens varied by age, habitat, landscape context (ground cover, forest canopy cover, etc.), and socioeconomic demographics of the gardeners. All the gardens used organic gardening practices, where pesticides and insecticides are prohibited.

The team collected data from the urban garden sites in the summers of 2015 and 2017. Plant, bee and bird data were obtained in 2015 and urban gardener data were collected two years later.

Using the data on plants, bees and birds, the team modeled the correlation between gardener demographic information, the rare plants grown by the gardeners, and the rarity of bird and bee species.

A plant, bee or bird species was considered “rare” if it appeared in one of the 18 urban gardens or in two of the 185 gardener survey responses. Of the 295 total plants observed, more than half the plants were rare, with 159 plants representing 156 distinct species.

“This project starts to unravel what rarity means in different ecological settings and how it functions to support biodiversity,” says Amanda Simcox, a program director in NSF’s Division of Biological Infrastructure. “These scientists discovered that rare plants attract rare species of birds and bees, another positive role for green spaces in cities.”