Rare plants attract rare bees and birds in urban gardens
Bachelor's buttons (Centaurea cyanus). Credit: Azucena Lucatero

It's not known what drives the diversity of species existing at the smallest frequencies or rare diversity in urban gardens. The study found that rare bee and bird species are attracted to rare plant species in urban gardens. The results show that people who live near gardens tend to find more rare plants.

There appears to be a cascading effect of people planting rare species on other rare species.

In the urban gardens, over half of the plants were rare. People are planting a lot of unusual plants. What we found is that what is rare in an urban garden can be quite common elsewhere. Some rare species are at greatest risk of extinction. In urban gardens, rare organisms may be at higher risk of extinction, but they could also be less adapted to the urban environment. "Rare status could be a sign that urban gardens are acting as an important conserved habitat for rare species in cities but could also be a sign of what's to come for the future of the species."

A Dartmouth-led study on urban gardens in northern California (Santa Clara, Santa Cruz and Monterey) finds rare plants attract rare bees and birds. Women, older gardeners, and gardeners who live near the urban gardens planted more rare plants. The rare species found in the gardens included the poppy, leafcutting bee, Bachelor's button, purple finch, and taro. “What is rare in an urban garden can be quite common elsewhere,” says Theresa Ong, assistant professor of environmental studies at Dartmouth. Credit: LaDarius Dennison

One rare bird that was observed in the urban gardens that were part of the study was the purple finch. The species is on the Red List of Threatened species. If it shows that their populations are declining, that could be a cause for concern. It is1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556

There are 18 community gardens in Santa Clara, Santa Cruz and Monterey, California. Local habitat, landscape context, and age are some of the factors that affect the urban gardens. Pesticides and insecticides are not allowed in all of the gardens.

Data was collected from the urban garden sites over two summers. The plant, bee and bird data was obtained in 2015. Different methods were used to sample the organisms.

Rare plants attract rare bees and birds in urban gardens
Leaf cutting bees (Megachile apicalis) and thistle (Cirsium sp.). Credit: Malcolm Bowey

The researchers visited the urban gardens and took random samples, noting the plants that were cultivated, as well as the weeds and other vegetation present. Crop or ornamental species were measured and categorized. Plants that aren't cultivated for consumption are called ornamental plants. Researchers stood in the center of each garden for 10 minutes and documented all the birds that were seen or heard. Pan traps elevated off the ground and aerial nets were used to catch bees. Gardeners were asked what plants they planted.

The team used the field data on plants, bees and birds to model the correlation between demographic information and rare plants grown by gardeners.

If a plant, bee or bird was found in one of the 18 urban gardens or in 2 of the 185 gardener survey responses, it was considered rare. Over half the plants were rare, with 159 plants representing 156 different species.

Rare plants attract rare bees and birds in urban gardens
Purple finch (Haemorhous purpureus) at Miami University Bird Blind, Oxford, Ohio. Credit: Andrew Cannizzaro (CC x 2.0)

One of the rare plants found in the urban gardens is Teo, which is cultivated. A lot of Asian cultures planted it in Hawaii. It requires a lot of space to grow and is not a common food in California. It is thought that gardeners who grow less common species of crops like taro cultivate less common habitats for other species.

Some rare plants have a special relationship with rare bees and serve as hosts to the other. "Bachelor's buttons" and the leafcutting bee are two rare plant-bee duos that were observed in the study.

The American kestrel was one of the rare birds that the team encountered. The most common and widespread falcon in the world is a small one. The population decline has been a cause of concern. It's an important predator of garden pests like mice and voles, so it's a good sign when it is found in some urban gardens, as it means that gardens can be managed in a way that provides habitat for a species that is rare in cities. According to researchers, gardens with more canopy cover give better habitat for rare birds.

"Previous research has shown that women tend to display more pro-environmental attitudes, and with age comes wisdom and knowledge of the environment, which may explain why rare plants are planted more often by women, older individuals and community members living in close proximity to the urban gardens." A lot may have to do with people just wanting to take care of their own neighborhood and having pride in that landscape and community, which encourages more biodiversity in terms of rare species.

A lot of the rare plants found in the urban gardens were weeds and leaving some in your garden could be a good thing.

More information: Theresa W. Ong et al, Rarity begets rarity: Social and environmental drivers of rare organisms in cities, Ecological Applications (2022). DOI: 10.1002/eap.2708 Journal information: Ecological Applications