Urban gardens can be hotspots for biodiversity in cities, but little is known about the drivers of the biodiversity of species that occur in the smallest frequencies, or rare biodiversity. Rare plant species in urban gardens attract rare bee and bird species, according to a Dartmouth-led study of urban gardens in Northern California. The results, published in Ecological applicationsshow that women, older gardeners and those who live near the gardens tend to manage rarer plants.
“There appears to be a cascading effect of people planting unusual species on the accumulation of other unusual bee and bird species,” said lead author Theresa Ong, an assistant professor of environmental studies at Dartmouth.
More than 50% of the plants observed in urban gardens were rare. “This means that people are planting a wide variety of unusual plants. What we also discovered is that what is rare in an urban garden may be very common elsewhere and not necessarily how we would define rarity in less managed systems,” says Aan. “In less managed systems, rare species are often those most at risk of extinction.” She notes that while rare organisms in urban gardens are at greater risk of extinction, they may simply be less adapted to the urban environment, or, in the case of plants, less popular to grow. “Rare status can be a sign that urban gardens act as an important habitat for the conservation of rare species in cities, but can also be a harbinger of what is to come for the future of the species,” says Ong.
The purple finch (Haemorhous purpureus), for example, was a rare bird observed in the urban gardens that were part of the study, but is considered a fairly common species elsewhere. It is on the IUCN Red List of Threatened Species as one of the least concern species. Ong says, “Their rare status in urban gardens could be a cause for concern if it indicates their populations are declining. However, urban gardens may also provide more habitat, such as tree cover for purple finches and other birds that cities otherwise lack.”
The research was based on fieldwork from 18 community gardens in the counties of Santa Clara, Santa Cruz and Monterey, California. The urban gardens vary by age, local habitat, landscape context (ie ground cover, foliage, etc.) and socio-economic demographics of the gardeners. All gardens use organic gardening where pesticides and insecticides are prohibited.
The team collected data from the urban gardens during two summers in 2015 and 2017, from May to September. Data on plants, bees and birds were collected in 2015, and data from urban gardeners was collected two years later. Each of the organisms was sampled by a different method depending on the organism.
For planting data, the researchers visited the city gardens and took random samples, including from the garden plots and surrounding walkways, noting which plants were grown, as well as the weeds and other vegetation present. Plants were measured and classified into crop or ornamental species and cultivars. Ornamental plants are plants that are grown not for consumption but mainly for aesthetic reasons. For bird data, all birds seen or heard were documented, as researchers stood in the center of each yard for 10 minutes. For bee data, bees were captured with pan-traps raised from the ground and aerial nets and were later identified down to the species level. Researchers also directly asked gardeners what plants they were growing in the gardens.
Using field data on plants, bees and birds, the team modeled the correlation between horticulturists’ demographic information, the rare plants grown by the horticulturists, and the rarity of bird and bee species.
A plant, bee or bird species was only considered “rare” if it appeared in one of the 18 urban gardens or in 2 of the 185 responses to the gardener survey. Of the 295 total plants observed, more than half were rare, with 159 plants representing 156 different species.
“Taro (Colocasia esculenta), was one of the rare plants found in urban gardens, but is of no importance for conservation, especially as it is cultivated,” Ong says. “It’s a traditional crop planted in Hawaii and by many Asian cultures. It requires a lot of space to grow and is cooked for its underground tuber, much like the tuber of a sweet potato, but it’s not a common food grown in California.” Ong suspects that gardeners who grow less common types of crops such as taro also cultivate less common habitats for other species, such as the heavily watered soil necessary for taro to grow well.
Previous research has shown that some rare plants are known to have a specific relationship with rare bees, which serve as hosts for the others. One such rare plant-bee duo observed in the study are “Bachelor’s buttons” (Centaurea sp., a genus of ornamental plants with showy ray florets belonging to the thistle aster family), and the leafcutter bee (Megachile apicalis), whose female bees neatly cut off leaves to make nests for their young, usually in rotting wood.
One rare bird species the team encountered was the American kestrel (Falco sparverius). “It is a small and very cute falcon that is actually the most common and widespread falcon on the continent. Populations have declined recently which is a cause for concern. It is a major predator of garden pests such as mice and voles, so it’s a good sign when found in some urban gardens, as it indicates that gardens can be managed in a way that provides habitat for a species that is rare in cities,” Ong says. Researchers found that gardens with more canopy, meaning more trees, provide a better habitat for rare birds.
As to explaining the results on the possible reason why rare plants are more often planted by women, elderly individuals and community members living near urban gardens, Ong says: “Previous research has shown that women tend to exhibiting a more eco-friendly attitude, and with age comes wisdom and knowledge of growing techniques, but a lot may have to do with people just wanting to take care of their own neighborhood and taking pride in that landscape and community, which we believe is actually encourages more biodiversity in terms of rare species.”
The team’s findings also showed that many of the rare plants in urban gardens were weeds, so leaving some weeds in your yard may be a good thing.
Urban gardens are a reliable food source for pollinators all year round
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
Provided by Dartmouth College
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