Urban gardens can be biodiversity hotspots, but little is known about the drivers of the biodiversity of species that occur in cities at low frequencies.
Plant species generally uncommon in urban areas but planted 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 applications, show that women, older gardeners and those who live near the gardens tend to manage rarer plants.
Theresa Ong (right), assistant professor of environmental studies and co-lead author of the study, and co-author Azucena Lucatero at Mi Jardin Verde, Watsonville, California, one of the community gardens in the study. (Photo courtesy of Theresa Ong)
“There appears to be a cascading effect of people planting unusual species on the accumulation of other unusual bee and bird species,” said co-lead author Theresa Ong, an assistant professor of environmental studies.
More than 50% of the plants observed in urban gardens were categorized as rare, meaning they were not typically found in the gardens in the study.
“This means that people are planting a wide variety of unusual plants. What we also found 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 Ong. “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 serve 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.
purple tick (Haemorhous purpureus) at Miami University Bird Blind, Oxford, Ohio. (Photo by Andrew Cannizzaro / CC x 2.0)
The purple finch (Haemorhous purpureus), for example, was a bird rarely seen in the urban gardens that were part of the study, but considered a fairly common species elsewhere. It is on the International Union for Conservation of Nature’s Red List of Threatened Species as one of the least concern species.
“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 would otherwise lack,” says Ong.
The research was based on fieldwork from 18 community gardens in the counties of Santa Clara, Santa Cruz and Monterey, California. The city gardens vary by age; local habitat; landscape context such as ground cover and foliage; and socio-economic demographics of the horticulturists. Organic gardening is practiced in all gardens, where pesticides and insecticides are prohibited.
There seems to be a cascading effect of people planting unusual species on the accumulation of other unusual bee and bird species.
Theresa Ong, assistant professor of environmental studies
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.
For bird data, all birds seen or heard were documented while 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 unusual 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 of the plants were classified as rare in the study, with 159 plants representing 156 different species.
Taro (Colocasia esculenta) at the Botanical Gardens of Madeira, Funchal, Portugal. (Photo by Nasser Halaweh / CC BY-SA 4.0)
“Taro (Colocasia esculenta), was one of the rare plants found in urban gardens, but is of no importance for conservation, especially since it is cultivated,” says Ong. “It is 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 a sweet potato tuber, but it is 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 seen in the study includes “Bachelor’s buttons” (who are in the genus centaurea ornamental plants with striking ray florets belonging to the aster family) and the leaf-cutting bee (Megachile apical)whose female bees neatly cut the leaves to make nests for their young, usually in rotting wood.
One bird species the team encountered and labeled as rare in urban gardens 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. Recently, populations have declined, which is a cause for concern. It is a major predator of garden pests such as mice and voles, so it is 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 rare in cities, says Ong. Researchers found that gardens with more foliage, meaning more trees, provide a better habitat for the unusual birds.
Male American Kestrel (Falco sparverius) at Raptor, Inc. (Photo by Greg Hume / CC BY-SA 3.0)
As for explaining the results of why rare plants are more likely to be planted by women, elderly individuals and community members living near urban gardens, Ong says: “Previous research has shown that women tend to have a more displaying eco-friendly attitudes, and with age comes wisdom and knowledge of cultivation techniques, but a lot may have to do with people simply looking to take care of their own neighborhood and take pride in that landscape and community, which we believe encourages greater 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 a garden can be a good thing.