Study shows the amount of city greenspace has a bigger influence on number of species than connectedness and other factors.
Hawks in Senate Park, Washington, D.C.
Image Credit: U.S. Capital / via Wikicommons
A recent study at the University of Maryland showed that the total area of greenspace was often the most important factor influencing the number of bird species in cities by a wide margin. The impact of greenspace area--parks, cemeteries and golf courses--exceeded the impacts of factors that are typically considered important to urban biodiversity, like whether the greenspace was broken up or fragmented, or the flexibility of species’ diets.
In addition, the data showed that as the amount of greenspace in a city increased over time, so did the number of birds with a wider range of characteristics that are traditionally considered ill-adapted to city living, like a limited diet or range.
The study appears in the September, 2024, issue of Landscape and Urban Planning.
“When we talk about what makes an urban species, we focus on what traits help them do well in cities,” said Daniel Herrera, a PhD candidate in UMD’s Department of Environmental Science and Technology, and the study’s lead author. “This analysis kind of flipped that script and found it’s not the traits that make an urban bird, it’s the city that allows a bird to live there. The amount of habitat is actually more important.”
Herrera and his colleagues analyzed historical city maps and data from an annual census of birds called the Christmas bird count for Washington, D.C., Pittsburgh, PA, and Minneapolis, MN, between 1900 and 2020 to reveal how greenspace and bird populations changed over time. In each city bird diversity increased as the total area of greenspace grew.
In the early 1900s, Washington, D.C.’s designated greenspace totaled 16.8-square-kilometers, and the Christmas Bird Count recorded only 13 species, mostly sparrows. In 2020, greenspace had increased to 34.2-square-kilometers, and the bird count reported 131 bird species, with Canada geese being the most observed followed by European starlings, ring-billed gulls, and American robins. Results were similar in the other cities.
Comparing bird diversity and greenspace over time provided Herrera and his team with a more accurate picture of the impact of city planning decisions. Trees and other greenspace features can take years to become established, and previous studies that assess the impact of current greenspace on bird diversity don’t account for long-term impacts of newer, less mature greenspaces that will provide more habitat in the future. This is the first study that addresses that issue by analyzing 120-years of data.
By analyzing three cities, they were also able to assess the impacts of other measures like how fragmented the greenspace is, or how dense the human population is. These measures are typically used in conservation management to determine the number of species and individuals a city can sustain without human-wildlife conflict.
Depending on whether they focused on the number of species, number of individual birds, or some other metric, the amount of historical greenspace had up to six times the effect of population density and 7 to 12 times more impact than connectivity, in which greenspaces are close to one another and form a sort of connected pathway or “flyway” through a city.
The results imply that even cities without large areas of undeveloped land can be designed to support more bird diversity.
“Most cities can’t simply add square miles of park, but we found that even small additions of greenspace, like pocket parks, can make a huge impact,” Herrera said. “Our study shows that mean growth of 0.16 sq km per year across an entire city is enough to increase bird diversity.”
That’s about the size of 25 football fields, just 0.01% of the area of Washington, D.C. Herrera said these findings suggest that urban planners should consider preservation of greenspace as properties become available even if they are not connected to current park systems.
