City parks are often oases for wildlife in otherwise hostile urban landscapes. Parks can serve important conservation roles both for the cities and the larger environments they’re situated in, but in order to design and manage urban parks to meet conservation goals, we must understand how different park features impact the biodiversity housed within.

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In a new paper published in Nature Cities this month and led by Dr. Frank La Sorte, a senior scientist at the BGC Center, researchers investigated how park features, such as park size or tree canopy cover, affect bird diversity by analyzing data from nearly one thousand different parks across 186 U.S. cities. While no clear “winning” combination of features stood out, the study suggested that careful and intentional park management could support a thriving and diverse array of birds. “To support bird diversity in cities year-round,” said La Sorte, “our study highlights the importance of having multiple parks distributed across the city that contain different collections of park features.”

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Birds are somewhat unique among other species groups thanks to their ability to fly: while small mammals and amphibians are more likely to call a park home year round, birds typically utilize much larger areas for food, nesting, and other activities, and many of them migrate seasonally. This means that throughout the year, the assemblage of bird species in a park will often change drastically. So, the researchers analyzed a 16-year time series of bird observation data pulled from the citizen science initiative eBird across the four seasons to investigate not just how bird diversity differed between parks, but also throughout the year. 

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The researchers focused on two sometimes overlooked components of biodiversity in addition to the more typical measure of species richness: phylogenetic diversity, which measures the uniqueness of evolutionary lineages amongst a species assemblage – for example, an assemblage of only passerine birds would have less phylogenetic diversity than an assemblage of passerines, hawks, owls, and waterfowl – and functional diversity, which measures the diversity in species traits and ecological roles – for example, an assemblage of only insectivorous birds would have less functional diversity than an assemblage of insectivorous, carnivorous, and fish-eating birds. While related, these three measures of diversity are not necessarily correlated and each capture different aspects of biodiversity; altogether, they paint a more complete picture of biodiversity, the researchers say. 

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The park features the researchers investigated included five describing the internal characteristics of the park – total area, the proportion of the park covered by trees, habitat variability, the amount of water, and the shape of the park – and two features describing the spatial location of the park – proximity to other parks and distance from the edge of the city. Previous studies had investigated the effect of park area, but none had so thoroughly examined such a broad set of features before.

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Across all U.S. city parks, the researchers found that species richness was highest in the spring and autumn, aligning with migration seasons, and lowest in winter and summer; conversely, phylogenetic diversity was highest in the summer and lowest in spring while functional diversity was highest in the spring and autumn and lowest in winter. Four park features – area, proportion covered by trees, habitat variability, and amount of water – were consistently identified as the most important predictors for all three components of bird biodiversity. 

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But, how these three features affected bird biodiversity along the three components varied in substantial and even unexpected ways. Larger area led to increased species richness, as expected, but from there, the results are more complicated and spatially varied. 

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More water meant higher species richness across seasons and generally higher phylogenetic diversity, but lower functional diversity across seasons. The researchers postulate this pattern could be due to large water bodies hosting many aquatic species, which may be diverse in species richness and evolutionary history but tend to fill similar functional roles. More variable habitat meant higher species richness across seasons, especially in the autumn, but lower functional diversity. Parks closer to the edge of the city had higher species richness in the summer and higher phylogenetic and functional diversity in the autumn. 

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“The most surprising finding was that tree canopy cover had a variable relationship with bird diversity across the US,” said La Sorte. La Sorte and coauthors revealed a sharp contrast in the role of trees between eastern and western U.S. cities: in the west, more trees were accompanied by higher species richness and phylogenetic diversity while the opposite relationship was true for the east. “Our results point to a largely positive role for trees in western cities where forests are often a novelty and a largely negative role for trees in eastern cities where forests are heavily managed to create a savannah like landscape with a few large, widely spaced trees,” said La Sorte. 

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Taken collectively, the researchers say these results emphasize that there is no one optimal park configuration that will support all three measures of diversity in all regions or seasons. Instead, careful design and management of city parks is needed to meet different biodiversity outcomes, and the results of this study can help city planners and park managers set and meet their goals for bird biodiversity. 

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Of course, urban parks are designed and managed to meet multiple different goals within and outside of conservation. “Urban parks are typically designed to provide a safe and attractive environment for human recreation, which can compete with biodiversity conservation,” said La Sorte. “For example, forests in eastern urban parks are often managed to minimize ground cover and dead wood, features that are essential for many bird species, such as woodpeckers, but are considered dangerous and unappealing by park designers. By allowing for the presence of mature forests within some parks, birds and many other taxa would benefit, and a unique educational opportunity would be created for park visitors.”

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There’s also a wide diversity of species beyond birds who use and inhabit city parks, from pollinating insects to burrowing mammals to aquatic species, if water bodies are present. Similar studies haven’t been done for these groups, but it’s likely the trends in biodiversity for these species would be very different from birds. “Birds display a broad diversity of form and function that allows them to occupy widely different ecological niches in urban parks,” said La Sorte. “Many other taxa lack this ecological breadth, and it is likely that they would require a more specific set of features to be able to occur in urban parks.”

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Nevertheless, birds are in many cases iconic inhabitants of city parks, much loved by birders and more casual park goers alike. This study has revealed that supporting a diversity of birds in city parks is possible if careful attention is paid to how those parks are designed and managed. 

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