In a study of 15 small community parks located in Phoenix neighborhoods with distinct socioeconomic classifications ranging from lower to upper income, Arizona State University ecologists Ann P. Kinzig and Paige Warren measured the abundance and diversity of both birds and trees. The researchers chose parks rather than residential yards because these city-controlled spaces offered comparable environments for the study sites, with a similar landscape (grass, athletic fields, facilities and scattered trees) but significant differences in the surrounding neighborhoods.
"What we are seeing is a pretty strong trend in the data," said Kinzig. "We can't explain bird diversity in the parks by the size of the parks, or the types or sizes of trees in the parks, which is what we might expect. Instead, the characteristics of the neighborhood, including the income of the residents, seem to play a significant role in influencing the number of species that live in the park ."
Trees and other vegetation are considered to be a major factor affecting bird populations. But the study's findings on diversity and abundance of park trees, which are the primary vegetation in the survey sites, do not correspond with the bird data. While bird populations were found to be most diverse in upper income neighborhood parks and progressively less diverse in parks in middle and lower income neighborhoods, tree diversity is actually highest in lower income neighborhood parks.
The lack of influence of park vegetation is even clearer when the researchers examined bird abundance (the total number of birds seen, regardless of species), with summer bird abundance actually being consistently lower where tree abundance is higher.
The lack of correlation between trees and the bird populations is important, Kinzig notes, because the variety and number of trees planted are among the few significant variables present in the make-up of the parks themselves, and could be affected by economic factors. Tree landscaping is done by the city, and could conceivably be influenced by the neighborhood's age and economic status.
Though the study eliminates park landscaping as a factor, it does not yet pinpoint specific explanations for how neighborhood economic status could affect bird populations.
"Something that happens in the radius of 200 meters from the park boundaries is influencing the diversity of birds," Kinzig noted. "Whatever people are doing is having an influence, because we can't explain it with the park itself. There's a variety of things - it could be what people are planting, it could be socio-economic differences in how often you feed birds, maybe the rich people have more bird feeders...
"It could be something as small as the feral and domestic cats and other predators that live in the neighborhoods - not just how many people have cats, but how many of them are prowling around wild and are good at catching birds, or whether or not people put bells on their necks... Or it could be zoning; what the city plants on the median strips; or how much industrial and commercial activity is allowed," she said. "We don't know, but it's something about the differences in people's lifestyle."
Though the mystery is hard to unravel, there are still some ecological factors that Kinzig thinks could be relevant.
"We still want to look at reproductive success in the parks. There may be something that's really influencing reproduction and that has an influence on the bird community," she said. "We'll look at food sources during the breeding season. What do people plant in their yards that birds eat when they breed? Maybe some neighborhoods are better than others for insects... we need to do arthropod surveys in these areas. Maybe it's water... dog dishes are important water sources for birds... maybe it's the distribution of dog dishes.
"We may not need to have all these absolute measures, but we need to learn fundamentally how these neighborhoods differ from each other," she said. "Cities are where people live now, and parks are going to provide their daily access to nature. We have to understand what kind of nature people will have in their parks, and what determines that."
The urban parks study is expected to continue for three more years, with periodic updates thereafter.
For more information on urban parks research and on other research of the Central Arizona-Phoenix Long Term Ecological Research Project, see http://caplter.
Source: Ann Kinzig, 480-965-6838
Note: This research will be presented at a session at the Ecological Society of America meeting in Tucson, Arizona on Thursday, August 8, 2002