Through experiments, the group found that insects, whose populations are controlled in the native desert mainly by seasonal scarcity of vegetation, instead are kept in check in the city by a larger population of birds. Bird populations are large because of ample water and the absence of their natural predators -- especially hawks (though they are partially replaced by cats). In turn, the lack of predation leads the city birds, most of whom are mainly seed-eating species, to spend more time hunting nutrient-rich insects, a behavior that would be risky around predators.
Other behavioral and trophic shifts also appear to follow. Particularly intriguing is the further implication that populations and trophic structures vary somewhat within the city depending on the economic status of the neighborhood. Economic issues appear to be a controlling factor for the urban ecosystem, much like climate and the abundance or absence of resources is in the surrounding desert environment.
"The urban ecosystem is driven very largely by the local economy," said Grimm. "The populations are systematically altered by the decisions that we make and the relationships of the animals have been shuffled."
"There's also a relationship our researchers are seeing between the spatial distribution of where the water is, where all the plants are and where the wealth is," she noted. "This is something that we still don't know the mechanisms of, but one of the most interesting findings is that the diversity of plants in the Phoenix area is related strongly to family income -- higher family income, higher plant diversity; lower family income, lower plant diversity. We don't know the mechanisms, but it's an interesting phenomenon."
Grimm and Redman note
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Contact: James Hathaway
hathaway@asu.edu
480-965-6375
Arizona State University
3-Jan-2005