Plant Diversity Has 'Luxury' Effect, Say Scientists; Biodiversity In Urban/suburban Yards Correlates With Household Income

Researchers at the National Science Foundation (NSF)-funded Central Arizona-Phoenix Long-Term Ecological Research (LTER) site found higher plant diversity in "upscale" neighborhoods. "The line flattens out, however, at about the $50,000 per year salary mark," said scientist Charles Redman. "When investigating urban systems, we must re-conceptualize biodiversity in terms of human choice, and in terms of how choices are made, and why."

These findings, published in this week's on-line issue of the journal Proceedings of the National Academy of Sciences, "are a result of analysis of baseline data gathered in the LTER site's 200-point survey, a major survey undertaken to measure ecological characteristics of the Central Arizona-Phoenix area, provide a baseline for future monitoring, and give an overview of features such as plant diversity, soil chemistry, and animal distributions," explained NSF's Henry Gholz, LTER program director.

For the research, the entire study area (some 6,400 square kilometers) was covered with a five-by-five-kilometer grid. Field measurements were done in 30-by-30-meter survey plots, and included identifying all native and exotic plants; mapping the area of surface cover; collecting samples of soils, insects, microbes and pollen; and taking photos from the center of each plot.

Since this survey, which was conducted in the spring of 2000, scientists have monitored bird abundance and diversity, along with human activity, at 40 of the 200-plus sites, four times a year.

The resulting data is intended to provide a comprehensive picture of ecological characteristics of the Phoenix metro area and surrounding agricultural and desert lands, at a single point in time. However, the sites will be resurveyed every five years to monitor how these variables are changing with continued development, according to scientist Diane Hope, director of the field study. "Another use of the data is to compare features such as plant diversity and soil nutrient status in the city, and in the outlying desert," said Hope. "For example, preliminary analyses show that total plant diversity in the desert becomes greater as the elevation of the site increases, but in the city resource abundance (wealth) is the key factor."

Human geographic and socioeconomic variables are important factors in explaining spatial patterns in plant diversity and soil-nutrient status, the scientists found. In general, average plant diversity was similar between desert and urban sites, although the total number of plant genera and the variation in composition from site to site was higher in the urban landscape. Interestingly, said Hope, "median family income, age of residential housing and whether a site has ever been farmed are all correlated with plant diversity. Wealthier, younger neighborhoods and those on sites that have never been cultivated tend to have a greater number of perennial plant types."

In a separate study focusing on parks located in neighborhoods of differing socioeconomic status, bird diversity was also found to be greater in the higher income residential areas.