Using Gradient Analysis to Determine and Compare Invertebrate Responses to Urbanization: Can We Achieve Understanding Without Defining Reference Conditions?

The U.S. Geological Survey's National Water-Quality Assessment Program uses a gradient design to investigate the effects of urbanization across the U.S. This design has successfully defined invertebrate responses in metropolitan areas associated with Boston, MA, Birmingham, AL, Salt Lake City, UT, and Raleigh, NC. An urban intensity index (UII) based on population, land use, land cover, and infrastructure is used to define the gradient without explicitly identifying reference sites, although the low end of the urban gradient may include such sites. Many invertebrate metrics (e.g., tolerance, biotic integrity, richness) are significantly related to UII. Detection of responses is not dependent upon reference conditions and this design can detect responses even when many low intensity (UII < 20) sites are excluded. Reference conditions can be inferred from regressions of metrics and UII by creating a "dummy" site where the components of the UII are set to background values (e.g., population = 0, road density = 0) and then extrapolating metrics for this site (UII = 0). Unfortunately, these end members (reference site conditions) tend to be highly variable and it is more difficult to determine these values than to detect the existence, form, and rate of response across the urban gradient.