Ecological homogenization of urban America

a research project funded by the U.S. National Science Foundation program on “MacroSystems Biology: Research on Biological Systems at Regional to Continental Scales.”

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Questions and hypotheses

Our overarching hypothesis is that similar management practices among cities leads to homogenization in ecological structure and functions relevant to ecosystem carbon and nitrogen dynamics, with continental scale implications.  Further, we suggest that understanding the nature and extent of urban homogenization will provide the conceptual basis, database and methodologies for scaling the impacts of urban land use change from local to continental scales.  We are addressing the following specific questions and hypotheses:

  1. Does urbanization lead to a homogenization of ecological structure such that residential land parcels and neighborhoods in different cities are more similar to each other than to native ecosystems in their own metropolitan area?  We hypothesize that:
    • While native ecosystems in biophysically dissimilar regions of the U.S. have significantly different levels of soil carbon and nitrogen (total and reactive), plant community composition and aboveground biomass, residential ecosystems in these different regions are more similar in these variables because of similar management choices among city dwellers across regions (e.g., nutrient and water inputs, vegetation choice).   Therefore urbanization decreases regional differences, increasing continental scale homogenization.  The N isotope composition of organic matter, which is affected by fertilizer additions, will also be homogenized by urbanization.
    • The C isotope composition of urban lawn grasses will be primarily related to water availability for C3, cool season grasses, and to temperature for mixtures of warm season and cool season grasses across all cities reflecting the homogenization of anthropogenic controls on production across the continent.  Controls will vary widely in native ecosystems across the continent.
    • Differences in hydrography (the physical characteristics of water bodies) among regions are reduced by urbanization.  Urbanization decreases the extent of lakes and wetlands in wet regions, and increases it in dry regions, resulting in increased homogeneity in hydrography across regions. 
    • Variation in microclimate of residential areas among regions is reduced by urbanization, in part due to more similar soil moisture regimes and impervious surface in managed residential parcels of diverse cities compared to their native landscapes.
    • Similar physical and biological urban/suburban landscapes across biophysically dissimilar regions are tied to similar social dynamics. Across biophysically dissimilar regions, neighborhoods with similar lifestyle characteristics among residents (e.g., age, socioeconomic status, life-stage, ethnicity) and social preferences (e.g., values, interests), will have more similar landscaping practices (e.g., watering, fertilization) than socially dissimilar neighborhoods within a city.
  2. What are the continental-scale effects of urban homogenization on ecosystem function? If residential ecosystems in biophysically dissimilar regions are more similar to each other than to their respective native ecosystems, we hypothesize the following continental-scale ecological effects:
    • An increase in ecosystem carbon sequestration.  This increase occurs because in humid regions, carbon stocks in unpaved soils (the largest reservoir) are not changed by urbanization.  In arid regions soil and vegetation carbon stocks are increased by urbanization, and this effect is larger than any declines in vegetation carbon in humid regions.  Therefore, the net effect of continental-scale urbanization is an increase in ecosystem carbon stocks. 
    • An increase in reactive nitrogen.  Additions of fertilizer, high rates of atmospheric deposition, improvements in litter quality, and addition of water associated with residential land use change will increase production (mineralization and nitrification) and pools of reactive nitrogen in all regions.
    • An increase in denitrification (the conversion of nitrate, an important water pollutant to nitrogen gas) and nitrous oxide flux (a greenhouse gas).  Homogenization of hydrography and creation of wet, nitrogen rich areas in residential landscapes will increase both denitrification  and nitrous oxide flux. 
  3. Can parcel-level information obtained from remote sensing and socio-demographic data be used to scale the effects of urban land use change and homogenization to regional and continental scales?
    • New remote sensing and aggregate socio-demographic data can accurately depict the ecosystem structure and function and socioeconomic dynamics of residential parcels.  Comparisons of structural and functional aspects of parcels derived from remote sensing with detailed household survey and vegetation and soil data will allow us to scale urban ecological responses from parcels to neighborhoods and whole regions (metropolitan statistical areas (MSAs)).
    • Broad-scale socio-demographic and remote sensing data will be predictive of variation in ecosystem structure and function within and among cities. If so, we will be able to evaluate the effects of urban land use change on ecosystem structure and function at the continental scale.

Experimental approach

We are addressing these questions and hypotheses across six MSAs that cover the major climatic regions of the U.S.:  Phoenix, AZ, Miami, FL, Baltimore, MD, Boston, MA, St. Paul, MN and Los Angeles, CA.  At the household/parcel scale, we are coupling homeowner surveys with intensive biophysical measurements to determine how land-management practices influence ecological structure and function.  We are compiling extensive, high-resolution (≤1.0m pixels) remotely sensed and socio-demographic data to assess the extent, spatial distribution and quality of lawns and other cover types at the parcel and neighborhood levels.  These data are being used to link social decisions and preferences with ecological patterns and processes at broader (MSA) geographic scales.  Conducting these MSA-scale analyses across diverse regions of the U.S. will allow us to determine if scaling tools based on parcel level data can be used to produce continental-scale assessment of the effects of urban homogenization on ecosystem structure and function.