Ecological Homogenization

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Figure 1. Residential Images. Ecological Homogenization of Urban America: testing the hypothesis that human impact in and around cities causes homogenization of the biota, altering functional and phylogenetic diversity patterns.

Neighborhoods across biophysically different regions have similar patterns of development leading to ecological homogenization of basic neighborhood structure and residential yards more ecologically similar to yards across the nation than to their respective nearby natural areas. Fewer lineages of plants occur spontaneously in urban environments than in natural areas, and urban plants are shorter lived, faster growing and promote fewer pollinators than their country counterparts. About half of the plants urbanites cultivate are exotics, driving homogenization of our regional and continental flora. The Ecological Homogenization of Urban North America study, involving six sites across the nation, focuses on the ecology and evolution of urbanizing ecosystems to reveal generalities in the ecosystem impacts of residential development.

Cedar Creek data provide a natural reference point for examining the effects of urbanization and suburbanization on plant community composition and diversity, however the applicability to other metropolitan regions is not yet known. Recent Cedar Creek work has begun to quantify regional-scale variation in ecosystem responses to change, including links between land conversion for residential uses, ecological communities, and ecosystem processes. The landscape surrounding CDR is changing rapidly, as agricultural lands are replaced by low-density residential land use at the fringes of the Twin Cities of Minneapolis and St. Paul. In a series of related projects, we have been examining the influence of residential development on ecological structure and function.

In one project, soil carbon levels in a 100-year chronosequence of residential lawns of 40 houses surrounding Cedar Creek was highly variable and nonlinearly related to age of development. Soil carbon accumulated in the top 40 cm at a rate of 68 g C m-2 yr-1, comparable to rates measured in lawns in Colorado and California and more than double rates measured in the CDR old-field chronosequence, likely because of lawn management.

A second project used Cedar Creek Social Science Supplement funds to extend the Twin Cities Household Ecosystem Project to the exurban region surrounding Cedar Creek (www.tchep.umn.edu). Among households, fluxes of C, N, and P were highly variable and often skewed, with a small number households disproportionately contributing to total fluxes across all households. Carbon fluxes were dominated by home energy use, motor vehicle travel, and air travel; N fluxes by human diet, fossil fuel emissions, and lawn fertilizer; and P fluxes by human diet, detergents, and pet diet. Both social and biophysical factors contributed to patterns of household biogeochemical fluxe: greater household income was associated with greater household C fluxes, while less easily quantifiable factors like values, norms, and attitudes also influenced specific behaviors such as maintaining a lawn.

In a third project, we used Cedar Creek International Supplement funds to enlist the collaboration of a German graduate student to compare phylogenetic diversity and functional trait composition of spontaneously occurring (not planted) species in privately managed yards change along a gradient of housing density in the Twin Cities and between yards and Cedar Creek. Yards had more spontaneously occurring plant species per hectare in urban than in exurban regions, but phylogenetic diversity and functional composition did not change with housing density.

In contrast, in comparison to natural areas, yard plant species were more closely related to each other, causing phylogenetic homogenization within yards, and were functionally distinct, being more often short-lived, self-compatible, and having higher specific leaf area than species of Cedar Creek. The total plant diversity per hectare in household yards increased with household wealth and education and decreased with higher use of fertilizer indicating an important role for human choices in driving biodiversity in human-dominated landscapes (Figure 1). Urban homogenization research is investigating implications that land use changes due to human dominance and current popular land-management practices in U.S. cities and urban areas may result in urban/suburban systems more similar to each other than to adjacent native ecosystems; even when they are geographically divergent (Figure 2).

These photos of residential yards in Minneapolis and Phoenix illustrate the phenomenon of homogenization
For further reading: 
Groffman, P.M., J. Cavender-Bares, N.D. Bettez, J. M. Grove, S. J. Hall, James Heffernan, S. Hobbie, K. L. Larson, J L. Morse, C. Neill, K. Nelson, J. O’Neill-Dunne, L. Ogden, D. E. Pataki, C. Polsky, R. Roy Chowdhury, M. K. Steele, Ecological Homogenization of Urban America, Frontiers in Ecology and Environment 2013. In press.
Cavender-Bares, J., J. Heffernan, E. King, S. Polasky, P. Balvanera, W. C. Clark, and K. N. Lee. 2013. Sustainability and Biodiversity. In: Encyclopedia of Biodiversity. Elsevier, Oxford, S. Levin (ed). ISBN: 9780123847195
Cavender-Bares, Jeannine, David D. Ackerly, and Kenneth H. Kozak. Integrating ecology and phylogenetics: the footprint of history in modern-day communities. Ecology 93:S1–S3. 2012
Knapp, S; Dinsmore, L; Fissore, C; Hobbie, Sarah; Jakobsdottir, I; Kattge, J; J, King; Klotz, S; Laughlin, D; McFadden, J; Cavender-Bares, J; Phylogenetic and functional characteristics of household yard floras and their changes along an urbanization gradient. Ecology 2012
Fissore, C.; Hobbie, S.E.; King,J.Y.; McFadden, J.P.; Nelson, K.C.; Baker, L.A.; The residential landscape: fluxes of elements and the role of household decisions.; Urban Ecosystems 15:1-18 2012
Fissore, C., L. A. Baker, S. E. Hobbie, J. Y. King, J. P. McFadden, K. N. Nelson, I. Jakobsdottir. Carbon, nitrogen, and phosphorus fluxes in household ecosystems in the Minneapolis-Saint Paul, Minnesota, urban region. Ecological Applications 21:619-639 2011
Fissore, C., S. Hobbie, J. King, J. McFadden, D. Nelson, and L. Baker., The residential landscape: fluxes of elements and the role of household decisions. Urban Ecosystems. Doi:10.1007/s11252-011-0189-0 2011b
Cavender-Bares, Jeannine; Kozak, Kenneth H.; Fine Paul V. A.; Kembel, Steven W.; The merging of community ecology and phylogenetic biology. Ecology letters 12:693. 2009
Knops, J. M. H. and K. L. Bradley. Soil carbon and nitrogen accumulation and vertical distribution across a 74-year chronosequence. Soil Science Society of America Journal 73:2096-2104.
Chapman, K. and P. B. Reich. 2007. Land use and habitat gradients determine bird community diversity and abundance in suburban, rural, and reserve landscapes of Minnesota, USA. Biological Conservation 135:527-541.
Cavender-Bares, J., D. D. Ackerly, D. A. Baum, and F. A. Bazzaz. 2004. Phylogenetic overdispersion in Floridian oak communities. American Naturalist 163:823-843.
For further information: 
Jeannine Cavender-Bares
Sarah Hobbie
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