A Role for Theory
The goals of BES are supported by three major kinds of theories. Theories are unifying frameworks that embrace many models and identify the fundamental postulates and relationships of a broad area of research. Each of the three empirical foci of BES has at least one overarching theory that justifies its use in urban ecology, and serves to relate the understanding of urban ecosystems to habitats beyond the urban realm.
Focus 1: Ecosystem Flux of Matter and Energy
The major foundations of ecosystem ecology include principles of thermodynamics, conservation of matter and energy, the law of the minimum, and chemical stoichiometry. A major prediction of ecosystem ecology is that limiting nutrients will be retained by intact ecosystems. A guiding question for ecosystem research in BES is to test the prediction of ecosystem retention in urban areas. Because urban systems are driven by human goals and structures for using and transporting water, materials, and energy, the assumptions that nutrient limitation drives retention may not hold. This assumption is tested by Focus 1 of BES.
The watershed approach is a powerful tool for facilitating this test, and for understanding the mechanisms of nutrient retention or loss from ecosystems. Furthermore, following the variable source area concept from hydrology, the watershed approach suggests that identifying sources, sinks, and flow paths of materials can be used to examine whether an ecosystem at the watershed scale is in fact retentive or leaky. Consequently, this theory provides a framework for addressing the mechanisms that may underlie an urban exception to limitation as a driver by identifying sources and sinks of nutrients and contaminants, and discovering their long-term relationships in space.
Focus 2: Biotic Community Organization and Change
The biota ̶ plants, animals, and microbes ̶ are the metabolic engines for ecosystem fluxes. Therefore, the second Focus of BES is on the patterns and mechanisms of assembly of the biological communities of the urban ecosystem. Biological community theory is founded on familiar theories of competition, niche partitioning, top-down versus bottom up control, disturbance, and succession, all operating in the three dimensional spaces of heterogeneous watershed landscapes. This focal research area rests on the theory of the metacommunity, cast in terms of regional versus local sources of species over space. The guiding question is whether or how does metacommunity theory apply in the fragmented, constructed, and highly managed mosaics of urban systems. The data collected in service to this theory also can be used to test relationships of biodiversity to ecosystem functioning in key habitat types in the metropolitan area.
Focus 3: Locational Decisions and Land Management
The theory of "urban land rents" or "bid rents" is a classic set of economic propositions to explain the distribution of various land uses with distance from urban cores. Bid rent theory is based on assumptions that land use decisions are driven by price, and that markets identify the relevant quality of land in different locations in the metropolitan area. Furthermore, competition among bidders recognizes these differences in quality, which may include such location-specific factors as transportation costs, materials, inputs for production, and infrastructure. Land rent theory assumes that amenities such as climate and soils are uniformly distributed across the territory.
Over its long history, land rent theory has been modified by many factors, such as labor, speculative behavior in periurban agricultural areas, and so on. BES extends the testing and refinement of bid rent theory by taking explicit account of heterogeneous ecological structures and processes throughout a metropolitan area. In addition to identifying environmental features that act as amenities and disamenities, the application of this theory in BES examines the unintended negative effects of environmentally motivated regulation of subdivision size and density, of county-wide zoning regulations, and spillover effects of amenities and regulations. Finally, the prediction of land use decisions in the urban core, where de-industrialization and population loss have made use of classic transaction-based modeling of housing markets impossible, new models are being developed to extend and modify the theory. Because "shrinking cities" exist in many industrialized regions and countries, this theoretical refinement is widely relevant.
An Integrative Frontier for Theory
The three theoretical areas are linked by shared processes. For example, the natural and constructed features of watersheds influence transacted housing prices, with nearby stormwater detention basins decreasing housing prices, and stream restoration enhancing value. These same features influence the degree of watershed nutrient retention, and also serve as habitat for native and exotic aquatic organisms. The failed housing market in some neighborhoods in the residential core of Baltimore results in abandoned buildings that, as ruins, provide habitat for exotic mosquitoes that vector human and bird diseases, and stimulate demolition with consequent fine scale heterogeneity and management opportunities. Vacant lots, although they contribute to social disamenities and biophysical hazards, provide stepping stones that can facilitate vegetation metacommunity dynamics, and support generalist native bird species. Policy and management options can be evaluated based on the plusses and minuses of those habitats. Other examples of the interactions of conditions predicted by the three theoretical realms exist, but the key point here is that the three areas areas motivate data that bridge among them, and address interacting human-natural drivers of urban ecosystem change over the long term.