Global Change and the Ecology of Cities

Review: Global Change and the Ecology of Cities

Introduction

• Urban areas are environmental change drivers at multiple scales.
• Cities impact land use, biodiversity, and hydrosystems locally and regionally.
• Urban waste affects biogeochemical cycles and climate from local to global levels.
• Global environmental changes are overshadowed by local environmental changes for urbanites.
• Urban ecology integrates natural and social sciences to study altered local environments and their effects.
• Cities present both problems and solutions for sustainability in an urbanized world.
• Shift to urban living: In 1900, 10% of the global population were urban dwellers; now it exceeds 50%.
• Projected increase: >95% of the net increase in the global population will be in developing world cities, reaching 80% urbanization.
• Megacities: Most new megacities (>10 million) are in the developing world.
• Demands on ecosystems: Economic growth and demographic changes will increase demands on ecosystem services.
• Ecologists' involvement: Increasing collaboration with scientists, planners, and engineers to understand and shape urban ecosystems.
• Urban ecology has begun to change the discipline of ecology.
• Urban ecology integrates natural and social sciences to study urban ecosystems.
• Cities are viewed as heterogeneous, dynamic landscapes and complex, adaptive, socioecological systems.
• Ecosystem services link society and ecosystems at multiple scales.
• Urban ecologists seek commonalities among city ecosystems and their role in environmental change.
• Focus on five major types of global environmental change: land use, biogeochemical cycles, climate, hydrosystems, and biodiversity.
• Cities are microcosms of global changes, serving as test cases for understanding socioecological system dynamics.

Land-Use and Land-Cover Change Accompanying Urbanization

• Urban population growth has occurred on <3% of the global terrestrial surface, with global impacts.
• Cities account for 78% of carbon emissions, 60% of residential water use, and 76% of industrial wood use.
• Land change to build cities drives other environmental changes.
• Urban dwellers rely on ecosystems beyond city boundaries for resources and waste absorption.
• Ecological footprints are tens to hundreds of times the area occupied by a city.
• Urban areas are centers of human innovation but may require fewer resources per capita than smaller towns or rural areas.
• Historical context: Excessive demands led to degradation and societal collapse (e.g., Mesopotamia).
• Globalization: Consumer demands led to deforestation of colonial lands and rainforest conversion for grazing land.
• Regional scale: Land-use changes are driven by population movement.
• Migration: Perceived opportunities in urban centers drive migration from degraded rural landscapes.
• China: 300 million more people likely to move to cities, transforming landscapes.
• Constraints: Shortages of construction materials may constrain urbanization and pressure global infrastructure growth.
• Urbanization increases patch fragmentation and diversity.
• Increased edges: More interfaces between distinct land-cover types.
• Smaller patch sizes: Urban, residential, and desert land-use patches vary in size (e.g., central Arizona).
• Urban land use leaves a legacy in ecological characteristics.
• Phoenix: Formerly agrarian lands exhibit unique soil biogeochemical properties after 40 years.
• Agricultural legacies: Some locations reveal agricultural impacts after centuries.
• Urban planning: Assumption that city form follows land-use patterns.
• Convergent urban form: Chinese cities show similar shape, size, and growth rates despite varying drivers.
• Land-use policies: Zoning and growth boundaries determine urban form and impact.
• Unintended consequences: Growth-management efforts led to low-density housing sprawl beyond boundaries.
• Urban ecology at the local scale: Relationships among urban design, ecosystem services, and human responses.
• Edge expansion: City edges extend into rural landscapes, impacting soils, structures, and ecosystems.
• Peri-urban environments: These areas link core cities in extended urbanized regions.
• Megapolitan regions: Cities are part of dominant metropolitan regions, coalitions of urban centers.
• Next frontier: Understanding urbanization in biophysical, economic, and political contexts through continental or global comparisons.

Altered Biogeochemical Cycles in Cities and Their Regional-to-Global Effects

• Urban areas both cause and respond to biogeochemical cycle changes.
• Cities are point sources of CO_2 and other greenhouse gases, affecting Earth’s climate.
• They also emit trace gases like NO, NO2, O3, SO_2, and organic acids.
• Regionally, air pollution influences nutrient cycling and primary production in adjacent ecosystems.
• Cities along rivers and coastlines contribute to eutrophication.
• Urban wastes affect biogeochemical cycles from local to global scales.
• Influence extent depends on transport vectors.
• Major CO2 contribution: The 20 largest U.S. cities contribute more CO_2 than the continental United States can absorb.
• Urban metabolism: Analogy of a city to an organism consuming resources and releasing wastes.
• Scientists debate the analogy's appropriateness.
• Utility in quantifying consumption and waste generation trends.
• Increased throughput: Large increases in food-waste stream, paper and plastics, and building materials.
• Beijing: Total carbon emissions from solid-waste treatment increased by a factor of 2.8 from 1990 to 2003.
• Pollution generation: Increasing concern as urbanization outpaces pollution-control measures.
• U.S. emissions controls counterbalance increased driving distances from urban sprawl.
• China: Increased coal burning and automobile use lead to serious air-pollution consequences.
• Nutrient loads: Rapidly urbanizing regions increase nutrient loads to rivers and coastal ecosystems.
• Developing world: Sewage treatment is lacking or inadequate.
• Affluent cities: Waste from affluent cities is a primary driver of altered biogeochemical cycles globally.
• Cities show symptoms of biogeochemical imbalances.
• High acid and N deposition and elevated atmospheric concentrations of CO2, CH4, and O_3. have both growth-enhancing and growth-inhibiting effects on organisms.
• Elemental mass balances: Identify potential excesses of inputs over outputs and sinks within the urban landscape.
• Cities as accumulation zones: Hot spots of N, P, and metals.
• Resource potential: Cities harbor a pool of material resources.
• Wastewater reuse: Using treated wastewater as a substitute for commercial N fertilizers.
• Phoenix example: Nitrate-rich groundwater could reduce fertilizer needs by >100 kg/ha.
• Metal recycling: Increasing the reuse and recycling of copper and other metals to stem demand and alleviate soil accumulation.
• Human management heterogeneity: Varies within cities based on financial resources and land cover.
• Soil-nutrient concentrations: Vary across desert metropolitan regions due to legacy factors, urban structure, and landscape choices.
• Stream features: Some stream features are more effective in retaining nutrients.
• Atmospheric pollutants: Localized behavior is less important than collective behavior.
• Driving habits: Produce daily or weekly cycles of particulate, CO2, NOx, or O_3 plumes.

Urbanization and Climate Change

• Urban centers, especially in the developed world, are primary sources of greenhouse-gas emissions.
• Global climate change impacts on cities may be overshadowed by local climate changes from urbanization.
• Local changes: Increased minimum temperatures, reduced maxima, reduced or increased precipitation, and weekly cycles.
• Urban heat island (UHI) effect: Cities tend to have higher temperatures than rural surroundings.
• Characteristics influencing UHI:
  • Land-cover pattern.
  • City size (related to urban population size).
  • Increased impervious surfaces (low albedo, high heat capacity).
  • Reduced vegetation and water (reduced evaporative cooling).
  • Increased surface areas for solar energy absorption.
  • Canyon-like morphology of high-rises.
• UHI is a local phenomenon with negligible effect on global climate.
• Magnitude and effects may represent harbingers of future climates.
• Temperature increases within cities exceed predicted global temperature rise.
• Kalnay and Cai (41) estimated that urbanization and land-use changes accounted for half of the observed reduction in diurnal temperature range and an increase in mean air temperature of 0.27°C in the continental United States during the past century.
• Downtown temperatures for the United States have increased by 0.14° to 1.1°C per decade since the 1950s.
• Research on elevated temperature effects on remnant ecosystems within cities can inform predictions of ecosystem response to global climate change.
• UHI affects local and regional climate, water resources, air quality, human health, and biodiversity.
• Heat stress on organisms, including humans.
• Influence on water resources by changing surface-energy balance.
• Induction of photochemical smog and promotion of pollutant dispersion.
• Increased energy consumption for cooling in warm regions.
• About 3 to 8% of electricity demand in the United States is used to compensate for UHI effects.
• Mitigation strategies: Increasing vegetation cover and albedo.
• Trade-off: Greater water use, especially in arid regions.
• Other climate change risks to cities: Coastal cities exposed to rising sea level and increased hurricane frequency.
• Urban sustainability requires strengthening ability to respond to the changing relation between urbanization and climate.
• Mitigation and adaptation strategies—and economic markets for them—will be required.

Human Modifications of Hydrologic Systems

• Cities historically sprung up along rivers and deltas due to water availability.
• Waterways are seldom left unmodified within cities.
• Water is linked to domestic use, industrial processes, sanitation, and protection from natural disasters.
• Hydrosystems modified to meet conflicting goals.
• Designed or altered streams, rivers, flood channels, and canals do not replicate aquatic ecosystems or preserve lost ecosystem services.
• Few model systems for comparison.
• Calls for restoration of streams in urban areas.
• Advocacy for study and management of designed ecosystems to optimize services to urban populations.
• Services include flood protection, habitat for diverse biota, nutrient retention, and a sense of place.
• Impervious cover increases: Changes hydrology and funnels pollutants into streams.
• Point-source pollution reduced by regulation in the United States but remains an issue in developing countries.
• Industrial discharges and sewage contaminate rivers and lakes.
• Stormwater infrastructure: Separate from wastewater discharges in newer cities, mixed in older cities.
• Storms and low flow-discharge contribute to localized or regional pollution downstream.
• Changes in chemical environment, pollutant exposure, simplified geomorphic structure, and altered hydrographs create an urban stream "syndrome".
• Low biotic diversity.
• High nutrient concentrations.
• Reduced nutrient retention efficiency.
• Often elevated primary production.
• Other ecosystem functional attributes respond less consistently.
• Countering urban stream syndrome may require designed ecosystems rather than “restored” streams.
• Ecologically based designs of novel urban aquatic ecosystems are becoming more common.
• Stream-floodplain protection.
• Retrofitting of neighborhood stormwater flowpaths.
• Low-impact stormwater/water capture systems as creative solutions to urban stormwater management.

Biodiversity Changes in Cities

• Urbanization and suburbanization usually reduce species richness and evenness for most biotic communities.
• Increases in abundance and biomass of birds and arthropods.
• Urban sprawl reduces native species diversity at regional and global scales.
• Northern latitudes: Urban sprawl related to declines in abundances in some migratory birds in southern latitudes.
• Exceptions:
  • Plant species richness and evenness often increase in cities relative to wildlands.
  • Heterogeneous patchwork of habitats, human introductions of exotic species, and preferences for species with few individuals of each in landscaped yards.
  • Bird species richness may peak at intermediate levels of urbanization.
  • Increased heterogeneity of edge habitats.
• Humans directly control plant richness, evenness, and density.
• Indirect control over other functional groups of species (herbivores, predators, parasites, omnivores, detritivores) or their trophic interactions.
• Human-dictated urban plant communities form the template for other functional groups of species.
• Proposed mechanisms for changes in richness and evenness:
  • Increased rate and seasonal variability in productivity.
  • Relaxed predation on the dominant species.
  • Increased competitive abilities of some urban species.
  • Increased parasite pressure on less successful urban species.
• These hypotheses are not mutually exclusive.
• Certain species may become better urban competitors because they are released from natural enemies.
• Urbanization alters the species composition of communities.
• Urban communities are dissimilar to surrounding communities as urban species become reshuffled into novel communities.
• Bird communities shift to more granivorous species at the expense of insectivorous species.
• Arthropod communities shift from more specialized to more generalist species.
• Soil nematode diversity does not vary between rural and urban riparian soils, but functional composition changes to fewer predaceous and omnivorous species in urban than in rural soils.
• Cities as homogenizing forces: Some “urban-adapted” species become common worldwide.
• Subset of native species, usually species adapted to edges, become locally and regionally abundant at the expense of indigenous species.
• Homogenization of terrestrial and aquatic communities via urbanization proceeds at different rates in different geographic areas.
• Urban environment as a powerful selective force that alters behaviors, physiologies, and morphologies of city-dwelling organisms.
• Anthropogenic changes (direct and indirect) cause short-term changes in phenotypes of urban-dwelling organisms.
• Urban environments act as a potent evolutionary force on population genetics and life-history traits of urban species.
• Human organisms are not immune to selective action of the urban environment.
• Social structure and interactions, physiology and health, morphology (e.g., increased obesity), and even long-term changes in genetics of human urban residents may be associated with urban living
• Prognosis for maintaining diversity and function of biological communities within and near cities seems dire.
• Intensified conservation efforts to preserve or reconstruct habitats within or near cities may ameliorate these biological changes.
• Introduction of nonnative species combined with the UHI may enhance ecosystem services.
• Reconciliation ecology: Habitats altered for human use are designed, spatially arranged, and managed to maximize biodiversity while providing economic benefits and ecosystem services.
• Ecologists will be increasingly called upon to help design and manage new cities and reconstruct older ones.
• Cities offer real-world laboratories for ecologists to understand fundamental patterns and processes.
• Collaboration with city planners, engineers, and architects to implement policies that maximize and sustain biodiversity and ecosystem function.
• Human connections and encounters with urban nature have supplanted experiences with natural biodiversity.
• Experiences with nonnative, global “homogenizers” may be essential for conserving global biodiversity.

Prospects

• Cities are concentrated centers of production, consumption, and waste disposal that drive land change and global environmental problems.
• Locally, they represent microcosms of global environmental change and offer opportunities for enriching both ecology and global-change science.
• The totality of human activity occurs on a biophysically constrained planet.
• Urban ecology can elucidate the connections between city dwellers and the biogeophysical environment.
• As our ecological footprint expands, so should our perception of greater scales and broader impacts.
• Cities concentrate industry and creativity, making them hot spots for solutions as well as problems.
• Urban ecology has a pivotal role in finding those solutions and navigating a sustainable urban future.