III.18 Responses of Communities and Ecosystems to Global Changes
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III.18 Responses of Communities and Ecosystems to Global Changes Erika Zavaleta and Nicole Heller OUTLINE 1. What are global changes? 2. Global climate change 3. Elevated CO2 in the atmosphere and oceans 4. Global nitrogen fertilization 5. Ozone depletion in Earth’s stratosphere 6. Interacting global changes Increases in the scale and extent of human activity in the last two centuries have brought about environmental changes that affect most of the globe. These global changes include directional shifts in climate, greenhouse gas concentrations , nitrogen fixation, and stratospheric ozone depletion . They also include biotic changes such as land cover change, biological invasions, and global loss of biodiversity. In this chapter, we focus on the responses of ecological communities and ecosystems to directional changes in climate , atmosphere, and global biogeochemistry. Our understanding of these responses comes from observations of trends in nature, experiments manipulating global change factors at small scales over years to decades, and predictive models. We consider all of these sources, with an emphasis on the empirical knowledge derived from observations and experimentation. GLOSSARY biodiversity. The totality of the inherited variety of all forms of life across all levels of variation, from ecosystem to species to gene (E. O. Wilson). biogeochemistry. The cycles of matter and energy that transport the Earth’s chemical components through time and space, and the chemical, physical, geological , and biological processes and reactions that govern the composition of the natural environment. biomes. Generalized regional or global community types, such as tundra or tropical forest, characterized by dominant plant life forms and prevailing climate. community. A group of interacting species living in a specified area. Communities are often defined by the dominant vegetation types, such as maple-oak or sagebrush. However, community composition is dynamic as species dominance and diversity shift in space and time. ecosystem. An ecosystem is a complex system formed by the interactions of living (biotic) and nonliving (abiotic) components, which shape each other through exchange and material flows. An ecosystem can be bounded more or less arbitrarily and can range in scale from an ephemeral pond to the entire globe but most often refers to a landscape-scale system characterized by one or a specified range of community types (e.g., grassland ecosystems). eutrophication. An increase in an ecosystem’s plant production resulting from nutrient inputs, often with undesirable effects such as excessive plant decay , oxygen deprivation, and water quality declines in aquatic systems. nitrogen fixation. The conversion of inert atmospheric dinitrogen (N2) to nitrate and ammonia that can be taken up by organisms. phenology. The timing of recurring biological phenomena , ranging from annual budburst and senescence in plants to the onset of animal migrations, egg laying, and metamorphosis. 1. WHAT ARE GLOBAL CHANGES? Humans, like all other organisms, modify their environment . Human modification, unlike that of other organisms, however, is drastically altering ecosystems over the entire globe through an explosion in human numbers and the scope of their activities. As anthropogenic impacts on the environment cause changes in biological systems, those changes in turn cause further changes in the environment, resulting in complex feedbacks and interactions. This process is described as global change. Many, although not all, global changes originate from transported chemical effects of human activity, such as carbon dioxide (CO2) emissions, aerosols, CFCs, and NOx emissions. Global changes also include direct effects on biological systems, such as habitat destruction, invasive species, and biodiversity loss. Finally, global changes interact extensively—for example, tropical forest destruction releases large amounts of greenhouse gases that contribute to global climate change (figure 1). In this chapter, we focus on climate change, CO2, nitrogen (N) deposition, and ozone depletion. Habitat loss and degradation, invasive species, and loss of biodiversity are discussed elsewhere in this volume, but we address them in the context of interactions with climate and atmospheric change. Communities and ecosystems respond to global changes interactively (figure 2). Global changes can affect ecosystem processes, such as through altered biogeochemical cycles, in ways that then influence the resident community. Conversely, global changes can drive community shifts directly, in ways that then affect ecosystem processes, such as when a shift from tropical forest to savanna vegetation drastically reduces local rainfall. The result is a feedback loop between community and ecosystem change, one that can buffer or amplify global change effects over time and result in long-term, ongoing changes even after a particular global change driver has stabilized. For example , an abrupt increase in temperature might reduce grassland soil moisture...