238 Energy Flow and Ecosystems of abundance (or local extinctions) following natural and anthropogenic disturbances can sometimes illustrate how certain key species regulate nutrient cycling and other ecosystem functions Field testing of many concepts related to understanding the importance of key species in ecosystem functioning is just beginning Results of these long-term ecosystem studies across a range of spatial and temporal scales can provide guidelines for the stewardship of biodiversity Precipitation Energy Land use Ecosystem Boundaries: Inputs, Outputs, and Transformations of Energy Ecosystem Structure An ecosystem approach can be used to address many different questions spanning scales from the global biosphere to small ponds or patches of habitat As the questions change, so the boundaries and the complexities of species interactions within and among different compartments or across trophic levels Ecosystem boundaries are often defined to include natural species assemblages and to analyze inputs and outputs of energy and materials for cross-site comparisons of efficiencies in energy transfers and studies of changing conditions These analyses often take the form of mathematical models such as computer simulations or individual-based models of species and their specific functions within the biotic assemblage and environmental conditions under study One ecosystem may export nutrients and organic matter (stored energy) to other ecosystems so that cross-site linkages often become important For example, in studies of nutrient cycling in terrestrial ecosystems, the definition of boundaries would likely have some compartments of organic matter production by living plants and their relationships with herbivores and carnivores Macro- and micronutrient inputs would likely be derived from the atmosphere through dry deposition of particulates – nitrogen gas being taken up by some nitrogen-fixing species of microbes Other sources of nutrients, especially phosphorus, would come from weathering and erosion of soil and bedrock deposits, with movement among other compartments derived from actions of wind and water This combination of interactions illustrates the importance of defining clear boundaries for subsystems within the complete ecosystem so that measurements of movements (fluxes) among compartments can be measured accurately Generally, ecosystems and their boundaries are abstractions that can only be useful and insightful when combined with sufficient knowledge regarding the natural history and general ecology of communities and their physical environment There is wide recognition that a combination of direct field observation, experimentation, and modeling is essential when conducting ecosystem studies Currently, fundamental questions dealing with relationships between energy flow and the species-specific roles of organisms are attracting increased attention For example, can results of controlled small-scaled (fine-grained) experimental studies of productivity be used to predict responses of other natural assemblages at larger scales (coarse grained) of ecosystem dynamics? Does energy flow through an ecosystem increase, decrease, or remain the same if one species goes locally extinct but the abundances of other ‘‘similar species’’ change rapidly to compensate for the lost species? Under what Terrain Biodiversity Functions Nutrients Eco s y s t e m c o n s tr a i n t s Figure Interactions among various factors constrain how ecosystems function The climatic controls over inputs of solar energy and precipitation are two ‘‘forcing functions’’ which strongly influence how ecosystems operate Land-use changes are also important in determining how water and nutrients move through the ecosystem and influence biodiversity Nutrients moving among biotic and abiotic components are a type of ‘‘transfer function’’ that depends on abiotic and biotic factors (Modified from Schulze and Mooney, 1994) environmental conditions species substitute for one another and compensate functionally for the lost species? These and many other questions are beginning to be answered, but studies related to biodiversity and the persistence of species assemblages remain incomplete A series of symposia during the 1990s dealt with the relationships between biodiversity and ecosystem functions (Figure 1) and stimulated many new ideas (Lawton, 1997; Palmer et al., 1997; Naeem, 1998) A Historical Perspective: The Ecosystem Concept How energy moves from one group of species to another has been an active area of study at least since Charles Darwin and Alfred Russel Wallace first wrote in the 1850s about the interconnections among species They were intrigued with the general proportions of population abundances that were thought to exist among different groups of large predators and their prey, and they emphasized competition and predation as important factors for regulating species interactions These early observers lacked a conceptual approach to what was later viewed as ecosystem-level dynamics Darwin’s studies of earthworms and their roles in soil development and his views on the roles of multiple species in the ‘‘tangled bank’’ metaphor stimulated others to consider how these many interactions could be viewed holistically In 1887, Stephen Alfred Forbes described material cycles within lakes and used a table of predator–prey data to examine which fish species