Ecology

It is tempting to overstate the case for ecosystem services, to try to find them everywhere simply because anywhere is in one or another ecosystem. But it is important not to confuse ecosystem functions, which are ubiquitous, with ecosystem services, which are the consequence of only some ecosystem functions. The critical difference between the two, and which makes the development of ecosystem services policy both complicated and controversial, is that ecosystem services have relevance only to the extent human populations benefit from them. They are purely anthropocentric. The ecology of ecosystem services, therefore, must be carefully defined in order to begin considering how to formulate a policy foundation for their management.

Ecosystems and Ecosystem Processes

Since Tansley's (1935) early description of the ecosystem as part of a continuum of physical systems in nature, decades of research and literature have been devoted to forging the concept into a scientific discipline (Brooks et al. 2002; Golley 1993). Modern ecologists describe ecosystems as the complex of organisms that appear together in a given area and their associated abiotic environment, all interacting through the flow of energy to build biotic structure and materials cycles (Blair et al. 2000; Millennium Ecosystem Assessment 2005). Ecosystems thus move and transform energy and materials through basic processes such as those listed by Virginia and Wall (2001):

Photosynthesis

Plant nutrient uptake

Microbial respiration

Nitrification and denitrification

Plant transpiration

Root activity

Mineral weathering

Vegetation succession

Predator–prey interactions

Decomposition

These and other ecosystem processes operate according to fundamental internal rules and constraints of physical and biotic systems. Energy transformation processes are essentially one-way flows, preventing reuse or recycling of the energy units. But nutrients can circulate through different components of an ecosystem, leading to what ecologists call nutrient cycles and nutrient pools. At its most fundamental level, ecology as a discipline is interested in describing and quantifying the factors that regulate energy transformation and nutrient cycling within an ecosystem as defined. And because these processes operate at many scales, ecological studies also take place at many scales. For example, photosynthesis can be measured and studied at scales ranging from the individual cell to the canopy of a forest ecosystem as defined. Often, therefore, it is as much a question of how to define an ecosystem as it is to understand how these processes work within it.

Ecosystem Functions

The process-based description of ecosystems has led to improved understanding of the functions ecosystems perform in natural settings. The transformation of energy and materials into vegetation structure, for example, provides habitat for other organisms. The decomposition of materials in the ecosystem builds soil structure. Each process under way in an ecosystem thus contributes to one or more of a set of functions associated with the ecosystem and with its relation to other ecosystems (Virginia and Wall 2001).

The same basic biological and chemical processes occur in all ecosystems, but different conditions yield different functional representations (Blair et al. 2000). It is like electronic circuitry—the same principles of electromagnetism apply in all cases, but different combinations of circuitry and voltage produce different functional applications. An inventory of just some of the functions typically associated with different ecosystem processes, and which we should expect to observe in different forms and magnitudes across ecosystems is provided in Table 1.1.

As this representation suggests, there is no one-to-one correspondence between ecosystem processes and ecosystem functions. In reality, many processes are needed to produce any of the defined functions. For example, a farm, which can be thought of as a highly modified and highly managed ecosystem, relies on biotic production, energy flow, decomposition, and nutri-ent cycling to make possible its basic function of producing, say, corn. It is no different in the remote undisturbed depths of a rain forest. Hence, another key study theme of ecology is to improve our understanding of how the basic ecosystem processes work together to generate the functions vital to sustaining the ecosystem within its environment.

Ecosystem Structure and Natural Capital

Ecosystem functions contribute to the building of the ecosystem's physical structure, such as biomass (e.g., vegetation and wildlife) and abiotic resources (e.g., soil and water), which in turn supports the sustainability of the functions (Christensen et al. 1996; Daly and Farley 2003). Events that degrade ecosystem structure (e.g., overfishing in coral reef ecosystems) consequently disrupt the integrity of the associated ecosystem functions (Roberts 1995). These effects are important not only to the sustainability of the ecosystem but also to the sustainability of humans, given the importance of ecosystems to human well-being (Millennium Ecosystem Assessment 2003, 2005). This property—that ecosystem structure and functions provide for human needs and wants—is what makes ecology inevitably relevant to economics.

Ecologists thus analogize ecosystem structure to capital as that term is used in economic theory—the stock that possesses the capacity of giving rise to the flow of goods and services (Costanza et al. 1997; Ekins et al. 2003). Ecological capital, or "natural capital" as many ecological economists call it, consists of the ecosystem structure and functions that support the creation and flow of goods and services valuable to humans (Clark 1995; Costanza and Daly 1992; Daily and Dasgupta 2001). Other than in...