FIGURE 7.1 Concepts of hierarchy and scale in ecosystems. The relationship between scales (indicated by double-headed arrows) is important in assessing the impact of function at a lower scale on the processes at higher scales. Source: From Friese et al. (1997).

In a recent article, Pickett and Cadenasso (2002) discussed their ideas of what we think about the concept of an ecosystem. They started their discussion with the basic definition

of Tansley, which states that an ecosystem consists of an assemblage of organisms (the biotic component) and the associated physical environment in which the organisms live. They further suggest that the interaction among the component parts of an ecosystem, both among the organisms and between the organisms and the physical environment, is another important aspect of the ecosystem. These interactions provide a hierarchical structure through which material (energy and nutrients) flow. They further show that the evolution of the use of the term ecosystem incorporated the idea that ecosystems are scale-independent and are dynamic in nature (meaning that they are not static), and changes in time reflect changes in the complexity and degrees of divergence from equilibrium or stability.