Gradients in ecological systems

In this paper, the potentials of a holistic and hierarchical gradient approach to ecosystem analysis and ecosystem theory are discussed, using some examples from the ecosystem research project in the Bornhoved Lakes Region (Northern Germany). In the gradient concept, which originates in the thermodynamic non-equilibrium principle, structural ecosystem properties are comprehended as concentration gradients in space and time. They build up potentials to carry out mechanical work, chemical reactions, or biological interactions. Ecosystem function is defined as the general characteristic of the systems' gradients dynamics. The gradient concept is theoretically discussed as an integrating tool for the aspects of thermodynamics, self-organization, and hierarchy theory. It helps to avoid inadequate reductions from holistic data sets to non-representative theoretical variables. Also, it can be used as an indicator to test theoretical hypotheses which are often based on non-measureable variables, and it may improve the cooperation between theoreticians and empirical ecologists. The necessary interfaces between this strategy and important ecosystem theoretical ideas are briefly described in this text. As an illustration, three aspects of the gradient concept are presented in empirical case studies: spatial, temporal, and functional gradients. In these examples, the gradient systems of nature-near ecosystems are compared with those of stressed ecosystems. On this basis, the applicability of the gradient concept in theory and practice is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.