What are indicators? On the definition of indicators in ecology and environmental planning

The term "indicator" is frequently used at the interface between science and policy. Although there is a great demand for clear definitions of technical terms in science and policy, the meaning of indicator is still ambiguous. In this contribution, we analyze different meanings of the term in ecology and environmental planning, suggest a general definition, and make recommendations for its appropriate use. We determined that the ways in which indicator is defined differ greatly, and some definitions are mutually exclusive. We arrived at the conclusion that a broad definition of the term is feasible. We recommend distinguishing between indicators as ecological components, i.e., ecological units, structures, or processes and as measures, i.e., properties of a phenomenon, body, or substance to which a magnitude can be assigned, and between descriptive and normative indicators. This clarification prevents the term "indicator" from becoming a meaningless buzzword, improves communication among stakeholders, and assures the testability of theories that include indicators. To avoid problems based on different understandings of the term and to maintain integrity in its use, we advise always providing a definition of the indicator term. (C) 2009 Published by Elsevier Ltd.