ABSTRACT
Defining ecosystem integrity and choosing indicators for it would be a relatively simple task if the science of ecology was able to provide the reader with simple, rigorous models for describing and predicting these states of ecosystems. There are three key steps to developing useful and ecologically meaningful indicators: selecting the appropriate state variables, setting critical limits, and testing the indicators by following them through monitoring programs. In the past, indicators have been developed haphazardly, often reflecting the interests of specific user groups and value systems, rather than according to more broad scale ecological criteria. Selecting indicators and setting critical limits is obviously an evolutionary process. There is a huge ecological literature on diversity, including quantitative measures for it and the implications of different levels of diversity for ecosystem function. Guilds or functional groups of organisms are defined by their sharing certain traits which allow them to perform a similar function in a given ecosystem.
