Causality and the choice of measurements for detecting human impacts in marine environments

The choice of biological indicator variables to be measured in detecting human impacts on the environment is a critical one. The usual community-level measures (species richness, diversity) generally have questionable theoretical justification, have no demonstrable causal links to the impact, and are dependent on the taxonomic expertise available. Results from trampling experiments on an intertidal rocky shore demonstrate that these measures are also insensitive in detecting impacts that clearly affected populations of individual species. The need for experimental work that identifies which indicator variables are causally linked to human impacts and therefore which will be useful in monitoring is emphasized.