Modifying Kempton's species diversity index for use with ecosystem simulation models

The Q-90 statistic, a variant on Kempton's Q index, is developed to measure the effects of mortality from fishing or climate on species diversity in whole ecosystem simulation models that group functionally similar organisms. The statistic represents the slope of the cumulative species abundance curve between the 10- and 90-percentiles. In applying Kempton's method to the Ecopath with Ecosim modeling system, functional groups in the model are considered equivalent to ‘species’, and their biomasses, sorted into bins, are analogous to the number of individuals in field sampling studies. Our algorithm generates a Q-90 value from a time series of model output; this allows biodiversity to be monitored over the course of a simulation. Comparing the biodiversity trajectory generated by different fishing strategies, this technique provides an additional method to evaluate the outcomes and meets policy requirements for describing impacts on biodiversity. This methodology complements previously described evaluation tools used in the ‘Back to the Future’ approach to restorative marine ecology.