Question

How to determine the index of association between two species?

Answer 1

Abstract. Ecologists often face the task of studying the association between single species
and one or several groups of sites representing habitat types, community types, or other
categories. Besides characterizing the ecological preference of the species, the strength of the
association usually presents a lot of interest for conservation biology, landscape mapping and
management, and natural reserve design, among other applications. The indices most
frequently employed to assess these relationships are the phi coefficient of association and the
indicator value index (IndVal). We compare these two approaches by putting them into a
broader framework of related measures, which includes several new indices. We present
permutation tests to assess the statistical significance of species–site group associations and
bootstrap methods for obtaining confidence intervals. Correlation measures, such as the phi
coefficient, are more context-dependent than indicator values but allow focusing on the
preference of the species. In contrast, the two components of an indicator value index directly
assess the value of the species as a bioindicator because they can be interpreted as its positive
predictive value and sensitivity. Ecologists should select the most appropriate index of
association strength according to their objective and then compute confidence intervals to
determine the precision of the estimate.