On quantitative measures of indirect interactions

Indirect effects, whether density-mediated (DMII) or trait-mediated (TMII), have been recognized as potentially important drivers of community dynamics. However, empirical studies that have attempted to detect TMII or to quantify the relative strength of DMII and TMII in short-term studies have used a range of different metrics. We review these studies and assess both the consistency of a variety of different metrics and their robustness to (or ability to detect) ecological phenomena such as the dependence of forager behaviour on conspecific density. Quantifying indirect effects over longer time scales when behaviour and population density vary is more challenging, but also necessary if we really intend to incorporate indirect effects into predictions of long-term community dynamics; we discuss some problems associated with this effort and conclude with general recommendations for quantifying indirect effects.