Using experimental indices to quantify the strength of species interactions

Here we clarify some of the assumptions made in the application of the two most commonly used indices for measuring the strengths of species interactions with manipulative removal/addition experiments: Paine's index and the dynamic index. We explain how the values these indices are intended to measure - the per capita interaction strength between two species - are typically not estimated in common currencies. We then introduce extensions to these indices that alleviate a subset of previously made assumptions and limitations. These include a reformulation of Paine's index appropriate for an open-recruitment system, and an extension of the dynamic index applicable to interactions of a particular nonlinear form. While we focus our language on predator-prey interactions, our discussions are pertinent to the measurement of species effects in other types of interactions as well (Mitchell and Wass 1996, Freckleton et al. 2009).