Measuring plant interactions: A new comparative index

We propose an index to measure the relative interaction intensity in plants (RII) with strong mathematical and statistical properties which overcome problems shown by other frequently used indices. RII has defined limits [-1, +1]; is symmetrical around zero, with identical absolute values for competition and facilitation; is linear; and does not have discontinuities in its range. It is therefore safe to use in statistical and mathematical operations. RII distribution is approximately normal, with means equal to the true population index and a sampling variance that can be derived. Its strong statistical properties make RII proper for use in parametric meta-analyses. It can be applied to any kind of interaction (from competitive exclusion to symbiosis) and in commonly published ranges of interaction intensity it offers the most consistent results. Because RII uses basic arithmetical operators, it can be scaled up and used to measure multispecific interactions at the community level.