Importance: an overlooked concept in plant interaction research

1. Numerous approaches have been taken to analysing the relationship between plant interactions and environmental severity. Considerable debate surrounds this relationship and its implications for the evolution of plant strategies and traits. This debate is commonly associated with the names of two of its main protagonists: Grime and Tilman. 2. However, because researchers often fail to make explicit the distinction between two concepts - the importance and intensity of competition - they perpetuate this debate. To illustrate this point we discuss a new approach to examining plant interactions across gradients proposed by Wilson. 3. We support the main thrust of Wilson's work - improving the information provided by interaction indices. However, we highlight what we believe to be an oversight in Wilson's Discussion: the proposal that his new indices can shed light on the Grime-Tilman debate. We argue that this is not possible as Wilson's indices measure competition intensity (competition's absolute impact), whereas Grime's theories relate to competition importance (competition's relative impact). 4. We support our argument through re-analysis of the data explored by Wilson, demonstrating that the relationships between competition importance, competition intensity, Wilson's new indices and biomass (a surrogate of productivity) are both complex and variable. 5. Wilson's discussion is part of a body of literature that overlooks the distinction between competition importance and intensity. We examine a number of other well-known studies, demonstrating how this oversight and associated confusion are widespread, may have originated with the concept's original source, and unnecessarily perpetuate debate. 6. Synthesis. The concepts of competition intensity and importance are central to resolving key ecological debates, but are still overlooked in contemporary research. Their uptake and careful application would greatly aid advances in plant ecology.