Measuring the importance of competition in plant communities

Plant ecologists have developed numerous ways to measure competition and to rank the effects of competition relative to other factors. According to one line of logic, there is an important distinction between competitive intensity (broadly, the absolute, proximate effects on individuals) and the importance of competition (loosely, the relative effects on ecological or evolutionary processes). It has been argued recently that there is a great deal of confusion in the literature regarding these concepts. We agree and suggest that this arose because of loose logic in the initial formulation, and that this has been perpetuated and exacerbated in recent critiques. Using a simple example, we argue that recent analyses have generated new problems because of a failure to measure importance in terms of individual fitness (defined in an evolutionary sense) or per capita rates of population change. Only when calculated in this way can importance be measured relative to all other processes in the life cycle. It is not possible to directly measure the importance of competition from short-term experiments that last less than one generation, using data from artificial conditions, or using data from glasshouse experiments. Too often researchers use the term 'importance' without stating clearly what this is measured with respect to. We highlight, for example, that importance could be measured for population growth rate, community composition or community invasibility and that the appropriate measure would differ in each case. Synthesis. We ask whether a single index of importance is really useful in plant ecology. The concept focuses on index-based measures of competition using experimental data, narrowly concentrating on comparing two theories of plant competition. In the rest of the ecological world, researchers are using model-based analyses of field data and increasingly sophisticated fitting techniques to dissect out the various processes determining the dynamics of single and interacting populations and making a great deal of progress. It is obviously very useful to disentangle the effects of competition at different stages in the life cycle and to determine how these vary along environmental gradients. However, the measure of competition and the measure of importance should be tailored to the question being addressed.