Interspecific competition in plants: How well do current methods answer fundamental questions?

Accurately quantifying and interpreting the processes and outcomes of compe tition among plants is essential for evaluating theories of plant community organization and evolution. We argue that many current experimental approa ches to quantifying competitive interactions introduce size bias, which may significantly impact the quantitative and qualitative conclusions drawn fr om studies. Size bias generally arises when estimates of competitive abilit y are erroneously influenced by the initial size of competing individuals. We employ a series of quantitative thought experiments to demonstrate the p otential for size bias in analysis of four traditional experimental designs (pairwise, replacement series, additive series, and response surfaces) eit her when only final measurements are available or when both initial and fin al measurements are collected. We distinguish three questions relevant to d escribing competitive interactions: Which species dominates? Which species gains? and How do species affect each other? The choice of experimental des ign and measurements greatly influences the scope of inference permitted. C onditions under which the latter two questions can give biased information are tabulated. We outline a new approach to characterizing competition that avoids size bias and that improves the concordance between research questi on and experimental design. The implications of the choice of size metrics used to quantify both the initial state and the responses of elements in in terspecific mixtures are discussed. The relevance of size bias in competiti on studies with organisms other than plants is also discussed.