On the Meaning of Competition and the Mechanisms of Competitive Superiority

Thompson (1987) evaluated my theory of plant competition, community structure and succession (Tilman, 1980, 1982, 1985, 1986a) by considering how five assumptions or predictions of my work compared with the ideas and data of Grime (1979). Thompson's predisposition toward believing the ideas in Grime (1979) is clearly indicated by his closing sentence, which stated that my work 'opens up arguments about the nature of competition (above and below ground and at low and high resource levels) and the evolution of life histories in response to stress, which I had hoped had been settled 10 years ago,' i.e. with the publication of Grime (1979). However, few issues in science are ever settled, once and for all time. Although Grime (1979) had a significant impact on the field, there is new observational, experimental and theoretical evidence that supports alternative conceptual approaches. In this paper, I shall show the flaws in Thompson's evaluation of my work, suggest an alternative interpretation of Grime's work and present additional evidence that supports my theory. In doing this, I want to stress that it is not my purpose to try to dichotomize my work and that of Grime. Indeed, when viewed in a dispassionate manner, there are some broad, qualitative similarities between Grime's approach and mine. As Austin (1986) pointed out, both Grime and I agree that soil nutrient levels, light, disturbance and competition are important factors influencing plant community structure. However, Grime (1979) and I disagree about the ways in which these factors interact to structure plant communities. These differences are the subject of Thompson's paper and thus of this reply. In responding to Thompson, I shall first discuss the five points that he raised, and then consider some broader issues.