An experimental test of the relationship between seed size and competitive ability in annual plants

Competition–colonization tradeoff models explain the coexistence of competing species in terms of a tradeoff between competitive ability and colonization ability. One class of such models is based on the idea that seed size determines competitive ability, seed number determines colonization ability, and the two traits are negatively correlated such that higher competitive ability of large‐seeded species compensates for their smaller seed number. According to such models, species inhabiting the same community should show a distinct ranking of competitive ability and this ranking should be correlated with seed size. We tested these predictions using a greenhouse competition experiment focusing on 25 annual species that coexist in sandy habitats of the Mediterranean region in Israel. Rankings of species based on their competitive effects on two independent phytometers were positively correlated. Corresponding rankings based on competitive responses were also correlated. Rankings based on competitive effects were correlated with those based on competitive responses. Yet, in spite of the clear hierarchy in all measures of competitive ability, none of the measures was correlated with seed size. While lack of correlation between seed size and competitive ability has been documented in some systems, our study is the first time that absence of such correlation is documented in a system where the existence of competition is well established and the component species show a clear hierarchy of competitive ability.