Relative Size Early in Population Development Determines Reproductive Status of Individual Erodium cicutarium Plants

Differences between reproductive and nonreproductive plants in terms of germination date, size before competition, size after competition and number of near neighbors (altered by seed predation by ants) of the annual Erodium cicutarium were examined. Seed production was best correlated with the size of the plant relative to its neighbors 5 wk before flowering began. Reproductive plants in the treatment with the highest plant density (initially 900 seeds m−2 with no seed predation) were 30% larger than nonreproductive plants in that treatment, but 33 to 36% smaller than nonreproductive plants in the low density treatments (initially 400 seeds m−2, with or without seed predation). Thus, reproductive status was linked to relative size of neighbors rather than absolute plant size. The need for a plant to be large relative to its neighbors before it reproduces could explain why similar numbers of plants reproduced in the populations (=replicate flats) despite very different densities and spatial heterogeneity.