Informed dispersal in plants: Heterosperma pinnatum (Asteraceae) adjusts its dispersal mode to escape from competition and water stress

Informed-dispersal theory (IDT) states that organisms may use information on the quality of the local environment when deciding whether to disperse or not. Dispersal is expected to occur from adverse patches where the costs of philopatry (lack of dispersal) are greater than those of dispersal. Evidence of informed dispersal in plants is scarce, and experiments under natural conditions are lacking. We tested IDT in a semiarid grassland using the annual forb Heterosperma pinnatum, which produces awned, zoochorous achenes and unawned, philopatric ones. We expected the proportion of awned seeds to increase under adverse conditions that reduce fitness, (i.e. under high competition and water stress). Heterosperma pinnatum was sown along natural moisture gradients with and without a shade that increased water availability. We found that competition (number of conspecific neighbors) and water stress reduced fitness. As expected, the proportion of highly dispersible seeds increased under such conditions, probably as a strategy to escape from unsuitable patches. Many plants species do not behave as expected by IDT. Our results suggest that experiments under natural conditions in systems where the assumptions of IDT are met may prove to be a wealthy source of evidence for informed dispersal in plants.