Growth, reproduction and weediness: testing four related species on a gradient of synanthropy

Background . The ability of weeds to thrive in the stressful environments created by human disturbance has been explained mainly by a set of life history traits, such as short life cycles, generalist habits, as well as early and sustained reproduction. However, the evidence that these traits are better represented in weeds than in related species of other environments is mixed. To explore the relationship between weeds and the life history traits, we used the fact that plants are weedy to different degrees because of the heterogeneous nature of environments produced by disturbance. In a group of four congeners, we studied some growth and reproduction parameters in relation to the degree of synanthropy of the species, determined previously. Methods. In a common garden experiment, we compared relative growth rate, time to flowering, and biomass distribution between four species of the genus Melampodium (Asteraceae) that are weedy to different degrees. Results. The most synanthropic species, M. divaricatum , stood out for its steady growth rate, but not for assigning more resources to reproduction, nor for early flowering. In general, we found no association between growth and reproductive parameters studied in the four Melampodium species and the degree to which they are weeds. Conclusions. Results suggest that traits such as fast growth and early reproduction may not be essential for life as a weed. Rather, weedy species exhibit a complex pattern of growth traits that could be affected by conditions independent of anthropogenic disturbance.