Dioecious species theoretically suffer a reproductive cost in comparison to cosexual (hermaphroditic or monoecious) species of equal abundance simply because populations of dioecious species contain fewer seed-producing individuals. To compensate for this population-level handicap, natural selection in dioecious populations should favor females that reallocate those reproductive resources, that in cosexual individuals are devoted to male function, towards the production of more or higher quality seeds, potentially resulting in a higher density of established individuals. We tested for two compensatory fitness advantages in dioecious species from several tropical forests. Using a phylogenetically corrected generalized linear modelling approach, we examined the phylogenetic dependence (Pagel's lambda) of (i) seed mass, growth form, wood specific density and (ii) species abundance, for several hundred species from Yasuni National Park, Ecuador, Tambopata Wildlife Reserve, Peru and Barro Colorado Island, Panama. Finally, we compared abundances of dioecious and cosexual species from a pantropical network of forest dynamics plots.
Seed mass was highly correlated with phylogeny, but abundance was not. We then used AIC values to test for models (including breeding system) that best explained seed mass and abundance, using the maximum likelihood values of lambda.
Results/Conclusions
Contrary to expectations, dioecious species exhibited no advantage with respect to either seed size or population density; there was no difference in mean seed size or species abundance between dioecious species and their most closely related cosexual species. It seems that the selective pressures on seed-producing individuals are similar, despite variation in sexual system. Therefore, breeding system may play a more limited role in community assembly of tropical forests than was previously considered.
