Seed mass, abundance and breeding system among tropical forest species: do dioecious species exhibit compensatory reproduction or abundances?

Abstract

Dioecious species may suffer a reproductive handicap compared to sympatric 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 disadvantage, natural selection in dioecious populations should favour females that reallocate those reproductive resources, that in cosexual individuals are invested in male function, towards the production of higher quality or more seeds, potentially leading to a higher density of established individuals. We tested for two compensatory fitness advantages (heavier seeds and higher population densities) in dioecious species in the flora of a high-diversity tropical forest in Western Amazonia. Using a phylogenetically corrected generalized linear modelling approach, we examined the phylogenetic dependence (Pagel's 03BB) of seed mass, abundance, growth form and wood-specific gravity for 641 species from Yasuní National Park, Ecuador. 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 species abundance, using the maximum likelihood values of 03BB. Contrary to expectations, dioecious species exhibited no advantage with respect to either seed mass or population density; there was no difference in mean seed mass or abundance between dioecious species and their most closely related cosexual species. We compared our results with those from a similar forest in the Tambopata Wildlife Reserve, Peru. There, dioecious shrubs had larger seeds than hermaphroditic shrubs, but (as for Yasuní) there was no difference between breeding systems in seed size among trees. Differences in the composition of species sampled may have contributed to differences in the results between the two sites. We suggest that other individual fitness advantages, such as reduced inbreeding, increased dispersal, superior seed quality (e.g. better defences) or increased seed number contribute to the persistence of dioecious populations at Yasuní. Synthesis. Similarities in the statistical models of seed mass and abundance for plant species in a Western Amazonian rain forest indicate that selective pressures on the 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.