Sexual system, abundance, range size and survivorship bias in the Amazonian tree flora

Dioecy (separate sexes) creates difficulties for pollination, seed production and dispersal in plants. Cosexual species, those with bisexual flowers (hermaphroditism) or unisexual flowers on the same individual (monoecy), should therefore have a competitive ecological advantage. However, we show, using a dataset of 5000+ Amazonian tree species, that for animal-dispersed trees, dioecious species tend to be more abundant with larger ranges than hermaphrodites. Moreover, with increasing abundance and range size, the probability of being dioecious increases. We suggest that survivorship bias explains these anomalous outcomes. As the reproductive handicap of dioecy cannot be fully compensated, extinction probability increases for restricted dioecious trees, leading to the over-representation of dioecy among abundant and wide-ranging species but under-representation among uncommon and rare species. Abiotically dispersed trees in Amazonian forests may also face increased extinction probability through dispersal difficulties in dense evergreen settings, and survivorship bias may likewise have increased their average abundance and dominance