Aim The factors affecting the distribution of dioecious species have received surprisingly little attention and, as a consequence, are poorly understood. For example, there is a well-documented negative association between dioecy and latitude, for which we have no candidate mechanisms. Conversely, it has been suggested that the relative proportion of dioecious species should be positively correlated with changes in elevation.
Location Costa Rica, Central America.
Methods We made use of data on the distribution of 175 seed plant species from a series of plots along a transect in Costa Rica that ranged from 30 to 2600 m a.s.l. to test the prediction that dioecy is correlated with elevation. Specifically, we examined relationships between the proportion of dioecy, at the species and individual levels, and the elevation, species richness, number of individuals, and phylogenetic diversity (PD) of plots. For comparison, we repeated all analyses with monoecious species, which also have unisexual flowers but do not suffer from reduced mate assurance and the seed shadow effect that may be the outcomes of having spatially separated sexes.
Results The relative proportions of dioecious species and individuals displayed a unimodal relationship with elevation, both peaking at 750 m a.s.l. In contrast, the relative proportions of monoecious species and individuals displayed negative associations with elevation. In addition, the proportion of dioecious species was significantly positively associated with species richness and with the PD of plots. The proportion of monoecious species was not associated with species richness and was weakly positively associated with the PD of plots.
Main conclusions Our results suggest that the selection pressure of elevation on the pollinators and life-history strategy of dioecious species is much less than expected, and is overshadowed by the as-yet unexplained correlation of dioecy with species richness. Additional studies of the ecology of cosexual and unisexual (i.e. dioecious and monoecious) species along large environmental gradients are needed.
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