Reproductive traits explain occupancy of predicted distributions in a genus of eastern North American understory herbs

Abstract

Aim: Abiotic, biotic and dispersal factors interact to shape species distributions. At broad geographic extents, abiotic factors are thought to exert the greatest influence on the distribution, while biotic and dispersal factors strongly influence the distribution regionally and locally. We test whether reproductive traits relating to biotic and dispersal factors explain differences between estimated potential and occupied geographic distributions for 21 species of Trillium. Location: Eastern North America. Methods: Fundamental niches and associated predicted suitable distributions were estimated using climate-calibrated ecological niche models. We defined occupied distributions as the intersection between known ranges and predicted suitable areas (PSAs). Proportional occupancy of the predicted suitable distribution (PO) was calculated by dividing the area of the occupied distribution by the PSA. We related reproductive traits (ovule number, seed set, number of seeds/plant, seed mass, adult biomass, flower type: sessile/pedicellate) to PO using beta regression models. AICc was used to assess model fit. Results: There was considerable variation in PO across species (1.1%–96%, mean = 51%). Eighty-five percent of species with PO < 60% were sessile; 88% of species with PO > 60% were pedicellate. The best-fit beta regression (pseudo R2 =.70) yielded significantly lower PO for sessile-flowered species; ovule number and seed mass were also significant predictors of PO. Main conclusions: Variation in PO among study species can be explained by flower type, ovule number and seed mass—biotic traits related to dispersal ability. We posit that variation in dispersal potential stemming from primary and long-distance dispersers is related to occupancy of the predicted suitable distribution in Trillium. We exemplify a scenario in which life history traits explain why some species are range-restricted when their close relatives are widespread. Our methodology constitutes a powerful comparative framework that can be applied to diverse biological systems to inform conservation of rare species.