Seed dispersal differences and determinants of ranges of narrow endemic and widespread eastern North American trilliums

Drivers of species rarity can be classified as abiotic (environmental) or biotic (species interactions), and the spatial scale at which factors impact rarity ranges from fine to coarse. This dissertation explores drivers of narrow endemism, a kind of rarity in which species are restricted to geographic regions or habitat types, in the genus Trillium, a group of perennial understory forest herbs in eastern North America. To identify factors characterizing narrow endemism in this system, I used a framework comparing ecological attributes of co-occurring endemic and widespread congeners. At a fine spatial scale, I explored whether seed dispersal effectiveness differed for endemic and widespread species. Trilliums rely on ants for seed dispersal; my work establishes that rates and probabilities of seed dispersal are lower in the field for endemic trilliums, and that ants display a preference for the seeds of widespread trilliums, suggesting that narrowly endemic species may be dispersal limited. To investigate underlying mechanisms, I quantified interspecific differences in the morphology and chemistry of seeds and elaiosomes (i.e. seed-coat appendages that are attractive to ants). I found that seed morphology, concentrations of fatty acids, and aspects of elaiosome phytochemistry were significantly related to dispersal probability for five species of Trillium, suggesting that endemic species produce less-attractive seeds. At a coarse spatial scale, I generated species distribution models (SDMs) using Maxent for 21 species of Trillium; SDMs produced areas of predicted suitability for each species based on abiotic conditions. Within this framework, I found that rarer species of greater conservation concern had lower proportional occupancy of the fundamental niche than common species of lower conservation concern; that SDMs performed best for species of higher conservation concern with larger predicted suitable areas; and finally, that flower type and ovule number were significantly related to model performance. These discoveries provide valuable insight into the ecology and biogeography of species in the genus Trillium, thus advancing the conservation-relevant knowledgebase for this horticulturally-important group of plants, and contribute to our understanding of factors that maintain plant endemism at fine and coarse spatial scales, from both abiotic and biotic perspectives.