Honors College Theses

Publication Date



Biology (B.S.B.)

Document Type and Release Option

Thesis (open access)

Faculty Mentor

Dr. John Schenk


Amphitropical disjunct plants are species that occur in both North and South America but not in the intermediate equatorial region. How amphitropical species dispersed across the Americas to their current distribution remains uncertain. To explain amphitropical distributions, three hypotheses were developed to test that (1) species dispersed successively through island hopping across the tropical zone in temperate microhabitats (= stepping stones), (2) species came to their current distribution through vicariance, or (3) species dispersed by a single long distance dispersal event. Twenty-five species were studied to infer their historical distributions with species distribution models in MaxEnt. Distribution models for each species were estimated under three different timelines: the current climate, the last glacial maximum (22,000 years ago), and the last interglacial maximum (120,000–140,000 years ago). Ecological niche models were generated in MaxEnt with 19 bioclimate variables from the WorldClim database. Across the three time slices, the vicariance hypothesis was rejected in all but one species for one of its time slices. In thirteen experiments, the long-distance dispersal and vicariance hypotheses were rejected in favor of the stepping stone hypothesis. In thirty- seven experiments, the stepping stone and vicariance hypotheses were rejected in favor of long distance dispersal. There were three species that could have dispersed by shorter-distance-dispersal events via intermediate dispersal from Mexico. Although our results suggest that long distance dispersal was the most dominant mechanism of dispersal among amphitropical species, they also convey that that numerous shorter-distance-dispersal events via intermediate, favorable populations are an underappreciated mode of dispersal.