Coexistence of Two Species of Bracken (Pteridium) in a Narrow Zone of Range Overlap

Abstract. Two closely related bracken (Pteridium) species, P. caudatum and P. aquilinum ssp. pseudocaudatum, have mostly separate ranges that overlap in Peninsular Florida. Congeneric species that occupy similar niches in distinct geographic ranges but whose native ranges narrowly overlap present an eco-evolutionary conundrum: How can species evolved to occupy similar niches in distinct ranges coexist? Research suggests that for a stable coexistence the species should differ at least subtly in their ecological niches, otherwise their coexistence would be unstable. We investigated whether these bracken species share a habitat and co-occur locally, and whether their coexistence is predicted to be stable. We surveyed bracken populations from seven sites at Archbold Biological Station (Venus, Florida, USA) and characterized their biotic and abiotic microhabitat. Both bracken species co-occur at three sites. The two species showed significantly different trends in canopy cover and in response to fire frequency, and they grew at different densities and biomass. These results suggest these two bracken species coexist stably by occupying distinct ecological niches.