Ecology Of Rudbeckia auriculata (Perdue) Kral (Asteraceae)

Abstract

Rudbeckia auriculata is a rare species endemic to three southeastern states: Alabama, Florida, and Georgia. Eight censuses of flowering individuals of the species were conducted from 1992 to 2002. Although the number of known populations increased during the census period, total counts of flowering stems remained relatively constant. Population sizes ranged from one individual with a single flowering stem to populations with over 1,000 flowering stems. Information on soils and associated species of vascular plants was collected at twenty of the thirty-two known sites. Typical sites are on wet soils along roadsides, power line right-of-ways or are otherwise disturbed. Associated species are characteristic of disturbed open wetland sites. Although some large colonies of R. auriculata still exist, only two populations, both in the northern portion of the species’ range, have been protected. Observations of insect abundance on flower heads and analysis of pollen loads on floral visitors indicated that the most likely pollinators are Andrena aliciae Robertson (in medium and large Rudbeckia auriculata populations) and Halictids (in small populations). Achene set varied from 0.24% to 16.9% in small populations (< 40 flowering stems) and from 26.5% to 31.4% in medium (40-999 flowering stems) and large (> 1000 flowering stems) populations. Achene set was significantly lower in the small populations. Exclusion of visitors from inflorescences showed that R. auriculata is probably self-incompatible and thus requires insect vectors for successful pollination and achene set. Achene dispersal appears to be highly localized and dependent upon gravity. Seedling recruitment is poor. The fungus Fusarium semitectum Berk. & Ravenel infects the flowering heads of Rudbeckia auriculata at two sites in Alabama. The fungus produces orangish or pinkish-white spores on the flower heads and renders infected flowers sterile. Fungal spores superficially resembled pollen and are picked up by the main pollinator, the composite specialist bee Andrena aliciae, which serves as a dispersal agent for the fungal pathogen. The fungus appears to pose no serious threat to the species at this time.