No Allee effects in Lyon's Pentachaeta, a federally listed endangered sunflower

Potential Allee effects were investigated in Pentachaeta lyonii, an annual Asteraceae that is listed as federally endangered. Since P. lyonii is self-incompatible and has no evident seed dormancy, pollination service is essential for persistence. Observations of floral visitors showed that insect composition varied by site, year, and over the flowering season. The most common visitors were the bee-flies Lepidanthrax sp. and Paravilla sp. and the bee Ashmeadiella californica subsp. californica. There were also late-season peaks of the bee-fly Exoprosopa doris and bee Exomalopsis sp. These generalist pollinators allowed ample pollination for P. lyonii. The absence of Allee effects was further supported by there being no difference in seed production between open-pollination and hand-augmentation treatments within patches. In 2008 there was a proportional increase in per-capita visitation rates with increasing density. Flower heads in low density quadrats did not suffer a significant reduction in seed production compared to flower heads in more dense quadrats. Flowering P. lyonii in pots were placed in patches of various densities and at distances up to several meters from a patch. The percent of quadrats visited was higher inside a patch (69%) than outside a patch (16%), but seed set was not reduced by being placed outside a patch. Visitation to lone potted plants was equal to or greater than visitation experienced by potted plants inside patches, possibly because an individual flower head is less attractive when surrounded by conspecifics. Isolated plants at one site were visited more by melyrid beetles and Ceratina bees than the visitors to patches of flowers. These results indicate that other factors such as habitat loss and competition with non-native plants are likely responsible for populations not expanding in size and in some cases declining.