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Abstract:
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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. |
