The Importance of Specialist Pollinators to Generalist Hosts

The number of generalist bee species visiting host plants that either DO or DO NOT also host
specialist bee species. Figure shows that specialists generally use the most popular floral
hosts used by generalists. Data sources listed below.

Many bee species are specialists, collecting pollen from a very narrow
taxonomic range of plants (a single species, genus, tribe or family). It is
very uncommon, however, that specialist bees are the only or even the
predominant visitor to their host plant. Instead, their host plants tend to
be utilized by various species of specialist and generalist bees, as well
as species from other animal groups. Along with collaborators Robert
Minckley (University of Rochester) and Neal Williams (Byrn Mawr), I am
studying whether specialists make up for their lack of exclusivity by
providing unusually good or unusually reliable pollination services
at a study site (Rancho San Bernardino) in the Chihuahuan Desert of
northwestern Mexico.

Data from Robertson, C. (1929). Flowers and Insects. Lancaster, Penn., Science Press Printing Co.;
Westrich, P. (1989). Die Wildbienen Baden-Württembergs. Stuttgart, Eugen Ulmer. Schlindwein, C.
(1998). "Frequent oligolecty characterizing a diverse bee-plant community in a xerophytic
bushland of subtropical Brazil." Studies on Neotropical Fauna and Environment 33(1): 46-59;
Minckley, R. L. and T.H. Roulston. 2006. Incidental mutualisms and pollen specialization
among bees, in "Specialization and generalization in plant-pollinator interactions," Waser, N. M.
and Ollerton, J. (eds.), Chicago Press, Chicago.

The 'squash bee' Peponapis pruinosa. Males
such as this search for females at squash/
pumpkin (Cucurbita spp) flowers during
the morning then sleep in the flowers after
they close. Females collect pollen exclusively
from plants in the genus Cucurbita.

A female tomatillo specialist bee, Colletes
, marked with typewriter correction
fluid and 'sharpie' markers. Males search for
mates and hold territories at tomatillo
(Physalis spp.) flowers. Females collect
pollen (and usually nectar) exclusively from
Physalis flowers.

Field crew at Rancho San Bernardino, Mexico,
spring 2005. Arturo Romero, Lucia Salas,
Rocio Lopez, and Esther Julier.

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