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Abstract:
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Specialization of a plant on a particular pollinator
may not evolve if co-pollinators are effective and
abundant. This is particularly evident if fruit set is resource
limited and cannot be increased above the levels
produced by the actions of co-pollinators. The pollinating
seed-consuming interaction between senita cacti and
senita moths in the Sonoran Desert presents a paradox
because it exhibits many traits resembling those of the
highly specialized yucca/yucca moth system, but also involves
co-pollinators. For 6 years, we studied how contributions
of nocturnal senita moths and diurnal co-pollinating
bees to fruit set depended on resource and pollen
limitation, time of flower closing, and the onset and phenology
of flowering. Fruit set was typically resource limited.
Fruit set of flowers exposed only to senita moths
was not different from resource-limited fruit set of control
flowers. When only co-pollinating bees were allowed
to visit flowers, however, fruit set became pollen
limited. Only in one year when fruit set was pollen limited
were bees able to increase fruit set beyond the level
resulting from senita moth pollination. High temperatures
commonly induced flowers to close before sunrise
so that diurnal bees were unable to visit flowers. This
was particularly important from 1998 to 2000, when
flowering did not begin until late in spring when temperatures
were already high enough to induce flowers to
close before sunrise. Bees were typically functionally redundant
with senita moths; excluding bees from visiting
flowers did not alter fruit set. Nevertheless, extreme specialization
of floral traits to exclude co-pollinators has
not evolved in senita, possibly because there are times
when bees do increase fruit set. This can occur when senita
moths are rare, fruit set is pollen limited, cool temperatures
prevent flowers from closing before sunrise,
and flowering begins early in spring. |
