Extended floral longevity does not increase seed production under experimental pollinator decline

Premise of researchIn plant species that depend on pollinators to produce seeds, declines in pollinator populations should strengthen natural selection for floral traits that increase outcross pollen receipt by increasing the probability of pollinator visitation. One such trait is floral longevity, the amount of time that a flower is open and functional. If flowers that are open longer have a higher probability of pollinator visitation and this pollination benefit outweighs the resource cost of maintaining floral tissue, then pollinator decline should strengthen selection for extended floral longevity. MethodologyTo test whether pollinator decline could strengthen selection for extended longevity, we studied the pollinator-dependent species Lobelia siphilitica. We exposed L. siphilitica plants to either an ambient open-pollination treatment or a reduced open-pollination treatment that simulates pollinator decline. Within each pollination treatment, we measured floral longevity and seeds per fruit and used these data to estimate directional selection on longevity. This experiment was repeated twice. Pivotal resultsIn one experiment, there was significant selection within both ambient and reduced pollination treatments for shortened (rather than extended) floral longevity. In the other experiment, selection on floral longevity within both ambient and reduced pollination treatments was not significantly different from zero. ConclusionsWe found no evidence of selection for extended floral longevity, regardless of the pollination environment. These results suggest that pollinator decline will not strengthen selection for extended longevity and thus that pollinator-dependent species such as L. siphilitica may not respond to pollinator decline by evolving extended floral longevity.