Pollinator limitation, autogamy and minimal inbreeding depression in insect-pollinated plants on a boreal island

We studied the pollination biology of 18 common insect-pollinated plant species on Kent Island, a boreal island in the Bay of Fundy, New Brunswick, Canada. Under natural conditions, fruit set was relatively high in most of the species studied [although it was very low (<1%) in Maianthemum canadense]. Hand-pollination increased fruit set in five of seven species examined (71%), indicating that fruit set may commonly be limited by the availability or behavior of pollinators on Kent Island. Twelve of 17 species examined (71%) were capable of substantial autonomous self-pollination (autogamy in the absence of pollinators), although fruit set averaged higher in open-pollinated flowers (65.7%) than in flowers from which insects were experimentally excluded (49.6%). The number of seeds per fruit was also less in autonomously self-pollinated flowers in two species (Rhododendron canadense and Ledum groenlandicum). In at least one species (Iris versicolor), rates of autonomous selfing were higher on Kent Island than on the mainland. Stamen-excision experiments in I. versicolor demonstrated that fruit set required pollen transfer in the absence of pollinators (i.e., agamospermy did not occur). In hand-pollination experiments, five of six species (83%) (R. canadense, L. groenlandicum, Smilacina trifolia, S. stellata and I. versicolor) showed no evidence of inbreeding depression in terms of percent fruit set, fruit size or number of seeds per fruit. Overall, our results demonstrate that for many insect-pollinated plant species on Kent Island, pollinators are likely to be limiting, autogamy is common and inbreeding depression is negligible. Although pollinator limitation and autogamy regularly occur in mainland habitats as well, a review of the literature suggests that they may be more common on islands such as Kent Island. If such island-mainland differences are general, they may arise because genotypes and species capable of self-fertilization are more likely than obligate outcrossers to colonize and become established in isolated habitats