Alternative Outcomes of Natural and Experimental High Pollen Loads

Seed production is usually assumed to be a positive monotonic function of pollen deposition and/or pollinator visitation. If this assumption were correct, there would be only two outcomes of excess pollen levels: an increase in fruit or seed set, or no increase. However, a substantial minority of the studies reviewed here has found that seed production declines with increased pollen loads, both under experimental and natural conditions. To explain this decrease, we propose the following mechanisms: pollen tube crowding, pollen removal or stigma damage by pollen thieves or pollinators, stigma damage during hand—pollination, application of low—diversity or local pollen, effects of bagging flowers, missed stigma receptivity, and the application of inviable pollen. These mechanisms can be distinguished through more complete and more careful experimental designs and incremental pollen supplementation.