Fitness impacts of herbivory through indirect effects on plant-pollinator interactions in Oenothera macrocarpa

The negative impacts of herbivores on plant fitness may include both direct and indirect effects. Direct effects on female plant fitness occur when de creased seed production is due to decreased resource availability from loss of leaf area and its attendant photosynthesis or through consumption of re productive structures. Indirect effects occur when folivore- and florivore- mediated changes in floral traits influence pollinator preference and/or po llinator efficiency, thus reducing pollen receipt. We examined the effects of both leaf and floral herbivory (bud damage) and determined the relative contribution of direct and indirect effects of damage on female fitness thr ough changes in floral traits for Oenothera macrocarpa (Onagraceae). The ex periment was a two-factorial design that manipulated both leaf damage (simi lar to 25% leaf area removed by hand) and pollen receipt (supplemental hand pollination). We then measured effects of leaf damage on floral traits (co rolla diameter, floral tube length, and flower number) and fruit and seed s et. Because herbivores damaged corollas in the bud stage in this experiment , we were also able to determine the effect of florivores on subsequent fem ale reproduction. Plants in the increased leaf damage treatment (33.4% leaf area loss) produc ed fewer flowers than natural leaf damage plants (6.5% leaf area loss), and their flowers had smaller corolla diameters and shorter floral tube length s. Experimentally damaged planes also had 18% lower fruit set and produced 33% fewer seeds compared with natural leaf damage plants. Hand pollination increased fruit set by 60% and total seed number by 38% above that of natur ally pollinated plants, demonstrating that female fitness was pollen-limite d. Bud damage significantly decreased corolla diameter and floral tube leng th, and it led to a 68% reduction in fruit set. Hawk moths were observed pr eferentially visiting flowers with larger corollas, and study flowers with larger corollas had significantly increased probability of setting fruit. A dditionally, seed number per fruit was positively related to floral tube le ngth. In path analysis, we found no significant path between percentage lea f damage and female reproduction, suggesting that herbivores did not reduce seed production directly through decreased resource availability. Instead, we found a significant indirect path from percentage leaf damage to seed n umber through corolla diameter. We conclude that decreased female reproduct ion was due to changes in floral traits from both leaf and floral bud damag e, which affected hawk moth preference (flowers with smaller corollas recei ved fewer visits) and hawk moth efficiency of pollen delivery (flowers with shorter floral tubes had fewer seeds). This study demonstrates that herbiv ores can mitigate the mutualistic relationship between plants and pollinato rs through their effects on floral traits.