Fitness Impacts Of Herbivory Through Indirect Effects On Plant-Pollinator Interactions In Oenothera Macrocarpa

Ecology, Jan, 2000 by Kristine Mothershead, Robert J. Marquis

Effects of increased leaf damage on female plant fitness: indirect or direct?

Increased leaf damage decreased both fruit set and seed number per plant. Initially, leaf damage reduced both flower number and flower size (corolla diameter and floral tube length). We consider the reduction in flower number and floral traits to be an initial direct effect of leaf damage through decreased resource availability. However, evidence suggests that ultimate female fitness of O. macrocarpa plants declined following experimental leaf damage due to an indirect effect of damage on plant pollinator interactions rather than through the effects of resource loss. Specifically, pollen availability in natural pollination plants was limited to such a degree that plants producing more flowers did not produce more fruits and seeds. In contrast, pollinator observations demonstrated that hawk moths visited flowers with larger corollas, and that the probability of producing a fruit for natural pollination plants increased with corolla diameter. Thus, differential female fitness between plants of increased and na tural leaf damage treatments appears to be due to discrimination by hawk moth pollinators between flowers of those plants, based on corolla diameter.

Path analysis results support the idea that herbivory had an indirect effect on female plant fitness. For increased leaf damage plants, there was a significant negative path from percentage leaf damage to corolla diameter and a positive path from corolla diameter to seed number, but no significant direct path from damage to seed number. Additionally, there was no direct path from initial leaf area to total seeds, indicating that plant size was not a factor affecting corolla diameter or plant fitness. Because the range of leaf damage to natural damage plants overlapped that found in increased damage plants, we believe path results apply to plants in the natural population that experienced higher levels of leaf damage.

Reproductive output is often positively correlated with plant size. Such a relationship would be expected in 0. macrocarpa if direct effects of herbivores were occurring because plants more heavily attacked (smaller plants) would produce fewer flowers and have fewer total ovules per plant to mature into seeds. However, this correlation may not exist when reproduction is limited by pollinator availability. Juenger and Bergelson (1997) also found no fitness benefit of larger plant size. If pollinator availability varies from year to year in 0. macrocarpa, as it does in ipomopsis aggregata (Juenger and Bergelson 1997), then we predict that the impact of leaf herbivores on plant fitness would shift from being an indirect effect to a direct effect with increasing pollinator abundance. In low pollination years, hawk moths discriminate among flowers of different sizes. Damaged plants in those years have lower fitness because their flowers are smaller. In high pollinator years, most flowers may be pollinated, but da maged plants would produce fewer seeds because they produce fewer flowers. Therefore, while flower number has the potential to influence female fitness greatly, the effect of leaf herbivory on flower number observed in our study may be realized only when pollinators are abundant.