Abstract
Plant responses induced by herbivore damage can provide fitness benefits, but can also have important costs due to altered interactions with mutualist pollinators. We examined the effects of plant responses to herbivory in a hummingbird-pollinated distylous shrub, Palicourea angustifolia. Through a series of field experiments we investigated whether damage from foliar herbivores leads to a reduction in fruit set, influences floral visitation, or alters floral traits that may influence pollinator preference or pollinator efficiency. Foliar herbivory by a generalist grasshopper led to reduced fruit set in branches that were directly damaged as well as in adjacent undamaged branches on the same plant. Furthermore, herbivory resulted in reduced floral visitation from two common hummingbird species and two bee species. An investigation into the potential mechanisms behind reduced floral visitation in induced plants showed that foliar herbivore damage resulted in shorter styles and lower nectar volumes. This reduction in style length could reduce pollen deposition between different floral morphs that is required for optimal pollination in a distylous plant. We did not detect any differences in the volatile blends released by damaged and undamaged branches, suggesting that foliar herbivore-induced changes in floral morphology and rewards, and not volatile blends, are the primary mechanism mediating changes in visitation. Our results provide novel mechanisms for how plant responses induced by foliar herbivores can lead to ecological costs.
Highlights
Plants interact simultaneously with diverse communities of antagonistic and mutualistic organisms
Our results suggest that P. angustifolia is an obligate outcrosser (Table 1)
Plant responses induced by herbivore damage are a defense mechanism that can protect plants against further damage [39]
Summary
Plants interact simultaneously with diverse communities of antagonistic and mutualistic organisms. These interactions are often studied in isolation, plant traits that influence multiple interactions are common, and plant responses to one organism can mediate interactions with multiple other organisms [1,2]. Leaf herbivory imposes fitness costs these changes can negatively affect interactions with mutualists, including pollinators, seed dispersers, and mycorrhizal fungi [4,5,6]. The negative effects of induced plant defenses on mutualistic interactions are examples of ‘ecological costs of defense’, which are costs that arise when the expression of defense traits alters interactions between plants and their environment in a way that reduces plant fitness [7]. We focus on costs of foliar herbivory that can result from a reduction in the visitation of pollinators
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