Benefits and Costs of Defense in a Neotropical Shrub

Abstract

Benefits and costs are central to optimality theories of plant defense. Benefit is the gain in fitness to reducing herbivory and cost is the loss in fitness to committing resources to defense. We evaluate the benefits and costs of defense in a neotropical shrub, Psychotria horizontalis. Plants were either exposed to herbivores or protected within a cage of fine mesh in three gardens planted in large light gaps on Barro Colorado Island, Panama. Two cuttings of each of 35 clones were paired based on initial masses and assigned randomly to a garden and to either the exposed or protected treatment. Annual rates of herbivory were 37% for exposed plants and 17% for protected plants. Among the exposed plants, high tannin concentration and tough leaves reduced herbivory. Twenty months after planting, the protected plants had a 10—fold higher growth rate, and 5.5 times more leaf area and dry mass than the protected plants. Twenty—two percent of the protected plants flowered, whereas no exposed plants flowered. Within the protected treatment, both total tannin concentration and leaf toughness varied among clones, which suggests that these traits are heritable. Total tannin concentration, but not toughness, was negatively correlated with growth for protected plants, which implies a cost to producing tannins. High tannin concentration and tough leaves reduced herbivory, however, these benefits of defense were balanced by the costs such that there were no significant differences in growth among exposed plants. These results are consistent with the hypotheses that there are benefits to being defended, that defense can be costly, and that herbivores act as selective agents in the evolution of plant defense.