Defense of Ripe Fruit from Pests: Its Significance in Relation to Plant-Disperser Interactions

Ripe fruits of vertebrate-dispersed plants are susceptible to irreversible damage by pathogens and invertebrates while they are attached to the parent plant "waiting" for the visit of a dispersal agent. Adaptations of plants to decreasing fruit damage include temporal escape from pests, reducing exposure time, and decreasing fruit profitability to pests by means of secondary compounds and reduced and/or unbalanced nutrient content. A variety of secondary compounds has been reported from the ripe fruits of many vertebrate-dispersed plants, including alkaloids, saponins, volatile oils, resins and phenolics. Toxic fruits containing poisonous substances that potentially are defensive agents against pests may be very frequent, as revealed by an analysis of their occurrence among European plants. As judged from the literature, fruits containing secondary substances are at a competitive disadvantage relative to nontoxic ones. It is suggested that a hierarchy of fruit defense methods should exist, with the least desirable ones selected for only when better mechanisms are precluded or rendered ineffective. A simplified model is proposed to explain the occurrence of toxic mature fruits; the roles of pest pressure and plant competition for disperser services are considered, and some testable predictions are suggested. It is finally argued that nutritionally unbalanced fruits or those with secondary compounds probably serve indirectly as a means of increasing the potential for plant coexistence through their diversifying effects on diet selection by individual frugivores.