Avian Interference of Insect Frugivory: An Exploration into the Plant-Bird-Fruit Pest Evolutionary Triad

It is hypothesized that insect frugivores experience interference from avian seed dispersers while they are on fleshy, bird-dispersed ripe fruits, as a result of exposure to potentially insectivorous birds attracted to fruits. Several predictions emanating from this hypothesis are tested, and verified, for insect frugivores recorded on ripe and unripe fruits in three contrasting Spanish habitats. Frugivorous insects were rare at all sites on ripe fruits and their abundance was inversely related to the abundance of avian dispersers. This pattern was reversed for insects on unripe fruits. Insects tend to occur mostly on unripe fruits where dispersers are abundant, and predominantly on ripe ones where avian frugivores are most scarce. Insect frugivores lacking especial adaptations against bird predation (aphids) were confined to unripe fruits, while those having some discouraging mechanisms (hemipteran bugs) largely predominated on ripe fruits. The finding that avian seed dispersers interfere with insect frugivory supports the notion that bird-dispersed plants, avian seed dispersers and frugivorous insects integrate a complex evolutionary triad in which each component simultaneously interacts with the other two. Further studies on the detrimental effects to plants of insect frugivory are needed before the significance to the plant-disperser mutualism of avian interference to insect frugivores can be correctly assessed.