Determinants of Plant-Animal Coevolution: The Case of Mutualistic Dispersal of Seeds by Vertebrates

Abstract

Patterns contradicting expectations from earlier coevolutionary models, and the observation that optimality and fine scale adjustments are conspicuously absent in many instances, suggest that coevolved, interdependent plant-vertebrate seed dispersal systems are, at best, very rare in nature. Weak selective pressures of dispersers on plants, temporal and spatial unpredictability of favourable germination sites and disperser behaviour, extensive plant gene flow derived from distant seed dispersal and frequent dioecy, and slow evolutionary rates relative to recurrence of environmental perturbations (animal species' extinctions), all combine to render close coevolution between particular plant and disperser species unlikely. Slower species turnover of woody plants over geological time relative to vertebrate disperses may have favoured a sort of very coarse diffuse coevolution, through long-lived plant species having acted as carriers of information through evolutionary time. This may suffice to explain patterns of successful plant-disperser interaction commonly observed not only in natural habitats, but also in artificial systems recently assembled by man and lacking a common historical background to participant organisms. This background is indispensable for non-diffuse, species-to-species coevolution to have actually taken place.