Bird-mediated seed dispersal and the decline and regeneration of fleshy-fruited forests: a review

Fruit-eating birds are important seed dispersers of many woody plants (trees, shrubs and vines) worldwide, and thereby control the plants’ movements and regeneration, especially in tropical regions, where 70–90% of rainforest tree species produce fleshy fruits. Among these fleshy-fruited forests, there are notable global similarities in bird–plant interactions, even though different bird families dominate on different continents. This review describes how frugivorous birds interact with plants and disperse their seeds, how these bird–plant relationships have responded to environmental changes, and how they influence forest regeneration. It draws information from global literature on birds and plan ts in fleshy-fruited forests, including Asia-Pacific examples. The most effective avian seed dispersers are generally those which: do not crush or digest seeds; consume fruits often or most of the time; and have wider gapes. Some 20–30% of trees in fleshy-fruited forests produce large seeds (>10 mm wide). The most effective avian dispersers of larger seeds are larger-bodied, larger-gaped species that often eat fruit, and many of these are sensitive to human impacts. Consequently, in landscapes where forests are fragmented and disturbed, larger-seeded trees may regenerate poorly even when fruiting adults are present, potentially leading to their long-term decline. Disturbed areas, regenerating fleshy-fruited forests and areas of active forest restoration support mainly smaller-gaped dietary generalist birds that disperse small seeds. The effects of feedback cycles and climate change are likely to amplify these trends over time. Understanding how to manage bird communities in a manner that can promote forest regeneration is a promising but underdeveloped field.