In neotropical forests, 50–90% of the canopy trees bear fruits adapted for animal dispersal whereas close to 100% of the shrubs and sub-canopy trees produce fleshy fruits (Howe & Smallwood 1982). It has been suggested that during periods of fruit scarcity, some plant species perform a critical role in the forest ecosystem by sustaining frugivorous animals that are important dispersal agents for seeds of many other trees during other seasons of the year (Howe 1984, Terborgh 1986a). Loss or absence of those seed dispersers would have strong negative consequences for tree species, thereby affecting the health of the entire ecosystem, over time. For that reason, plant species that sustain frugivores (= seed dispersers) during periods of resource scarcity have been called ‘keystone species’ (Gilbert 1980, Howe 1984, Terborgh 1983, 1986a, b; van Schaik et al. 1993). For example, in the tropical rain forest of Manu National Park, Peru, Terborgh (1983) found that figs, palm nuts and nectar are resources of great importance for the primate community during periods of fruit scarcity. During that season, three species of fig were identified as keystone resources maintaining nearly 40% of the animal biomass in the ecosystem (Terborgh 1983).
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