Identifying keystone plant resources in tropical forests: the case of gumsfrom Parkia pods

Keystone plants that produce seasonally critical trophic resources comprise one of the main classes of keystone species, yet no studies have attempted to examine the ecological attributes that might help us recognize them and evaluate their importance in species-rich plant assemblages. In this paper the concept of keystone plant resources is reviewed using potential candid ates proposed in the literature for neotropical forest sites. A poorly know n example of a potential keystone resource-the gums produced by mature pods of two emergent tree species (Parkia nitida and P. pendula, Leguminosae: M imosoideae)-is described for primates and other arboreal vertebrates in Ama zonian forests. In particular, the fruiting phenology, tree density, patter ns of vertebrate consumption, and nutritional quality of Parkia gums in Ama zonian terra firme forests are considered. Putative neotropical keystone re sources are then divided into four intersecting ecological attributes defin ing their community-wide importance to vertebrate frugivores: (1) temporal redundancy, (2) degree of consumer specificity, (3) reliability, and (4) ab undance. From a vertebrate perspective, keystone plants are here defined as those producing reliable, low-redundancy resources that are consumed by a large proportion of the bird and mammal assemblage with which they coexist. Plant populations proposed to date as keystone species range widely across two of these four variables, which may disqualify most putative taxa (incl uding Parkia spp.) from a more formal definition of keystone resources. Oth er importance attributes, the context-dependent role, the taxonomic refinem ent, and removal effects of the keystone plant resource concept as applied to tropical forests are also discussed.