Michael J. Shanahan
Centre for Biodiversity and Conservation, School of Biology, University of Leeds, Leeds LS2 9JT, UNITED KINGDOM.
Ficus (Moraceae) is a large plant genus of considerable ecological, evolutionary and conservation interest. This study focused on the interaction between Ficus species and the vertebrates that eat their fruit (figs), thereby acting as potential seed dispersers. Specifically, the study considered this interaction with regard to the mechanisms that allow different guilds of sympatric Ficus species to attract differentially subsets of frugivore communities.
Fig packaging and presentation was studied in Borneo and on an island off New Guinea, the former study, of 43 Ficus species represents the largest ever characterisation of the fruiting ecology of sympatric congeners. The Ficus species studied exhibited considerable diversity in the size, colour, texture, height, water content, seed burden and relative yield of their figs. In both sites, figs were eaten by a large proportion of the vertebrate fauna present. However, that frugivores were not equally attracted to each Ficus species provided evidence of Ficus dispersal guilds. Guild membership was determined by differences in fig packaging and presentation, with fig size, colour and height of presentation being the strongest predictors of frugivore attraction. These traits, and therefore guild membership, showed strong phylogenetic associations. Nonetheless, apparent adaptations to seed dispersers were also documented.
The differential attraction of frugivores means not only that competition for dispersal agents is reduced between guilds but also that guilds of Ficus species experience markedly different seed dispersal services from the frugivores they attract. These differences occur in terms of the number and diversity of frugivores attracted, the proportion of figs that are eaten by seedpredatory frugivores and the distance, density and heterogeneity of seed rain. Ficus species that attract relatively small subsets of frugivore communities (such as fruit bats, or large arboreal mammals) which are prone to anthropogenic threats face decreased seed dispersal associated with the continuing decline of these frugivores. Dependence for dispersal on subsets of frugivore communities also means that Ficus species differ in their abilities to colonise degraded habitats. A 12 month study of the phenology of fig production in the Bornean field site demonstrated that fig crops are initiated year-round and are thus a valuable resource for fruit-eating animals. However, patterns of fig production were not equal between Ficus species (because of a lack of pollination for some) and, so, the availability of the fig resource varied for the different animal groups attracted to each of the Ficus guilds.
Globally, figs are eaten by at least ten percent of all bird species and six percent of all mammal species, many of which are capable of dispersing Ficus seeds. That these frugivores also disperse the seed of many other plant species appears to support the suggestion that Ficus species are of great conservation status. However, the 'keystone resource' epithet applied to figs as a whole need to be re-assessed as I show that, because of fig-frugivore partitioning, Ficus species are not equal resources for vertebrate frugivores.