Traits, Trade-offs and Tropical Forest Diversity

Speaker: David Burslem (University of Aberdeen)

Abstract

According to niche theory, resource partitioning by competing plant species contributes to the maintenance of species richness in plant communities. Hyper-diverse tropical lowland rain forest tree communities present a challenge to this theory because ecologists have difficulty defining sufficient distinct niches to accommodate the large number of competing species. Trade-offs among multiple resource axes may help to resolve this problem, especially when biotic interactions are considered. In this seminar I discuss mechanisms of coexistence among competing species of Dipterocarpaceae, which is the dominant family of trees in Southeast Asian rain forests.  Among dipterocarps, per capita seed production appears to be equalised across species despite orders of magnitude variation in flower production among species. This equalization of seed production can be interpreted as contributing to an equalization of fitness among species that, all else being equal, would aid the coexistence of multiple competing species.  Our data suggest that this equalization of per capita seed production among species is determined by interactions of flower size, pollinator body size, pollination success and pollen dispersal distance. Maximum flower production was two orders of magnitude greater in small-flowered than large-flowered species of Dipterocarpaceae, but small-flowered species also had smaller-sized pollinators, lower average pollen dispersal distances and lower mean pollination success than large-flowered species. Paternity analysis revealed that mating between related individuals was more frequent in a smaller-flowered species. We conclude that a trade-off embedded in the relationship between flower size and pollination success contributes to equivalence of fruit production among species. Our findings also demonstrate the potential for differential vulnerability of species to the deleterious ecological and genetic consequences of forest fragmentation. They have clear implications for forest restoration, as seed collection from small-flowered species may be especially vulnerable to cryptic genetic erosion.  Our interpretation highlights the importance of maintaining pollinator communities and gene flow in order to sustain the species richness of tropical tree communities.