Linking life-history trade-offs to population genetic structure in tropical forest trees: implications for maintenance of species richness

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The tropical forests of Borneo are dominated by more than 250 tree species in one family, the Dipterocarpaceae, many of which have a similar form, architecture and ecology. Because of these similarities among species, it is uncertain what prevents a small number of species from dominating the forest and driving other species to extinction. One possibility is that differences in fruit production between species are minimized by biotic interactions that we don't yet understand. For example, we have found that the dipterocarp species at one site adopt different flowering strategies: species with small flowers tend to produce many of them, and vice versa. This 'trade-off' could equalize the average number of offspring produced by different species, and thereby prevent a small number of species from dominating the forest, but this interpretation requires us to invoke a differential likelihood that small and large flowers will develop into fruits and established seedlings. We will investigate how flower size compensates for flower number to equalize fruit production among species. We will focus on two potential costs for small flowers: (a) the reduced likelihood that a small flower will be pollinated, as the trees that produce small flowers tend to produce many more of them and pollinators are a scare resource, and (b) the increased chance that small flowers will receive pollen from a genetically-related near neighbour, leading to an increased potential for inbreeding to reduce the viability of offspring. Our guiding hypothesis is that competition for insect pollinators during flowering underlies the evolution of flowering strategy, which is credible because dipterocarp trees tend to flower synchronously at irregular intervals during intense 'general flowering' (GF) events separated by periods of 4-7 years. During GF events most large trees in the forest flower simultaneously over a 4-6 wk period, and during this time the demand for pollinators suddenly increases. Dipterocarps are pollinated by insects that vary in size from tiny flower thrips (< 1 mm in length) to very large giant honey bees (45 mm in length), and the average size of pollinator increases with flower size. The mobility of these insect pollinators is important, because their foraging determines the movement of genetic material from the father to the mother tree. We know that thrips can only fly very short distances, whereas giant honey bees can travel a few tens of km in search of food, and we will find out whether these differences in mobility translate into differences in the average distance that pollen gets transported between flowering trees. Furthermore, because dipterocarps are large-seeded and poorly dispersed, the movement of genes within a population is likely to be more strongly associated with dispersal of pollen than fruits. Therefore we predict that neighbouring individuals of species pollinated by small relatively immobile insects will be more closely related to each other than neighbouring individuals of species pollinated by large, mobile insects. We will check for evidence of inbreeding depression by hand pollinating the flowers of mother trees with pollen collected from fathers at different distances away from the mother and assessing effects on fruit development and seedling performance. This project will establish whether flower size and flower number are linked to pollinator size and mobility, the dispersal of pollen and the clustering of genetically related individuals in the forest. It offers an exciting new way of thinking about how diversity is maintained in an important tropical forest community and has general applicability to any plant community where competition for pollinators might underlie the evolution of flowering strategies. The work also has relevance to the management of disturbed and fragmented tropical forest communities where plant-pollinator interactions are disrupted by changes in mean distance between flowering trees.