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

婆罗洲的热带森林主要由一个家庭的250多个树种，龙脑香科，其中许多具有相似的形式，建筑和生态。由于物种之间的这些相似性，目前还不确定是什么阻止了少数物种统治森林并使其他物种灭绝。一种可能性是，物种之间水果产量的差异被我们还不了解的生物相互作用最小化了。例如，我们已经发现，在同一地点的龙脑香属物种采取不同的开花策略：花小的物种往往会产生许多花，反之亦然。这种“权衡”可以使不同物种产生的后代的平均数量相等，从而防止少数物种统治森林，但这种解释要求我们调用一个微分可能性，即小花朵和大花朵将发育成果实和幼苗。我们将研究花的大小如何补偿花的数量，以平衡不同物种的果实产量。我们将重点关注小花的两个潜在成本：（a）小花授粉的可能性降低，因为产生小花的树木往往会产生更多的小花，授粉者是一种稀缺的资源，（B）小花从遗传相关的近邻获得花粉的机会增加，导致近亲繁殖的可能性增加，从而降低后代的生存能力。我们的指导假设是，开花期间对昆虫传粉者的竞争是开花策略进化的基础，这是可信的，因为龙脑香树在强烈的“一般开花”（GF）事件期间往往以不规则的间隔同步开花，间隔时间为4-7年。在GF事件中，森林中的大多数大树在4-6周的时间内同时开花，在此期间，对传粉者的需求突然增加。龙脑香类的传粉昆虫大小不等，从微小的花蓟马（长度< 1毫米）到非常大的巨型蜜蜂（长度45毫米），传粉昆虫的平均大小随着花朵的大小而增加。这些昆虫传粉者的流动性很重要，因为它们的觅食决定了遗传物质从父树到母树的移动。我们知道蓟马只能飞很短的距离，而巨型蜜蜂可以旅行几十公里寻找食物，我们将发现这些移动性的差异是否会转化为花粉在开花树木之间运输的平均距离的差异。此外，由于龙脑香种子大，分散性差，种群内基因的移动可能与花粉的传播比果实的传播更密切相关。因此，我们预测，由较小的相对不移动的昆虫授粉的物种的相邻个体将比由较大的移动的昆虫授粉的物种的相邻个体彼此更密切相关。我们将检查近亲繁殖衰退的证据，用从离母亲不同距离的父亲收集的花粉手工授粉母树的花，并评估对果实发育和幼苗表现的影响。该项目将确定花的大小和花的数量是否与传粉者的大小和流动性，花粉的传播和森林中遗传相关个体的聚集有关。它提供了一个令人兴奋的新的思考方式如何保持多样性在一个重要的热带森林社区，并具有普遍适用性的任何植物群落的传粉者的竞争可能是开花策略的演变。这项工作也有相关的干扰和支离破碎的热带森林社区的植物传粉者的相互作用被破坏的开花树木之间的平均距离的变化的管理。

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.