Generalization versus Specialization in Pollination Systems: Visitors, Thieves, and Pollinators of <i>Hypoestes aristata</i> (Acanthaceae)

Many recent studies have suggested that the majority of animal-pollinated plants have a higher diversity of pollinators than that expected according to their pollination syndrome. This broad generalization, often based on pollination web data, has been challenged by the fact that some floral visitors recorded in pollination webs are ineffective pollinators. To contribute to this debate, and to obtain a contrast between visitors and pollinators, we studied insect and bird visitors to virgin flowers of Hypoestes aristata in the Bamenda Highlands, Cameroon. We observed the flowers and their visitors for 2-h periods and measured the seed production as a metric of reproductive success. We determined the effects of individual visitors using 2 statistical models, single-visit data that were gathered for more frequent visitor species, and frequency data. This approach enabled us to determine the positive as well as neutral or negative impact of visitors on H. aristata’s reproductive success. We found that (i) this plant is not generalized but rather specialized; although we recorded 15 morphotaxa of visitors, only 3 large bee species seemed to be important pollinators; (ii) the carpenter bee Xylocopa cf. inconstans was both the most frequent and the most effective pollinator; (iii) the honey bee Apis mellifera acted as a nectar thief with apparent negative effects on the plant reproduction; and (iv) the close relationship between H. aristata and carpenter bees was in agreement with the large-bee pollination syndrome of this plant. Our results highlight the need for studies detecting the roles of individual visitors. We showed that such an approach is necessary to evaluate the pollination syndrome hypothesis and create relevant evolutionary and ecological hypotheses.近年来诸多研究表明，绝大多数动物传粉植物的传粉者多样性，要高于根据其传粉综合征（pollination syndrome）所预期的水平。这一基于传粉网络（pollination web）数据得出的广泛结论，却受到了一项事实的挑战：传粉网络中记录的部分花访客并非有效的传粉者。为参与这一学术争论，并对比花访客与有效传粉者的差异，我们于喀麦隆巴门达高地开展研究，针对假杜鹃（Hypoestes aristata）的未遭访问的原生花朵，调查了其昆虫与鸟类访客。我们以2小时为周期观察花朵及其访客，并以结籽量作为繁殖成功率的衡量指标。我们采用两种统计模型，结合针对高频访客类群收集的单次访问数据与访客频率数据，明确了单个花访客的作用效应。该方法使得我们能够量化花访客对假杜鹃繁殖成功率的正向、中性乃至负向影响。我们的研究结果显示：其一，该植物并非泛化传粉系统，而是特化传粉系统；尽管我们记录到15个花访客形态类群，但仅有3种大型蜂类可作为重要的有效传粉者；其二，木蜂（Xylocopa cf. inconstans）既是出现频率最高的访客，也是传粉效率最优的传粉者；其三，西方蜜蜂（Apis mellifera）扮演了盗蜜者的角色，对该植物的繁殖产生了显著的负向影响；其四，假杜鹃与木蜂之间的紧密共生关系，与该植物的大型蜂传粉综合征特征相符。我们的研究结果凸显了开展针对单个花访客角色的研究的必要性。本研究证实，此类研究方法对于评估传粉综合征假说、构建合理的进化与生态学假说均不可或缺。