Pollinator-mediated facilitation alleviates pollen limitation through effects on the quality of plant reproduction

Facilitation and competition among plants sharing pollinators have contrasting consequences for each species' fitness. However, it is unclear whether pollinator-mediated indirect interactions between plants affect plant fitness in a multispecies context. Here, we investigated how pollinator sharing affects pollen limitation (contribution of pollination to fitness) in a tropical hummingbird-pollinated community marked by facilitation. We employed indices describing how much a plant species is important to shared pollinators (acting degree) and have its shared pollinators influenced by other plants (target degree) within the plant-hummingbird network. Since facilitation often increases pollination quantity but not necessarily quality, we expected both indices to be associated with reductions in pollen limitation estimates that depend on pollination quantity (fruit set and seed number) rather than estimates more strictly related to quality (seed weight and germination). Instead, we found that both indices were associated with reductions in pollen limitation only for seed weight and germination. Thus, facilitation acted on qualitative estimates of pollen limitation. Our results suggest that facilitation may enhance plant fitness even if quantitative components of plant fecundity are already saturated. Overall, we showed that pollinator-mediated indirect effects in a multispecies context are important drivers of plant fitness with consequences for coexistence in diverse communities.

共享传粉者（pollinator）的植物之间的传粉易化（facilitation）与竞争作用，对各物种的适合度会产生截然不同的影响。然而，在多物种群落背景下，传粉者介导的植物间间接相互作用是否会影响植物适合度，目前尚不清楚。本研究针对以传粉易化为特征的热带蜂鸟（hummingbird）传粉群落，探究了共享传粉者如何影响花粉限制（pollen limitation，即传粉对适合度的贡献）。我们在植物-蜂鸟网络中采用了两类指标，分别描述某一植物物种对共享传粉者的重要程度（作用度，acting degree），以及该物种的共享传粉者受其他植物影响的程度（靶标度，target degree）。由于传粉易化通常会提升传粉数量，但未必能改善传粉质量，因此我们曾推测，两类指标均会与依赖传粉数量的花粉限制评估指标（坐果率与种子数）的降低相关，而非与更严格关联传粉质量的评估指标（种子重量与发芽率）相关。但实际研究结果显示，两类指标仅与种子重量与发芽率这两类传粉质量相关指标的花粉限制降低存在关联。由此可见，传粉易化作用于花粉限制的质量评估维度。本研究结果表明，即便植物繁殖力的数量组分已达饱和，传粉易化仍可提升植物适合度。总体而言，本研究证实，多物种背景下传粉者介导的间接相互作用，是影响植物适合度的重要驱动因素，其结果会对多样群落的物种共存产生影响。