Sexual system, abundance, range size and survivorship bias in the Amazonian tree flora

Dioecy (separate sexes) creates difficulties for pollination, seed production and dispersal in plants. Cosexual species, those with bisexual flowers (hermaphroditism) or unisexual flowers on the same individual (monoecy), should therefore have a competitive ecological advantage. However, we show, using a dataset of 5000+ Amazonian tree species, that for animal-dispersed trees, dioecious species tend to be more abundant with larger ranges than hermaphrodites. Moreover, with increasing abundance and range size, the probability of being dioecious increases. We suggest that survivorship bias explains these anomalous outcomes. As the reproductive handicap of dioecy cannot be fully compensated, extinction probability increases for restricted dioecious trees, leading to the over-representation of dioecy among abundant and wide-ranging species but under-representation among uncommon and rare species. Abiotically dispersed trees in Amazonian forests may also face increased extinction probability through dispersal difficulties in dense evergreen settings, and survivorship bias may likewise have increased their average abundance and dominance

雌雄异株（dioecy，即植株个体仅具有单一性别）会对植物的传粉、种子生产及种子扩散造成显著阻碍。因此，兼具雌雄生殖功能的物种——即拥有两性花的类群（雌雄同体，hermaphroditism），或在同一植株上着生单性花的类群（雌雄同株，monoecy）——理应具备更突出的生态学竞争优势。然而，基于涵盖5000余种亚马逊树木物种的数据集，我们的研究表明：对于依靠动物传播种子的树木而言，雌雄异株物种的种群丰度往往更高、分布范围也更广，远超雌雄同体物种。此外，类群的种群丰度越高、分布范围越广，其为雌雄异株的概率也随之升高。我们认为，存活偏倚（survivorship bias）可以解释这些看似反常的研究结果。由于雌雄异株的生殖缺陷无法得到完全弥补，分布受限的雌雄异株树木的灭绝风险会随之升高，这就导致在种群丰度高、分布范围广的类群中，雌雄异株物种的占比偏高；而在种群稀少、分布狭窄的类群中，雌雄异株物种的占比则偏低。亚马逊森林中依靠非生物媒介传播种子的树木，同样会因在茂密常绿生境中扩散受阻而面临更高的灭绝风险，同理，存活偏倚也可能提升了这类树木的平均种群丰度与优势度。