The contrasting and idiosyncratic changes in biodiversity that have been documented across urbanization gradients call for a more mechanistic understanding of urban community assembly. The reproductive success of organisms in cities should underpin their population persistence and the maintenance of biodiversity in urban landscapes. We propose that exploring individual-level reproductive traits and environmental drivers of reproductive success could provide the necessary links between environmental conditions, offspring production, and biodiversity in urban areas. For 3 years, we studied cavity-nesting solitary bees and wasps in four urban green space types across Toronto, Canada. We measured three reproductive traits of each nest: the total number of brood cells, the proportion of parasite-free cells, and the proportion of non-emerged brood cells that were parasite-free. We determined (a) how reproductive traits, trait diversity and offspring production respond to multiple environmental variables and (b) how well reproductive trait variation explains the offspring production of single nests, by reflecting the different ways organisms navigate trade-offs between gathering of resources and exposure to parasites. Our results showed that environmental variables were poor predictors of mean reproductive trait values, trait diversity, and offspring production. However, offspring production was highly positively correlated with reproductive trait evenness and negatively correlated with trait richness and divergence. This suggests that a narrow range of reproductive traits are optimal for reproduction, and the even distribution of individual reproductive traits across those optimal phenotypes is consistent with the idea that selection could favor diverse reproductive strategies to reduce competition. This study is novel in its exploration of individual-level reproductive traits and its consideration of multiple axes of urbanization. Reproductive trait variation did not follow previously reported biodiversity-urbanization patterns; the insensitivity to urbanization gradients raise questions about the role of the spatial mosaic of habitats in cities and the disconnections between different metrics of biodiversity.

中文翻译：

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生殖性状差异驱动城市空巢蜜蜂和黄蜂的后代生产

在城市化梯度中记录的生物多样性的对比和特殊变化要求对城市社区集会有更机械的理解。城市中生物的繁殖成功应支持它们的种群持久性和城市景观中生物多样性的维护。我们建议探索个体水平的生殖特征和生殖成功的环境驱动因素可以提供环境条件、后代生产和城市地区生物多样性之间的必要联系。3 年来，我们研究了加拿大多伦多四种城市绿地类型中的空巢独居蜜蜂和黄蜂。我们测量了每个巢穴的三个繁殖特征：育雏细胞总数、无寄生虫细胞的比例、以及没有寄生虫的未出现的育雏细胞的比例。我们确定了 (a) 生殖性状、性状多样性和后代生产如何对多种环境变量做出反应，以及 (b) 生殖性状变异如何解释单个巢穴的后代生产，通过反映生物体在资源收集之间进行权衡的不同方式和接触寄生虫。我们的结果表明，环境变量对平均生殖性状值、性状多样性和后代产量的预测效果不佳。然而，后代产量与生殖性状均匀度呈高度正相关，与性状丰富度和分化度呈负相关。这表明繁殖性状的范围很窄，是繁殖的最佳选择，并且个体生殖性状在这些最佳表型中的均匀分布与选择可能有利于不同生殖策略以减少竞争的想法是一致的。这项研究在探索个体层面的生殖特征和考虑城市化的多轴方面是新颖的。生殖性状变异不遵循先前报道的生物多样性-城市化模式；对城市化梯度的不敏感性提出了关于城市栖息地空间镶嵌的作用以及不同生物多样性指标之间的脱节的问题。这项研究在探索个体层面的生殖特征和考虑城市化的多轴方面是新颖的。生殖性状变异不遵循先前报道的生物多样性-城市化模式；对城市化梯度的不敏感性提出了关于城市栖息地空间镶嵌的作用以及不同生物多样性指标之间的脱节的问题。这项研究在探索个体层面的生殖特征和考虑城市化的多轴方面是新颖的。生殖性状变异不遵循先前报道的生物多样性-城市化模式；对城市化梯度的不敏感性提出了关于城市栖息地空间镶嵌的作用以及不同生物多样性指标之间的脱节的问题。