Data from: Interspecific competition counteracts negative effects of dispersal on adaptation of an arthropod herbivore to a new host

Dispersal and competition have both been suggested to drive variation in adaptability to a new environment, either positively or negatively. A simultaneous experimental test of both mechanisms is however lacking. Here, we experimentally investigate how population dynamics and local adaptation to a new host plant in a model species, the two-spotted spider mite (Tetranychus urticae), are affected by dispersal from a stock population (no-adapted) and competition with an already adapted spider mite species (Tetranychus evansi). For the population dynamics, we find that competition generally reduces population size and increases the risk of population extinction. However, these negative effects are counteracted by dispersal. For local adaptation, the roles of competition and dispersal are reversed. Without competition, dispersal exerts a negative effect on adaptation (measured as fecundity) to a novel host and females receiving the highest number of immigrants performed similarly to the stock population females. By contrast, with competition, adding more immigrants did not result in a lower fecundity. Females from populations with competition receiving the highest number of immigrants had a significantly higher fecundity than females from populations without competition (same dispersal treatment) and than the stock population females. We suggest that by exerting a stronger selection on the adapting populations, competition can counteract the migration load effect of dispersal. Interestingly, adaptation to the new host does not significantly reduce performance on the ancestral host, regardless of dispersal rate or competition. Our results highlight that assessments of how species can adapt to changing conditions need to jointly consider connectivity and the community context.

扩散与竞争均被提出可正向或负向驱动物种对新环境的适应性差异，然而目前尚缺乏对这两种机制的同步实验验证。本研究以模式物种二斑叶螨（Tetranychus urticae）为研究对象，通过实验探究未适应宿主的原种群扩散，以及与已适应宿主的叶螨（Tetranychus evansi）的种间竞争，如何影响其种群动态以及对新宿主植物的局部适应性。针对种群动态的分析结果显示，竞争通常会降低种群规模并提升种群灭绝风险，但此类负面效应可通过扩散得到缓解。而在局部适应性层面，竞争与扩散的作用则发生逆转：在无竞争条件下，扩散对物种在新宿主上的适应性（以繁殖力为衡量指标）产生负向影响，接收移民个体数量最多的雌性个体，其繁殖力与原种群雌性个体无显著差异；与之相反，在存在竞争的条件下，增加移民个体数量并不会导致繁殖力下降，且在存在竞争且接收移民个体数量最多的种群中，雌性个体的繁殖力显著高于同扩散处理下无竞争种群的雌性个体，同时也显著高于原种群雌性个体。本研究认为，通过对适应性进化中的种群施加更强的选择压力，竞争可抵消扩散带来的迁移负荷效应。有趣的是，无论扩散速率与竞争条件如何，对新宿主的适应性均未显著降低物种在原始宿主上的适合度表现。本研究结果表明，在评估物种如何适应环境变化时，需同时考虑种群连通性与群落背景因素。