Data from: Plant sex alters Allee effects in aggregating plant parasites

Species interactions are central to our understanding of population dynamics. While density typically strengthens competition, reducing absolute fitness, Allee effects can reverse this pattern, increasing fitness with density. Allee effects emerge in host-parasite systems when higher parasite densities dilute immune responses or increase resource-mobilization. The optimal density of individuals in these systems should be influenced by how host quality alters the rates at which facilitative and competitive effects change across densities. We tested these ideas using sumac (Rhus typhina) and a gall-forming parasite (Melaphis rhois) that attacks sumac leaves. Fitness peaked at intermediate densities , indicating an Allee effect, but the fitness peak depended on host sex. Patterns of abundance mirrored fitness patterns, with galls clustered on leaves and female hosts supporting greater numbers of galls. Within leaves, galls near the stem were more fit, and gall-makers preferentially oviposited near to the stem. The patterns of fitness and abundance are consistent with Allee effects caused by increased resource mobilization at higher gall-maker densities rather than diluted immune responses. Our results suggest that Allee effects in parasites can be described as the summative effects of competitive and facilitative processes and, because both are common, Allee effects are likely common in host-parasite systems.

物种相互作用是理解种群动态的核心基础。尽管种群密度通常会加剧竞争并降低个体绝对适合度，但Allee效应（Allee effect）可逆转这一趋势，使个体适合度随种群密度升高而提升。在宿主-寄生虫系统（host-parasite system）中，当寄生虫密度升高时，会稀释宿主免疫应答或增强资源动员，此时便会出现Allee效应。此类系统中个体的最优密度，应取决于宿主品质如何调控促进效应与竞争效应随密度变化的速率。

本研究以盐肤木（*Rhus typhina*）及其寄生叶片的虫瘿形成寄生虫（gall-forming parasite）Melaphis rhois为实验材料，对上述假说进行了验证。研究结果显示，适合度在中等密度下达到峰值，表明存在Allee效应，但该适合度峰值会因宿主性别不同而产生差异。虫瘿的丰度分布模式与适合度模式高度一致：虫瘿在叶片上呈聚集分布，且雌性宿主所能承载的虫瘿数量更多。在单张叶片内部，靠近茎部的虫瘿适合度更高，造瘿昆虫也优先选择在茎部附近产卵。

本研究观测到的适合度与丰度模式，与造瘿昆虫密度升高引发的资源动员增强所导致的Allee效应相符，而非免疫应答稀释所致。研究结果表明，寄生虫的Allee效应可被表述为竞争过程与促进过程的综合效应；鉴于这两类过程均广泛存在，宿主-寄生虫系统中的Allee效应大概率也普遍存在。