Data from: Aging alters interspecific competition between two sympatric insect-parasitic nematode species

Interspecific competition can vary depending on the stage, age, or physiological state of the competitors. Competitive ability often increases with age or size; alternatively, senescence can lead to a loss of viability and reduced competitive success. Differences between species in their age-specific competitive abilities can promote coexistence in the face of substantial niche overlap. We examined two sympatric species of nematodes (genus Steinernema) to determine whether their competitive relationship changes as a function of age. These obligately killing insect parasites are known for their broad host ranges and are transmitted from insect to insect via a juvenile stage propagule that is free-living in the soil. Here, we tested whether the two species differed in the effects of age by examining the mortality of insect hosts infected with young or old transmission stage nematodes of each species. We also performed mixed infections, where an equal ratio of both species was simultaneously exposed to a host, to determine the effect of age on competitiveness. One species showed reduced performance with age, as older propagules were slower at inducing host mortality. In contrast, the other species increased in killing speed with age. In competition, insect mortality rate was predictive of competitive outcome, such that if one species induced considerably faster host death in a single-species infection, it was competitively dominant in the coinfection. Accordingly, we found a shift in the competitive relationship between the two species with age. Our work demonstrates that species differences in the effects of aging can lead to dramatic shifts in reproductive success. As these effects are realized solely in a competitive environment, both spatial patchiness and temporal niche partitioning may be important for promoting coexistence.

种间竞争的强度与模式，会随竞争者的发育阶段、年龄或生理状态发生动态变化。竞争能力通常随年龄增长或体型增大而提升；与之相对，衰老过程会导致个体生存力衰退，进而降低其竞争成功率。不同物种间的年龄特异性竞争能力差异，即便在生态位（niche）高度重叠的情境下，也可成为促进物种共存的关键机制。

我们以两种同域分布的斯氏线虫属（Steinernema）线虫为研究模型，旨在探究二者的竞争关系是否随线虫年龄发生改变。这类专性致死性昆虫寄生物以宿主范围广泛为典型特征，其种群传播依赖于在土壤中自由生活的幼虫阶段繁殖体（propagule），通过该繁殖体在不同昆虫宿主间完成传播。

本研究通过分别接种两种线虫的年轻与衰老传播阶段繁殖体，监测昆虫宿主的死亡率，以此检验两个物种的年龄效应差异。此外，我们设置了混合感染（mixed infection）实验组：将两种线虫以均等比例同时暴露于宿主昆虫，以此明确年龄对竞争能力的影响。

实验结果显示，其中一种线虫的繁殖体随年龄增长，诱导宿主死亡的速度显著减慢，表现出随衰老而性能下降的特征；与之形成鲜明对比的是，另一种线虫的致死速度则随年龄提升而加快。在竞争环境中，宿主昆虫的死亡率可有效预测竞争结局：若某一物种在单种感染（single-species infection）实验中能更快诱导宿主死亡，其在混合感染实验中便会占据竞争优势。基于上述结果，我们发现两种线虫之间的竞争关系随线虫年龄发生了显著转变。

本研究证实，不同物种间衰老效应的差异，可导致种群繁殖成功率发生剧烈变化。由于这类效应仅在竞争环境中得以显现，空间斑块性（spatial patchiness）与时间生态位分化（temporal niche partitioning）或许均对促进物种共存具有重要意义。