Data from: Aggregation of infective stages of parasites as an adaptation and its implications for the study of parasite-host interactions

The causes and consequences of aggregation among conspecifics have received much attention. For infecting macroparasites, causes include variation among hosts in susceptibility, and/or whether infective stages are aggregated in the environment. Here, we link these two phenomena and explore whether aggregation of infective stages in the environment is adaptive to parasites encountering host condition-linked defenses, and what effect such aggregations have for parasite-host interactions. Using simulation models, we show that parasite fitness is increased by aggregates attacking a host, particularly when investment into defenses is high. The fitness benefit of aggregation remains despite inclusion of factors that should curb the benefits of aggregation: namely, mortality of low condition hosts (those hosts expected to be most susceptible to parasitism) and costs of high coinfection. Using sample sizes common in studies, aggregation of infective stages reduces the likelihood of detecting host condition-parasitism relations, even when host condition is the only other factor in models affecting parasitism. Thus, it is not surprising that the expected inverse relations between host condition and parasitism, commonly a premise in studies of parasite-host interactions, are inconsistently found. An understanding of how parasites encounter hosts is thus needed for developing theory for parasite-host ecological and evolutionary interactions.

同种个体间聚集行为的成因与后果，长期以来广受关注。对于寄生性大型寄生虫（macroparasites）而言，其聚集行为的成因包括宿主间易感性的差异，以及（或）感染期虫体（infective stages）是否在环境中发生聚集。本研究将这两种现象相结合，探讨环境中感染期虫体的聚集是否能使寄生虫更好地应对宿主状态关联的防御机制，同时探究此类聚集对寄生虫-宿主互作产生的影响。本研究通过模拟模型（simulation models）发现，当多虫体聚集攻击宿主时，寄生虫适合度（parasite fitness）会得到提升，尤其当宿主防御投入水平较高时，该效应更为显著。即便纳入本应抑制聚集收益的因素——即低状态宿主（这类宿主通常被认为最易感染寄生虫）的死亡率与高共感染（coinfection）带来的代价——聚集仍能为寄生虫带来适合度收益。采用研究中常用的样本量进行分析时，即便宿主状态是模型中唯一影响寄生情况的其他因素，感染期虫体的聚集仍会降低检测到宿主状态与寄生程度之间关联的概率。由此可见，作为寄生虫-宿主互作研究中常见的预设前提，宿主状态与寄生程度间预期存在的负相关关系，其检测结果往往并不一致，这一现象也就不足为奇了。因此，若要构建寄生虫-宿主生态与进化互作的相关理论，就需要先厘清寄生虫与宿主的接触方式。