Hosts and their parasites are under reciprocal selection, leading to coevolution. However, parasites depend not only on a host, but also on the host’s environment. In addition, a single host species is rarely infested by a single species of parasite and often supports multiple species (i.e., multi-infestation). Although the arms race between a parasite and its host has been well studied, few data are available on how environmental conditions may influence the process leading to multiple infestations. In this study, we examine whether: (1) environmental factors including altitude, temperature, vegetation cover, human disturbance, and grazing by livestock affect the prevalence of two types of ectoparasites, mites and ticks, on their host (the common lizard, Zootoca vivipara) and (2) competition is evident between mites and ticks. We found the probability of mite infestation increased with altitude and vegetation cover, but decreased with human disturbance and presence of livestock. In contrast, the probability of tick infestation was inversely associated with the same factors. Individuals with low body condition and males had higher mite loads. However, this pattern was not evident for tick loads. The results from a structural equation model revealed that mites and ticks indirectly and negatively affected each other’s infestation probability through an interaction involving the environmental context. We detected a direct negative association between mites and ticks only when considering estimates of parasite load. This suggests that both mites and ticks could attach to the same host, but once they start to accumulate, only one of them takes advantage. The environment of hosts has a strong effect on infestation probabilities and parasite loads of mites and ticks. Autecological differences between mites and ticks, as indicated by their opposing patterns along environmental gradients, may explain the pattern of weak contemporary interspecific competition. Our findings emphasize the importance of including environmental factors and the natural history of each parasite species in studies of host–parasite coevolution.

中文翻译：

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环境变化介导了普通蜥蜴Zootoca vivipara中寄生虫的流行和共存

宿主及其寄生虫处于相互选择之下，导致共同进化。但是，寄生虫不仅取决于宿主，还取决于宿主的环境。另外，单个寄主物种很少受到单个寄生虫的侵害，并且经常支持多种物种（即，多次侵染）。尽管已经对寄生虫与其宿主之间的军备竞赛进行了深入研究，但很少有数据可用于环境条件如何影响导致多种侵染的过程。在这项研究中，我们检查是否：（1）包括海拔，温度，植被覆盖，人为干扰和牲畜放牧在内的环境因素是否会影响其寄主（普通蜥蜴，人兽共患病）这两种类型的体外寄生虫（螨虫和tick虫）的患病率vivipara）和（2）螨虫和壁虱之间的竞争明显。我们发现螨虫侵袭的可能性随着海拔和植被的覆盖而增加，但随着人为干扰和牲畜的存在而降低。相反，tick虫侵扰的可能性与相同因素成反比。身体状况低下的个体和男性的螨虫负荷较高。但是，这种模式对于滴答负荷并不明显。结构方程模型的结果表明，螨虫和s虫通过涉及环境的相互作用而间接地对彼此的侵扰概率产生负面影响。仅在考虑对寄生虫负荷的估计时，我们才发现螨虫和壁虱之间存在直接的负相关性。这表明，螨虫和tick虫都可以附着在同一宿主上，但是一旦它们开始积累，它们中只有一个会利用。寄主的环境对螨虫和tick虫的侵袭概率和寄生虫负荷有很大的影响。螨和tick之间在生态学上的差异，如它们沿环境梯度的相反模式所表明的那样，可以解释当代种间竞争薄弱的模式。我们的发现强调了在寄主－寄生虫协同进化研究中包括环境因素和每种寄生虫物种自然史的重要性。