1. Parasites are known to respond strongly to changes in host diversity and may ultimately reflect changes in ecosystems. However, it remains unclear whether strong relationships between host and parasite richness also reflect those at the level of community composition. A complementary approach is to test for concordance between host and parasite composition to evaluate whether shifts in parasite community structure mirror patterns of their hosts and potentially the environment.
2. I tested for concordance between patterns of community similarity in parasites, their fish hosts, and environmental factors sampled across three sites and four seasons within each of two river ecosystems. I constructed ordinations of parasites, fish, and environmental variables to establish their patterns of similarity in multivariate space and used Procrustes analysis to evaluate whether patterns in community structure were concordant.
3. Spatial and seasonal patterns in fish and parasite community structure were concordant when analysing patterns in community composition (i.e. presence–absence), but not community abundance (i.e. numerical density). Patterns in fish communities were concordant with the physical river environment. However, despite finding concordance between fish and parasite communities and fish and their environment, parasites were concordant with the river environment in only one ecosystem.
4. Re‐evaluating the relationship between host and parasite diversity under a community concordance framework showed that patterns among host and parasite community composition exist, but this relationship is weaker than expected. The looser connection between patterns in host and parasite community composition can be mediated by parasite life history and parasites responding both indirectly and directly to changes in host communities and the local environment.

中文翻译：

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寄主群落结构的空间和季节变化与寄生虫组成的模式弱相关

1. 已知寄生虫对寄主多样性的变化有强烈的反应，并可能最终反映出生态系统的变化。然而，尚不清楚寄主和寄生虫丰富度之间的牢固关系是否也反映了社区组成水平上的关系。一种补充方法是测试宿主和寄生虫组成之间的一致性，以评估寄生虫群落结构的变化是否反映了宿主的模式以及潜在的环境。
2. 我测试了寄生虫，它们的鱼类寄主和两个河流生态系统中三个地点和四个季节采样的环境因素之间的社区相似性模式之间的一致性。我构建了寄生虫，鱼类和环境变量的准则，以建立它们在多元空间中的相似模式，并使用Procrustes分析来评估社区结构中的模式是否一致。
3. 当分析群落组成的模式（即存在与否）时，鱼类和寄生虫群落结构的空间和季节模式是一致的，但分析群落的丰度（即数值密度）时却是一致的。鱼群落的模式与河流的自然环境一致。然而，尽管在鱼类与寄生虫群落以及鱼类及其环境之间找到了一致，但在仅一个生态系统中，寄生虫与河流环境是一致的。
4. 在社区协调框架下重新评估寄主和寄生虫多样性之间的关系表明，寄主和寄生虫群落组成之间存在模式，但是这种关系比预期的要弱。寄生虫的生活史和寄生虫可以间接和直接响应宿主社区和当地环境的变化，从而介导宿主与寄生虫群落组成之间的联系松散。