Data from: Species turnover and invasion of dominant freshwater invertebrates alter biodiversity-ecosystem function relationship

Freshwater ecosystems rely on allochthonous resources. Integration of these subsidies depends on diversity of both terrestrial resources and aquatic shredder and decomposer communities, but the diversity effects on leaf litter breakdown and decomposition are less clear in aquatic than terrestrial ecosystems. We need a better understanding of this relationship because aquatic communities are rapidly changing with species invasions and anthropogenic impacts. Here, we experimentally disentangled the effects of leaf and shredder richness on leaf litter breakdown by macroinvertebrates in mesocosm experiments using three species of amphipods, a dominant guild of crustaceans in European freshwater ecosystems. Increased leaf richness led to lower-than-predicted leaf consumption by native shredders, with mixed evidence of resource-switching or prioritization of preferred food items within a leaf mix. Higher shredder species richness never promoted leaf consumption rates compared to predictions from relevant single-species experiments, and instead sometimes substantially decreased leaf consumption. We then conducted a meta-analysis of leaf litter consumption rates by seven widely distributed amphipod species (the three used in the experiments and four additional species). As expected based on our own experiments, non-native amphipod species generally had lower biomass-adjusted leaf litter consumption rates, although their larger body size led to higher per-individual leaf consumption rates. Contamination of the water by metals, pesticides and other chemicals additionally significantly decreased leaf litter consumption by multiple native and non-native species compared to unpolluted systems. While the meta-analysis suggested that litter consumption, and thus breakdown, would decline if native shredders are replaced by non-native heterospecifics, complete species replacement is not the only outcome following immigration in a meta-community context. Our experiments suggest that breakdown rates could remain reasonably high where native species coexist with non-native arrivals. Experiments which neglect the ecological realism of species coexistence will necessarily mischaracterize effects on ecosystem functioning.

淡水生态系统依赖外源资源（allochthonous resources）。这类外源营养补贴的整合过程，同时取决于陆地资源的多样性与水生碎屑食性动物（shredder）、分解者群落的多样性；但相较于陆地生态系统，水生生态系统中生物多样性对叶片凋落物破碎与分解过程的影响仍不甚明晰。鉴于水生群落正随物种入侵与人为干扰发生快速变化，我们亟需深化对这一关联的认知。
本研究依托欧洲淡水生态系统中占优势的端足类（amphipods）甲壳动物群落的3个物种种群，通过中型实验生态系统（mesocosm）实验，拆解了叶片资源丰富度与碎食者丰富度对大型无脊椎动物（macroinvertebrates）介导的叶片凋落物分解过程的影响。结果显示，叶片资源丰富度提升后，本地碎食者的叶片取食率低于预期，同时存在混合证据表明其在混合叶片资源中会进行资源转换或优先取食偏好食物。与相关单物种实验的预测结果相比，更高的碎食者物种丰富度始终未提升叶片取食率，反而有时会显著降低叶片取食量。
随后，本研究对7种广泛分布的端足类物种（包含本实验所用的3种与另外4种）的叶片凋落物取食率开展了荟萃分析（meta-analysis）。正如本研究实验的预期结果，非本地端足类物种的生物量校正后叶片凋落物取食率普遍更低，但其体型更大，导致个体水平的叶片取食率更高。与未受污染的水体系统相比，金属、农药及其他化学物质造成的水体污染还会显著降低多种本地与非本地端足类物种的叶片凋落物取食率。
尽管荟萃分析显示，若用非本地异种替换本地碎食者，凋落物取食率（进而凋落物分解速率）会下降，但在元群落（meta-community）的背景下，物种完全替换并非物种入侵后的唯一结果。本研究实验结果表明，当本地物种与入侵的非本地物种共存时，凋落物分解速率仍可维持在较高水平。忽视物种共存的生态真实性的实验，必然会错误表征其对生态系统功能的影响。