Leaf litter of alder (Alnus glutinosa) is a key resource to detrital stream food webs. Due to its high quality and palatability, it is readily colonised by microorganisms and consumed by detritivores, contributing significantly to carbon and nutrient cycling and to ecosystem functioning. Given that this species has declined due to the spread of the pathogen Phytophthora alni, we investigated how its loss would alter leaf litter decomposition and associated stream assemblages of aquatic hyphomycetes and invertebrates, in a field experiment conducted in three streams. We compared litter mixtures containing alder plus three other species (Corylus avellana, Quercus robur and Salix atrocinerea; that is, 4-species treatments) with mixtures that excluded alder (3-species treatments) and all the monocultures (1-species treatments). The loss of alder reduced decomposition rates, despite the existence of an overall negative diversity effect after 3 weeks of exposure (that is, monocultures decomposed faster than mixtures) and no diversity effect after 6 weeks. Aquatic hyphomycete and detritivore assemblage structure in the mixture without alder differed from those of the mixture with alder and the monocultures, and the former had lower fungal sporulation rate and taxon richness. Our results suggest that alder loss from the riparian vegetation can significantly slow down the processing of organic matter in streams and produce shifts in stream assemblages, with potential consequences on overall ecosystem functioning. We highlight the importance of assessing the ecological consequences of losing single species, particularly those especially vulnerable to stressors, to complement the multiple studies that have assessed the effects of random species loss.

der木（Alnus glutinosa）的落叶是碎屑流食物网的重要资源。由于其高质量和适口性，它很容易被微生物定植，并被有害生物消耗，从而大大促进了碳和养分循环以及生态系统的功能。鉴于该物种由于病原体疫霉菌的传播而减少，我们在三个流中进行的田间试验中研究了其损失如何改变凋落物分解以及相关水生真菌和无脊椎动物的流组合。我们比较了含有al木和其他三种树种的杂种混合物（榛子（Corylus avellana），栎（Quercus robur）和柳柳）; 也就是4种处理）与排除al木（3种处理）和所有单一栽培（1种处理）的混合物。der木的损失降低了分解速率，尽管在暴露3周后存在总体负面的多样性影响（也就是说，单一培养物的分解速度快于混合物），而在6周后没有多样性的影响。没有al木的混合物中的水生菌丝和有害菌的组装结构与有der木和单培养的混合物中的不同，前者具有较低的真菌孢子形成率和丰富的分类群。我们的结果表明，沿河植被的vegetation木流失会大大减慢河流中有机物的处理，并导致河流集合体发生变化，从而对整个生态系统功能产生潜在影响。