Forest conversion is one of the greatest global threats to biodiversity, and land-use change and subsequent biodiversity declines sometimes occur over a variety of underlying geologies. However, how forest conversion and underlying geology interact to alter biodiversity is underappreciated, although spatial variability in geology is considered an integral part of sustaining ecosystems. We aimed to examine the effects of forest conversion to farmland, the underlying geology, and their interaction on the stream fishes' diversity, evenness, and abundance in northeastern Japan. We disentangled complex pathways between abiotic and biotic factors with structural equation modeling. Species diversity of stream fishes was indirectly shaped by the interaction of land use and underlying geology. Diversity declined due to nutrient enrichment associated with farmlands, which was mainly the result of changes in evenness rather than by changes in species richness. This impact was strongest in streams with volcanic geology with coarse substrates probably because of the differential responses of abundant stream fishes to nutrient enrichment (i.e., dominance) and the high dependency of these fishes on large streambed materials during their life cycles. Our findings suggest that remediation of deforested or degraded forest landscapes would be more efficient if the interaction between land use and underlying geology was considered. For example, the negative impacts of farmland on evenness were larger in streams with volcanic geology than in other stream types, suggesting that riparian forest restoration along such streams would efficiently provide restoration benefits to stream fishes. Our results also suggest that land clearing around such streams should be avoided to conserve species evenness of stream fishes.

中文翻译：

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森林转化对河流鱼类多样性的地质依赖性影响

森林转化是对生物多样性的最大全球威胁之一，土地利用变化和随后的生物多样性下降有时会发生在各种潜在地质上。然而，尽管地质的空间变异性被认为是维持生态系统的一个组成部分，但人们对森林转化和基础地质如何相互作用以改变生物多样性的认识不足。我们旨在研究森林转化为农田的影响、潜在地质以及它们对日本东北部河流鱼类多样性、均匀度和丰度的相互作用。我们用结构方程模型解开了非生物和生物因素之间的复杂途径。河流鱼类的物种多样性是由土地利用和底层地质的相互作用间接塑造的。由于与农田相关的养分丰富，多样性下降，这主要是均匀度变化的结果，而不是物种丰富度的变化。这种影响在具有粗糙基质的火山地质河流中最强，可能是因为丰富的河流鱼类对营养丰富（即优势）的不同反应以及这些鱼类在其生命周期中对大型河床物质的高度依赖。我们的研究结果表明，如果考虑土地利用与潜在地质之间的相互作用，对森林砍伐或退化的森林景观的修复会更有效。例如，农田对均匀度的负面影响在具有火山地质的溪流中比在其他溪流类型中更大，建议沿这些溪流恢复河岸森林将有效地为溪流鱼类提供恢复效益。我们的结果还表明，应避免在这些溪流周围进行土地清理，以保护溪流鱼类的物种均匀度。