Comparison of macroinvertebrate assemblages in a stream before and after fine sedimentation by deer‐induced forest floor degradation

Japan's Sika deer (Cervus nippon) population has steadily increased since the 1990s. Consequent overgrazing of forest floor vegetation has created denuded slopes and increased the fine‐sediment input to nearby headstreams. Given the continuous structure of rivers, it is likely that upstream forest and headstream ecosystems ultimately impact the ecology of downstream ecosystems. To test this prediction, I investigated the taxonomic composition and habitat characteristics of stream macroinvertebrates before and after fine sedimentation infiltration. Fine sedimentation resulted in coverage of highly productive coarse substrates with fine sand and reduced the macroinvertebrate abundance and taxonomic diversity. These changes were influenced by habitat preference; taxa with coarse substrate preference decreased in abundance, while the abundance of taxa that prefer fine substrates increased. However, large armored caddisflies (Goera spp.) dominated after the fine sedimentation, despite their preference for coarse substrates. This rise likely results from the concurrent reduction in the number of the predatory cyprinid fish species, Tribolodon hakonensis, as large armored caddisflies have often been reported to be invulnerable to predation by fishes such as salmonids and sculpins. However, large cyprinids, which are abundant in Japanese rivers, can effectively forage “hard‐armored” organisms using their well‐developed pharyngeal teeth. Collectively the results provide insight regarding the catchment‐scale impacts of deer‐induced forest floor degradation and will help to inform conservation and management efforts. Abundance of the top 10 macroinvertebrate taxa with the largest loadings on the nonmetric multidimensional scaling (NMDS) 3 dimension by season and sampling period. Asterisks show significant decrease (*1) or increase (*2) after fine sediment infiltration (p