RESEARCH

Stream fish communities in primary river systems of an island represent a suitable model system for studying the dynamics of metacommunity, a “community of communities” connected by dispersal of species. As a case study of metacommunity dynamics in nature, my colleagues and I explore the patterns and processes of metacommunity assembly of stream fish species in Sado Island, Japan using environmental DNA (eDNA) metabarcoding. The main focus of our study is on: (1) whether the insular stream fish metacommunity is spatially heterogeneous and, if so, (2) to what degree this spatial heterogeneity is explained by habitat heterogeneity among local communities (variation in river system size, catchment land cover, etc.), connectivity between localities (spatial configuration of river systems), and species' dispersal ability (the degrees of diadromy).

Rice paddies, a major component of Japanese rural landscapes harbor a rich diversity of species that drives multiple ecosystem functions (e.g., bioturbation and trophic interaction) and services (improved soil quality and biological pest control). However, once rich and diverse biological communities in the Japanese rice agroecosystem have been lost due to agricultural intensification, eliminating many of the ecosystem functions and services. In the era of global climate change and associated local weather extremes, the (re-)integration of rice production and biodiversity conservation is a significant challenge for maintaining and enhancing the stability and resilience of ecosystem functioning of rice paddies. As a potentially effective farming practice for this challenge, I focus on integrated rice-fish farming, or concurrent culture of rice and fish. Using intensive field sampling and subsequent stable isotope analysis of soil, rice plants, and aquatic organisms, we investigate qualitative and quantitative changes in aquatic community structure and nitrogen dynamics in the rice-fish agroecosystem.

Salmonid species exhibit substantial intraspecific variation in life-history traits such as growth, maturation, migration, and reproductive behavior.  Such life-history variation has multi-level ecological consequences, including those for individual fitness, population stability and resilience, trophic community structure, and nutrient dynamics in the stream and terrestrial ecosystems. I studied the evolutionary and population ecological aspects of alternative reproductive tactics in male salmon and charr in my earlier career. Recently, I have been collaborating with many colleagues on population dynamics of stream salmonids. One of the collaborative works is the long-term population survey of red-spotted masu salmon and white-spotted charr inhabiting a Japanese headwater system. In this work, we characterize spatial variation and covariation in life-history parameters (e.g., survival, growth, movement, and fecundity) among highly fragmented (dammed) mainstem habitat and small tributary habitat to provide insights into the ecology and conservation of the stream salmonid metapopulation in a human-modified streamscape.