Integration of ecology, demography and genetics to reveal population structure and persistence: a mini review and case study of stream-dwelling Dolly Varden

Spatial structures can drive population dynamics and persistence. The roles of space and dispersal on dynamics, however, remain largely unknown in wild populations, mainly because of the difficulty in extensive /intensive field surveys at multiple locations. Here, I integrate results of ecological, demographic and genetic studies to elucidate detailed population structure and to identify the mechanisms of population persistence in a stream fish. Spatial structures, such as habitat size and connectivity, are particularly important in river and freshwater networks because such ecosystems are highly vulnerable to human activities. These issues are explored by examining results from a decadal research on the stream-dwelling Dolly Varden charr in the Sorachi River basin, Japan. More than 100 local habitats (i.e. tributaries) were examined for spatial structure of populations by, such as, species presence, population census, redd counts, analysis of population synchrony and microsatellite DNA analysis. The results indicate that (i) population demography is largely independent in each tributary, (ii) some large tributaries support self-sustaining populations, (iii) despite small population sizes in most tributaries (<30 spawning females), local extinction is rare, except in human-influenced areas and (iv) probabilities of extinction are likely reduced by immigration of fish from neighbouring tributaries. The evidence suggests that Dolly Varden in this river system function as a source-sink or mainland-island metapopulation structure and that processes influencing these structures vary within the stream network. Overall, local populations may be highly persistent as long as local populations are well connected. I also discuss the vulnerability of stream fishes to habitat alterations.