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Life-history variation is fundamental to the long-term persistence of populations and species because it ensures their ability to adapt to changing environments. Many important studies have focused on life-history variation between habitats, but the variation maintained within a habitat has often been overlooked.

Unraveling this puzzle at the landscape level is critical for understanding the spatial scales at which adaption and population persistence operate in nature. This motivated a team of researchers at Kyoto University to investigate life-history variation in masu salmon. The findings have been published in Journal of Animal Ecology.

"We wanted to understand how the variation in life-history is partitioned within and among habitats across heterogeneous landscapes," says first author Takeya Shida.

Previous research has demonstrated salmonid fish inhabiting large temperate watersheds to be an ideal subject for this type of study. In salmonids, "fast-life" individuals achieve accelerated growth early in the growing season—initiating gonadal development in spring and maturing in autumn of a given year—whereas "slow-life" individuals postpone the maturation decision until the following year.

Armed with this knowledge, the research team examined the hierarchical structure of life-history variation of the salmon in a large temperate watershed. They predicted that each habitat would maintain a certain degree of variation, but that the tendency for fast-life would be more prevalent in downstream reaches while slow-life would be more prevalent upstream.

The team established six study reaches, or segments, along a certain stream and investigated seasonal changes in water temperature and food resources, along with the seasonal growth of salmon juveniles and the ages of the mature fish.

They then quantified the relative contributions of differences in age of mature fish between habitats and within habitats to the overall diversity of age of mature fish throughout the watershed, making separate calculations for males and females.

The results largely confirmed the team's hypothesis. Fast-life individuals tended to dominate downstream reaches—with warmer temperatures and abundant aquatic invertebrate prey—while slow-life individuals were more prevalent upstream, with colder temperatures and fewer aquatic prey but relatively abundant terrestrial invertebrate inputs into streams.

However, the scientists also found considerable variation in the age of mature fish in all areas, except in the farthest downstream and upstream reaches. This suggests the variation in life-history at the watershed scale to be more strongly dependent on within-habitat variation than on between-habitat.

"It's intriguing that the balance between within-habitat and between-habitat diversity may be influenced not only by environmental factors within the stream, but also by terrestrial invertebrate prey supplied from surrounding riparian forests," says Shida.

As habitat homogenization has continued, such within-habitat variation may become more important to support adaptation to environmental changes such as global climate change.

"Recently, we have been witnessing a loss of diversity within species far more quickly than species diversity," says team leader Takuya Sato. "This study reminded us of the importance of managing salmonid resources in a way that preserves this diversity."