Research

Global warming increasingly threatens biodiversity through local extinctions, yet empirical validations of predictive species distribution models remain rare. We validated a historical extinction model using site-level "disappearances"—defined as the current absence of a species at sites where it was recorded 20–60 years ago. Focusing on a cold-adapted salmonid, the southern Asian Dolly Varden (Salvelinus curilus) in Hokkaido, Japan, we enhanced risk models by incorporating seasonal thermal regimes, groundwater effects, and biotic interactions.

We electrofished 138 historical sites (2018–2024) to assess species presence, and analyzed long-term air and modeled groundwater temperatures to estimate warming trends. Generalized linear mixed models were developed to evaluate disappearance risks against summer temperature trends and competition with white-spotted charr, masu salmon, and rainbow trout.

Local disappearance occurred at 12 sites (8.70%), doubling the prediction of the previous annual-temperature-based model (6 sites, 4.35%). Disappearances did not correlate with latitude or elevation. Models incorporating summer warming and interspecific competition yielded the highest predictive performance, whereas annual mean or subsurface temperature models performed poorly.

Our findings demonstrate that seasonal thermal dynamics and biotic interactions—rather than annual means—drive local disappearances. Limited dispersal and habitat specificity likely exacerbate these climate impacts. Integrating these ecological complexities significantly improves the accuracy of risk forecasting, providing crucial guidance for international freshwater conservation strategies.

How ecologically similar species coexist remains one of the fundamental questions in ecology. My research focuses on two closely related stream salmonids with contrasting thermal preferences: the cold-adapted Dolly Varden (Salvelinus curilus) and the warm-adapted white-spotted charr (S. leucomaenis). Although water temperature is widely believed to determine their distributions, previous experiments showed that white-spotted charr consistently outcompete Dolly Varden regardless of temperature, leaving their dominance in cold streams unexplained.

To resolve this paradox, we combined replicated field surveys with enclosure experiments. Field surveys across multiple independent tributaries showed that Dolly Varden abundance was strongly associated with colder water, whereas white-spotted charr were not physiologically excluded from these habitats. Instead, white-spotted charr occurred frequently in cold streams when Dolly Varden were absent, suggesting that species interactions rather than temperature limit their distribution. Enclosure experiments further demonstrated that white-spotted charr can survive and grow successfully in cold habitats, supporting this interpretation.

These findings indicate that temperature alone cannot explain the distribution of the two species. Instead, competition from numerically dominant Dolly Varden—possibly reinforced by priority effects and reduced food availability—appears to discourage white-spotted charr from occupying cold streams. This study highlights the importance of biotic interactions, alongside environmental conditions, in shaping species coexistence and community assembly.

Artificial transplantation of species can lead to the extinction of native species not only through cascading effects in predator–prey interactions but also through unexpected hybridization, known as invasive hybridization. In western Japan, the distributions of two bitterling fish (subfamily Acheilognathinae), Tanakia lanceolata and T. limbata, overlap. Bitterlings deposit their eggs in the gills of freshwater bivalves, where the early juvenile stages develop. However, populations of freshwater bivalves are declining worldwide, reducing available spawning substrates for bitterlings. T. limbata has been artificially introduced into some rivers around Matsuyama, Japan, where it now coexists with native T. lanceolata, and hybrids between the two species have been observed.

We collected individuals of both species from multiple sites in western Japan and analyzed their genetic population structure using multilocus microsatellites and mitochondrial cytochrome b sequences. Population structure analysis identified three genetically distinct groups: T. lanceolata, T. limbata “West Kyushu,” and T. limbata “Setouchi.” These two clades of T. limbata were also supported by molecular phylogenetic analyses based on cytochrome b. In Matsuyama, most hybrids originated from crosses between male T. lanceolata and female T. limbata “West Kyushu,” comprising more than 10% of the collected specimens. This suggests that hybrids commonly arise between females of colonizing species and males of native species. In contrast, interspecific hybrids were detected at high frequencies in some rivers on Kyushu Island, where the two bitterlings naturally occur in sympatry. Furthermore, we detected a few T. limbata “Setouchi” individuals in the Midori and Kase Rivers, likely introduced from other regions, coexisting with native T. limbata “West Kyushu.” This cryptic invasion may have triggered interspecific hybridization.

These results suggest that the artificial introduction of fish, coupled with the decline of unionid mussels and habitat degradation, has driven extensive hybridization among bitterlings in western Japan.