Research

Increasing temperatures due to climate change can elevate the risk of local extinctions. For this reason, extinction risk predictions have been made for many taxa over the past several decades. Although such predictions have informed conservation efforts for endangered species, few studies have compared past predictions with actual extirpations.

About 30 years ago, an extirpation model was proposed for a stream salmonid, Dolly Varden charr, in Hokkaido (Nakano et al., 1996, Freshwater Biology). This model predicted that approximately 28% of local populations would disappear if air temperature increased by 1.0 °C due to global warming, and 67% would disappear with a 2.0 °C increase (Figure 4 in Nakano et al. 1996). Since that prediction, the air temperature in Hokkaido has risen by about 1.0 °C; however, it remains unclear how many populations have actually been extirpated.

To clarify the effects of rising temperatures and associated biotic and abiotic factors on the distribution of Dolly Varden charr, we surveyed more than 140 sites in regions where temperatures have increased over the past several decades, and documented their current status.

How and why ecologically similar species can coexist has been a central theme in ecology. A classic model system for exploring coexistence mechanisms is the congeneric charr with parapatric distributions along water temperature gradients: Dolly Varden charr (DV, a cold-water species) and white-spotted charr (WSC, a warm-water species). However, past experiments have shown that WSC always outcompetes DV regardless of temperature, which does not explain why DV dominates in cold waters in the wild.

To address this paradox, we conducted a field survey at 35 stream habitat mosaics. Species composition shifted from DV to WSC with increasing water temperature. Interestingly, we also discovered a new pattern: WSC was abundant even in cold habitats when DV was absent. Based on this finding, we proposed a novel coexistence mechanism in which the distribution is determined by the dominant species’ cost–benefit balance, independent of the subordinate species.

To test this hypothesis, we carried out enclosure experiments in four cold streams using a BACI design. We excluded DV and then released 50 WSC into two “impact” streams and 50 DV into two “control” streams. After one month, WSC persisted in cold streams at similar rates (26–46%) as DV. This result supports our hypothesis that costs imposed by DV, rather than water temperature, reduce WSC’s use of cold-water habitats.

Together with previous studies, our results suggest the following distributional process: DV first colonized cold habitats but could not expand into warmer habitats due to competitive disadvantage against WSC. Conversely, WSC avoid cold habitats when DV is present, owing to costs such as reduced food availability through density-dependent competition. Thus, strong asymmetric competition and resulting spatial segregation may drive the regional coexistence of these two ecologically similar species.

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.