Research

Increasing temperatures with climate change can increase the risk of population extinctions. For this reason, the predictions have been made for many taxa over the past several decades. Although these predictions have been used to conservation for endangered species, few studies have compared the past predictions with current-actual extinctions that have occurred.

About 30 years ago, a population extinction model was proposed for a stream salmonid, "Oshorokoma" in Hokkaido (Nakano et al. 1996 Freshw Biol). This model predicted that about 28% of the local population would disappear if the temperature increased by 1°C due to global warming, and 67% would disappear if the temperature increased by 2°C (Fig. 4 in Nakano et al. 1996). From this prediction to the present, the temperatures of Hokkaido has increased by 1°C; however, it is unclear how many populations have remained.

To clarify the of rising temperatures and biotic/abiotic factors affecting the distribution of "Oshorokoma", we surveyed more than 100 populations in areas where temperatures have increased over the past several decades and revealed the current status.

How and why ecologically similar species can coexist has been a central theme in ecology. Congeneric competitive charr with parapatric distributions along with water temperatures, southern Asian Dolly Varden (SADV, cold-water species) and white-spotted charr (WSC, warm-water species), are one of the most well-known models to explore coexistence mechanisms. However, past experiments have shown that WSC always dominates SADV regardless of the temperature, which could not explain why SADV is dominant in cold water in the wild.

To tackle this, we conducted a field survey at 35 stream habitat mosaics. The species composition was changed from SADV to WSC with increasing water temperatures. Interestingly, we found a new pattern: the dominant WSC was abundant even in cold habitats if the subordinate SADV was absent. Therefore, we proposed a new coexistence mechanism determined only by the dominant species' cost-benefit balance regardless of the subordinate species. To test this hypothesis, we installed enclosures in four cold streams with a BACI design. We excluded SADV and then released 50 WSC in two “impact” streams and 50 SADV in two “control”. After a month, WSC remained in the cold streams at a similar rate (26–46%) as the SADV. This supports our hypothesis that some costs from the SADV, not water temperatures, can reduce the WSC’s cold-water preference.

Together with some previous studies, the distribution process of both species can be considered as follows: the SADV firstly colonized the cold habitats but was unable to expand to warmer habitats because of the competitive disadvantage to the WSC. On the other hand, WSC avoid selectively cold habitats with SADV and some costs from them (e.g., reduction of food resources by density-dependent competition). Thus, such spatial segregation via highly asymmetric competition could drive the regional coexistence of both species.

Artificially transplanting species can trigger the extinction of native species not only through cascade effects in prey-predator interactions but also through unexpected hybridization; that is, invasive hybridization. The distributions of two bitterling fish (subfamily: Acheilognathinae), Tanakia lanceolata and T. limbata, overlap in western Japan. Bitterling fish lay their eggs in the gills of freshwater bivalves, and the early juvenile stage develops in the gills. Populations of freshwater bivalves are declining worldwide, which has limited the number of spawning substrate for bitterlings. T. limbata has been artificially introduced to some rivers in Matsuyama, Japan, where it coexists with native T. lanceolata, and some hybrids have been observed.

We collected both species from several sites in western Japan, and analyzed the genetic population structure based on multilocus microsatellite and sequences of the mitochondrial cytochrome b gene. Structure analysis identified three genetically distinct populations: T. lanceolata, T. limbata “West Kyushu”, and T. limbata “Setouchi”. Two clades of T. limbata were also supported by molecular phylogenetic analyses based on cytochrome b. The hybrids in Matsuyama originated mostly from interbreeding between male T. lanceolata and female T. limbata “West Kyushu”, and made up more than 10% of all collected fish, suggesting that hybrids occurred frequently between females of colonizing species and males of native species. On the other hand, interspecific hybrids were detected at high rates in some rivers on Kyushu Island, which are naturally the two bitterlings’ sympatric regions. Furthermore, we found a few T. limbata “Setouchi” in the Midori and Kase Rivers, which were supposed to be introduced from other regions, coexisting with native T. limbata “West Kyushu”, and this cryptic invasion may have triggered the interspecific hybridization.

These results suggest that the artificial introduction of a fish species, a decline in the unionid population, and degradation of habitat have caused a broad hybridization of bitterlings in western Japan.