生態学：米国河川における魚類の生物多様性の変化

Changing water temperatures and human-driven introductions of fish have altered the composition of fish populations in streams and rivers across the USA over the past three decades, a study in Nature suggests. A disproportionately large effect has been observed in cold-water streams, where fish abundance decreased by more than 50% over the three-decade study period. The findings highlight the need to develop strategies to mitigate warming and to control fish introductions to conserve biodiversity.

Freshwater systems are home to more than 18,000 fish species, which account for nearly 25% of all vertebrates on Earth. These fish have important roles in the functioning of freshwater systems and are vital sources of food and money for humans. However, fish biodiversity is under threat around the world; in the USA, some of the largest threats include climate change and fish introductions (either invasive species or game fish stocking).

Samantha Rumschlag, Michael Mahon and colleagues have tracked trends in freshwater fish biodiversity in streams and rivers across the USA, monitoring 389 species across 2,992 sites between 1993 and 2019. They find that in cold streams (below 15.4°C), species abundance (the number of individual fish) and richness (the number of unique species) declined by 53.4% and 32% respectively over the 27 years, whereas communities became more distinct. In these cold waters, the proportion of larger, late-maturing fish increased whereas that of smaller fish decreased, which may have been caused by the introduction of large game fish, the authors suggest. Trends were reversed for warm streams (above 23.8°C); abundance and richness increased by 70.5% and 15.6%, respectively, and communities became more similar. In these warm waters, smaller fish replaced larger fish. The authors note that temperature effects are likely to be greater in cold streams as fish in these ecosystems are more sensitive to increasing temperatures than fish in warm streams are.

Changes in stream temperature and abundance of introduced or game fish magnified the impacts of each other, resulting in increased rates of degradation to local fish biodiversity, the authors report. Given the size of these changes over less than three decades, the authors conclude that there is an urgent need to address the shifting composition of fish communities in the USA. They suggest that controls on the introduction of fish and ecosystem restoration efforts may mitigate the changes in fish biodiversity.