Decoding the impact of environmental shifts on snail density dynamics in the Yangtze River basin: a 26-year study.

Background: With the intensification of climate change and human engineering activities, environmental changes have affected schistosome-transmitting snails. This study explored the influence of environmental changes on the evolution of snail populations.

Methods: Data from annual snail surveys and related factors such as hydrology, temperature, vegetation, etc., on nine bottomlands from 1997 to 2022 were collected retrospectively from multiple sources. Interpretable machine learning and the Bayesian spatial-temporal model assessed the relationship between environmental change and snail density.

Results: Between 1997 and 2003, mean snail density was in a high-level fluctuation stage. From 2003 to 2012, it declined significantly from 0.773/0.1 m² to 0.093/0.1 m². However, it increased by 27.6% between 2013 (0.098/0.1 m²) and 2022 (0.125/0.1 m²). Since operation of the Three Gorges Dam (TGD) began in 2003, the duration of bottomland flooding decreased from 122 days (1997-2003) to 57 days (2003-2012) and then rebounded in 2012-2022, which was noticeable in the Anhui Section. The ground surface temperature and night light index of the bottomlands increased from 1997 to 2022. After adjusting for confounding factors (e.g. rainfall, temperature, and vegetation), the relative risk (RR) of increased snail density rose with flooding duration of between 20 and 100 days but decreased with flooding duration of > 100 days. Snail density showed an "L"-shaped relationship with the night light index, and the RR of increased snail density was lower at a higher night light index. Compared with bottomlands in the first quartile cluster of ground surface temperature, bottomlands in the second, third, and fourth quartile clusters of ground surface temperature had higher snail density RR values of 1.271 (95% CI 1.082-1.493), 1.302 (95% CI 1.146-1.480), and 1.278 (1.048, 1.559), respectively.

Conclusions: The TGD lowered the water level and flooding duration, which were not conducive to snail population growth. However, over time, the inhibitory effect of the TGD on snails may have been weakening, especially in areas far from the TGD. In recent years, the rebound of snail density may have been related to the rise in water levels and the change in the microenvironment. Establishing an efficient monitoring and response system is crucial for precisely controlling snails.