Key environmental factors determining life history strategies of river fishes and their historical changes in the Yangtze River

Life history theory predicts that species-specific life history traits are shaped by variations in environmental conditions. Understanding the key environmental factors determining life history strategies can provide insights into the mechanisms of community assembly and identify potential biodiversity threats by predicting selective responses to environmental perturbations. We collected life history traits for fish species from 14 river basins across 5 continents to identify their life history strategies and explore the related key environmental factors. Our results showed that fishes from all the river basins conformed to a triangular life history strategy model including periodic, opportunistic, and equilibrium endpoints. However, the proportions of these three strategies varied significantly amongst river basins. For example, the Rhine River basin had a high proportion of fish with the periodic strategy and a low proportion of fish with the opportunistic strategy, whereas the Mississippi River basin displayed the opposite pattern. At the river basin extent, linear regression analysis revealed that the proportion of species with the opportunistic strategy increased with mean annual temperature, annual variation in river discharge, and annual precipitation, and was negatively associated with latitude. In contrast, species with the periodic strategy were negatively related to annual variation in river discharge and positively with latitude. These findings corroborate that opportunistic species are adapted to unpredictable environments, whereas periodic species are adapted to predictable environments. We then used molecular phylogeny to reconstruct the ancestral state of Yangtze fishes and analyzed the percentage composition of the three life history strategies across geological times. We found dominance of the equilibrium strategy in the Upper Cretaceous followed by dominance of periodic fishes before the Paleocene. Opportunistic fish species began to dominate the Yangtze River basin after the Miocene, with more than half of the fish species being opportunistic. This implies that the environmental conditions in the Yangtze River basin evolved from stable (low annual variation in river discharge) to unstable (high annual variation in river discharge and high annual precipitation) states, possibly associated with the onset of seasonality expressed as prevalence of a monsoon climate and the overall alteration of precipitation patterns. Life history theory can give hints for assessing river ecological conditions because changes in strategy proportions may indicate the alteration of environmental conditions. Such changes can aid in developing suitable strategies for river ecological protection and fish resource management.