Unravelling 42 years of biodiversity and trophic changes in large-river ecosystems under multiple stressors

Freshwater biodiversity is declining faster than in any other ecosystems, yet the drivers of change in large rivers remain poorly understood. Disentangling these drivers is particularly challenging, as stressors frequently interact, and long-term, high-resolution datasets needed to detect and capture their effects are scarce. Using an exceptional 42-year dataset of fish and macroinvertebrate communities from two large rivers in France (the Rhône and Loire rivers), we addressed the relative influence of climate change (water temperature and river flow), changes in water quality and biological invasions on food web structure and function.

We found a gradual restructuring of community composition and taxa abundances from the early 1980s, with a drastic acceleration of changes in macroinvertebrate communities during the past two decades. Among explanatory variables, increasing water temperature and the expansion of non-native species emerged as the dominant drivers of change in both fish and macroinvertebrate assemblages.

To further explore ecosystem-level consequences of these community turnovers, we inferred the structure of food-webs and the fluxes of energy between taxa, and investigated how they changed across time. Our results reveal substantial shifts in food-web structure, including an increased number of species with more connections between them, as well as an unexpected lengthening of food chains. We found that the arrival and spread of non-native species accounted for much of the food-web structural reorganization over time. The proportion of energy fluxes controlled by non-native species rose sharply in recent decades, highlighting their growing influence within these ecosystems. Increased predation and competition from non-native species have likely exacerbated declines in native populations. In parallel, rising water temperatures associated with climate change also altered energy fluxes within the trophic network, notably by increasing metabolic demand and thus energy flux controlled by top predators.

By clarifying mechanisms underlying biodiversity changes in different large-river ecosystems, our study represents an important step toward disentangling multiple stressors impacts, an essential requirement for effective ecosystem management and conservation. Moreover, our findings underscore the need to consider entire communities, including non-native species, and to adopt food-web perspectives to better understand and anticipate the ecological consequences of global change.