Invasive Crayfish: Drivers or Passengers of Degradation in Freshwater Ecosystems?

ABSTRACT Invasive species, such as the freshwater crayfish Procambarus clarkii , reportedly negatively influence the abundance of various aquatic species. Moreover, these invaders are increasingly linked to ecological degradation of aquatic ecosystems, as invaded habitats show increased levels of turbidity, nitrogen, and organic matter concentration. P. clarkii has, among other impacts, been associated with eutrophication in invaded habitats. However, observations suggest that the presence of P. clarkii is often not accompanied by ecosystem degradation, raising the question of whether they are drivers of degradation or function as passive passengers, with the degradation being caused by other stressors. To investigate these contrasting hypotheses, we conducted a full factorial experiment in 24 mesocosms with P. clarkii and nutrient pollution (specifically N, P, and K), a ubiquitous stressor in aquatic ecosystems. Here, we assessed the effects on community compositions of morphologically identified macrophytes and chironomids, as well as the compositions of bacteria, phytoplankton, and diatoms identified using environmental DNA (eDNA) metabarcoding. Nutrient pollution induced significant shifts in macrophyte biomass and in the composition of the bacterial, diatom, and phytoplankton communities. All microbial communities exposed to nutrient pollution initially diverged from the control, after which the bacterial and phytoplankton communities converged back to the control in the final weeks. In contrast, we found only marginal effects of P. clarkii , rendering it unlikely as a significant short‐ to medium‐term driver of the tested biodiversity. As microbial communities respond quickly to changes in the environmental conditions, these results signify that the mesocosms used in the study were relatively stable in spite of the presence of P. clarkii . The crayfish density and timeframe studied may be leveraged as threshold values in the design and execution of freshwater management strategies that aim to avert potential negative impacts of P. clarkii on ecosystem structure. Ultimately, the importance of nutrient pollution is reinforced as a driver of environmental change in aquatic ecosystems.