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Multiple stressor effects on freshwater ecosystems trigger a complete rewiring of the food web

Researchers from the Ecotoxicology group of the IMDEA Water Institute have determined for the first time that the reorganization of the interactions’ strength in a food web, which led to a loss of strong interactions with predators, is sufficient to trigger a complete rewiring of the food web after multiple stress sources have been applied. The study has been published in Proceedings of the National Academy of Sciences and provides novel insights to unveil the mechanisms in which pesticides and other stressors affect complex freshwater communities in the long-term.

Aquatic ecosystems are increasingly stressed by human activities. Such human-induced pressures can alter the number of species living in an ecosystem, their total biomass, but also their identity and relative abundances. Studies have shown that, after a short-term disturbance, the number of species composing a community, as well as their total biomass can be regained rapidly, even after an initially significant decline. However, the same does not hold true for their identity and relative abundance, which might take substantially more time to be recovered, or might not be recovered at all. 

Often, anthropogenic activities result in multiple stressors impacting simultaneously natural communities and ecosystems. When these stressors act together, they can result in additive (simple sum of their individual effects), antagonistic (less than the sum), or synergistic (more than the sum) effects. Recent studies show that non-additive interactions between stressors are temporal scale dependent, meaning that they may appear only at a later stage, even after cessation of the short-term disturbance. Late synergistic or antagonistc effects have been reported often in natural systems. However, we still lack suitable methods to understand what processes may be driving the (late) non-additive effects of multiple stressors at the community level. Here, for the first time it has been applied quantitative ecological network analyses to try to elucidate the mechanisms driving long-term community composition dissimilarity and late-stage stressor interactions. A experiment was performed following a full factorial design at the freshwater mesocosms of the IMDEA Water Institute (Fig. 1). It was applied a long-term perturbation of nutrients (P and N, lasting for the whole experimental time) along with short-term perturbations of an insecticide (chlorpyrifos 1 µg/L) and an herbicide (diuron 18 µg/L), and focused on three time points related to the timing of pesticide application: before stress, maximum effects phase, and post-exposure recovery phase. 
 

Estación experimental de mesocosmos ubicada en el Instituto IMDEA Agua. 

Fig. 1. Mesocosms facilities at the IMDEA Water Institute.

At the moment of maximum effects, the number of species, total biomass, and food web structure were significantly impacted by the stressors in isolation. In the recovery phase, the number of species, total biomass, and food web structure were all recovered. Yet, the multivariate community composition of the communities treated with single pulse application of either of the pesticides were still significantly dissimilar from the control. This means that the relative abundances of the different species were still modified compared to the pre-disturbance conditions. Mirroring this multivariate dissimilarity, also the interaction’s strength between species was significantly modified by the single pesticides in the recovery phase. Researchers identified the reorganisation of interactions strength as the mechanism driving the long-term dissimilarity in community composition. 

Multiple stressors (insectice and herbicide) interacted non-additively only in the recovery phase, reducing the total number of species and changing their relative abundances. Analysing the changes in the interaction’s strength between the species in the recovery phase, we found that they were significantly modified by the mixture of the pesticides. We found that the outgoing energy fluxes (our way to measure species iteraction strength) in the pesticide mixture was dominated (> 80%) by the basal species, whereas top predators strongly declined in both biomass and in the interaction’s strength they exerted, which resulted in a complete reorganisation of the interaction strength in the food web (Fig. 2). 

Representación de la red trófica y los flujos de energía en la fase de recuperación del experimento
Figure 2. Link-weighted representation of the food web in the recovery phase. The networks, weighted links, and node distribution show how the energy fluxes within the community change in the presence of different stressors.  N: nutrients, I: insecticide, H: herbicide.

 

Francesco Polazzo, Tomás I. Marina, Melina Crettaz-Minaglia, Andreu Rico. This study has been published in Proceedings of the National Academy of Sciences 119, no. 17 (April 26, 2022): e2117364119. https://doi.org/10.1073/pnas.2117364119.

This work has been carried out within the framework of the projects:

  • ECORISK2050 "Ecological Risks of Chemicals in the Future". The project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 813124.
  • CICLIC-ECOREST "Smart tools and technologies to assess the environmental fate and risks of ContamInants under CLImate Change". Grant RTI2018-097158-A-C32 funded by MCIN/AEI/10.13039/501100011033 and by ERDF A way of making Europe