Simultaneous effect of temperature warming, fertilization and harvesting on food webs dynamics

Background/Question/Methods

Ecosystems are threatened by the simultaneous action of various stressors, with a growing intensity each passing decade. The diversity of mode and scale of action of these stressors often causes them to interact in unexpected ways, complexifying the prediction of ecosystems' future under different scenarios of disturbance. Yet, it is crucial for informing conservation and management efforts. Tackling this task requires a better understanding of the mechanisms and scales through which stressors affect ecosystems emerging processes and properties. We have used simulations of biomass dynamics in ecosystems, represented as their underlying food webs, under different scenarios of harvesting, temperature change and fertilization to investigate how these three stressors act on various mechanisms and interact at the different scales of the ecosystem. First, we have compared the strength and directionality of their individual and combined effects on various ecosystem processes and properties, and analyzed their distribution across ecosystem scales. Then, we have used structural equation models to better understand the mechanisms through which the stressors interact.

Results/Conclusions

The pattern analysis of effect sizes of the one-, two- and three-way interactions between stressors showed that simultaneous stressors have a consistent effect across trophic levels but not across ecosystem scales. This means that stressors have similar effects on processes, whether we measure these processes (e.g. consumption) at the population scale or averaged across populations of the same trophic level. However, a negative effect at the population scale might not translate into a negative effect on global emergent properties at the ecosystem scale. This makes understanding the mode of interaction of simultaneous stressors a crucial step. The structural equation models shed some light on how stressors interact through the main processes of biomass dynamics (which correspond to the terms of our biomass dynamic model): basal species growth, consumption and excretion, thus shaping the ecosystems emergent properties (stability, persistence, connectance, etc.). Overall, our results show that while the synergy between simultaneous stressors makes prediction of ecosystems functioning and stability complex, understanding the mechanisms through which they interact might help us in this endeavour.