Identifying important species that amplify or mitigate the interactive effects of human impacts to marine food webs
Some species may be more important in transferring the complex effects of multiple human stressors through marine food‐webs. Here we show a novel approach to help inform conservation management in identifying such species. Simulating changes in biomass between species from the interaction effects of ocean warming and ocean acidification, and fisheries to year 2050 on the south‐eastern Australian marine system, we constructed annual interaction effect networks (IEN's). Each IEN was composed of the species linked by either an additive (sum of the individual stressor response), synergistic (lower biomass compared with additive effects) or antagonistic (greater biomass compared with additive effects) response. Structurally, over the simulation period, the number of species and links in the synergistic IEN's increased and the network structure became more stable. The stability of the antagonistic IEN's decreased and became more vulnerable to the loss of species. In contrast, there was no change in the structural attributes of species linked by an additive response. Using indices of species importance common in food‐web and network theory, we identified the most important species within each IEN for transferring the interaction stressor effect on changes in biomass via local, intermediate and global interaction pathways. Mid trophic level mesopelagic fish species were most often identified as the key species within the synergistic IEN's and phytoplankton or zooplankton within the antagonistic IEN's. For the additive response commonly assumed in conservation management demersal fish species were identified by all of the indices. Apart from identifying the most important species, we also identified other important species for transferring the different interaction effects. Knowing the most important species for transferring synergistic or antagonistic responses may help inform conservation strategies for conserving ecosystems under increasing multiple stressor impacts.