Identifying important species that amplify or mitigate the interactive effects of human impacts on marine food webs.

Some species may have a larger role than others in the transfer of complex effects of multiple human stressors, such as changes in biomass, through marine food webs. We devised a novel approach to identify such species. We constructed annual interaction-effect networks (IENs) of the simulated changes in biomass between species of the southeastern Australian marine system. Each annual 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 to the interaction effect of ocean warming, ocean acidification, and fisheries. Structurally, over the simulation period, the number of species and links in the synergistic IENs increased and the network structure became more stable. The stability of the antagonistic IENs 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 common in food-web and network theory, we identified the species in each IEN for which a change in biomass from stressor effects would disproportionately affect the biomass of other species via direct and indirect local, intermediate, and global predator-prey feeding interactions. Knowing the species that transfer the most synergistic or antagonistic responses in a food-web may inform conservation under increasing multiple-stressor impacts.

Identificación de las Especies Importantes que Amplifican o Mitigan los Efectos Interactivos de los Impactos Humanos Resumen Algunas especies pueden tener un papel más importante que otras en la transferencia de los efectos complejos de múltiples estresantes humanos, como los cambios en la biomasa por medio de las redes alimenticias marinas. Diseñamos una metodología novedosa para identificar a dichas especies. Construimos una red de efectos anuales de interacción (IEN, en inglés) a partir de los cambios simulados en la biomasa entre especies del sistema marino del sureste de Australia. Cada IEN anual estuvo compuesta por las especies conectadas por una respuesta aditiva (la suma de las respuestas individuales al estresante), sinérgica (una biomasa menor en comparación con los efectos aditivos) o antagónica (una mayor biomasa en comparación con los efectos aditivos) ante los efectos de interacción del calentamiento oceánico, la acidificación oceánica, y las pesquerías. Estructuralmente, durante el periodo de simulación, el número de especies y conexiones en los IEN sinérgicos incrementó y la estructura de la red se volvió más estable. La estabilidad de las IEN antagónicas disminuyó y se volvió más vulnerable ante la pérdida de especies. En contraste, no hubo cambio en los atributos estructurales de las especies conectadas por una respuesta aditiva. Con el uso de índices comunes entre las redes alimenticias y la teoría de redes identificamos a las especies en cada IEN para las cuales un cambio en la biomasa por causa de los efectos estresantes afectaría desproporcionalmente a la biomasa de las otras especies por medio de interacciones de alimentación locales, intermedias y globales del tipo depredador - presa directas o indirectas. Si sabemos cuáles especies transfieren el mayor número de respuestas sinérgicas o antagónicas en una red alimenticia podemos informar a la conservación que está bajo impactos estresantes cada vez mayores.

Climate change alters stability and species potential interactions in a large marine ecosystem.

We have little empirical evidence of how large-scale overlaps between large numbers of marine species may have altered in response to human impacts. Here, we synthesized all available distribution data (>1 million records) since 1992 for 61 species of the East Australian marine ecosystem, a global hot spot of ocean warming and continuing fisheries exploitation. Using a novel approach, we constructed networks of the annual changes in geographical overlaps between species. Using indices of changes in species overlap, we quantified changes in the ecosystem stability, species robustness, species sensitivity and structural keystone species. We then compared the species overlap indices with environmental and fisheries data to identify potential factors leading to the changes in distributional overlaps between species. We found that the structure of the ecosystem has changed with a decrease in asymmetrical geographical overlaps between species. This suggests that the ecosystem has become less stable and potentially more susceptible to environmental perturbations. Most species have shown a decrease in overlaps with other species. The greatest decrease in species overlap robustness and sensitivity to the loss of other species has occurred in the pelagic community. Some demersal species have become more robust and less sensitive. Pelagic structural keystone species, predominately the tunas and billfish, have been replaced by demersal fish species. The changes in species overlap were strongly correlated with regional oceanographic changes, in particular increasing ocean warming and the southward transport of warmer and saltier water with the East Australian Current, but less correlated with fisheries catch. Our study illustrates how large-scale multispecies distribution changes can help identify structural changes in marine ecosystems associated with climate change.

Predicting interactions among fishing, ocean warming, and ocean acidification in a marine system with whole-ecosystem models.

An important challenge for conservation is a quantitative understanding of how multiple human stressors will interact to mitigate or exacerbate global environmental change at a community or ecosystem level. We explored the interaction effects of fishing, ocean warming, and ocean acidification over time on 60 functional groups of species in the southeastern Australian marine ecosystem. We tracked changes in relative biomass within a coupled dynamic whole-ecosystem modeling framework that included the biophysical system, human effects, socioeconomics, and management evaluation. We estimated the individual, additive, and interactive effects on the ecosystem and for five community groups (top predators, fishes, benthic invertebrates, plankton, and primary producers). We calculated the size and direction of interaction effects with an additive null model and interpreted results as synergistic (amplified stress), additive (no additional stress), or antagonistic (reduced stress). Individually, only ocean acidification had a negative effect on total biomass. Fishing and ocean warming and ocean warming with ocean acidification had an additive effect on biomass. Adding fishing to ocean warming and ocean acidification significantly changed the direction and magnitude of the interaction effect to a synergistic response on biomass. The interaction effect depended on the response level examined (ecosystem vs. community). For communities, the size, direction, and type of interaction effect varied depending on the combination of stressors. Top predator and fish biomass had a synergistic response to the interaction of all three stressors, whereas biomass of benthic invertebrates responded antagonistically. With our approach, we were able to identify the regional effects of fishing on the size and direction of the interacting effects of ocean warming and ocean acidification.