Following the old adage “the whole is greater than the sum of its parts,” emergent ecosystem properties, such as ecosystem structure and function, (the whole) support the diversity of life (the parts) on the planet and generate important ecosystem services (e.g., food, recreation, water quality). Yet as climate change alters species interactions in sometimes novel and unexpected ways, it can disrupt these vital ecosystem properties, for example by creating novel combinations of species or shifting ecological roles and interactions. Aquatic ecosystems, which are already sensitive to disturbance from both climate and non-climate stressors, are particularly vulnerable to disruption.

A recent synthesis article published in the September issue of Fisheries, the American Fisheries Society’s (AFS) member magazine, summarizes the effects of climate change on emergent ecosystem properties within aquatic systems. The paper is the product of research from the 4^th National Climate Assessment, a symposium at the 2018 AFS annual meeting, and a targeted literature review. The effort was led by Northeast CASC Science Coordinator Michelle Staudinger and included National CASC Research Fish Biologist Abby Lynch. The authors developed a framework to evaluate evidence for how freshwater and marine ecosystems are reshuffling, restructuring, and rewiring in response to climate change.  They also explored two case studies that documented long-term and large-scale effects on fish and fisheries in the United Kingdom and North America.

The authors identified three primary mechanisms by which climate change can affect emergent ecosystem properties in aquatic systems. First, they found strong evidence that climate change is causing species rearrangement and turnover in aquatic ecosystems, resulting in changes in competition and dominance and most often favoring warm-water species. Second, there was compelling evidence that shifts in range, phenology, and other responses from changing climate are creating novel species assemblages in many aquatic systems. This resulted in more intense competition, changes in energy flows, and new disease-host dynamics. In some cases, species exhibited adaptive behaviors that led to novel habitat use and new ecological network pathways. Third, they found mixed evidence for hypothesized decreases in biodiversity and increased prevalence of generalist species within aquatic systems. In transitional habitats where warm water species were moving in and cold water species still persisted, biodiversity could actually increase temporarily. However, biodiversity declines were observed in places where species were at the edge of their ranges. The authors only found a few examples of generalist take-overs caused by climate change, although there are many such examples caused by non-native species invasions. Overall the range of observed emergent responses in aquatic ecosystems were highly complex and it remains unclear how systems will organize and function in the future under climate change. There was widespread consensus among studies that changes in emergent properties will disrupt ecological function and ecosystem services; however, more sophisticated models, holistic monitoring, and other tools are still needed to fully understand emergent responses. The authors hope ongoing research on this topic will allow fisheries managers and conservation groups to better consider emergent properties in their assessments and develop holistic strategies to manage and track aquatic systems in the future.