Extreme events and habitats as comparative tools for assessing scaled ecosystem responses to stressors in aquatic systems

Two types of extremes are often used as indicators of future change: Extreme climate events and extreme habitats. These extremes can be useful gauges of environmental change, but only if biological, spatial, and temporal scopes are aligned between the extreme and the future change it is hypothesized to mirror. In general, short-lived extreme events provide insights towards acute responses but are not as suitable for assessing long-term ecological or evolutionary change. Extreme habitats can reveal potential long-term adaptations, but community structure and trophic interactions can be challenging to assess due to species often following habitat-dependent evolutionary paths, and these environments tend to undergo gradual rather than rapid ecological change. Community structure and species interactions may further differ for both types of extremes between biological response levels. In cases of a mismatch between the size of an extreme habitat or event and its comparative system, it will be more appropriate to study biological responses at smaller geographic scales than the habitat or event occurs. Here, we explore the opportunities and limitations of a variety of extremes in marine and freshwater systems.