Stressors
The basis for the analysis of multiple stressor effects is a profound understanding of the impacts of individual stressors. Worldwide, stressors that have been impacting freshwater ecosystems for decades or even centuries (such as flow alteration, habitat degradation and eutrophication) remain important, and are increasingly complemented by emerging stressors such as expanding hydropower, salinisation, emerging contaminants and light pollution (Reid et al., 2019; Palmer and Ruhi, 2019).
RESIST mainly addresses the effects of three stressors:
Stressors rarely occur in isolation but the vast majority of streams and rivers are affected by multiple stressors. There are five main effect types when two stressors affect species, communities or ecosystems:
- Stressor dominance occurs when one of the stressors has an overriding effect on the considered response variable or if one stressor masks the effects of others
- Additive effects describe a situation where the combined effects add up without strengthening or weakening each other
- Synergistic effects relate to interactions of stressors that strengthen the individual effects such that the joint effect is stronger than the additive effect
- Antagonistic effects refer to interactions of stressors that weaken the individual effects such that the joint effect is weaker than the additive effect
- Reversal occurs when the joint effect is in the opposite direction of the individual stressor effects
A key to better understanding and eventually predicting multiple stressor effects is to delineate the mechanism of how species (and ultimately communities) are affected by multiple stressors. ARC distinguishes five possible scenarios:
- Direct effects: both stressors directly affect the focal species.
- Indirect effects through an abiotic environmental variable: Both stressors jointly affect an environmental variable that in turn affects a focal species.
- Indirect effects through other species: Both stressors affect one or several species that interact with the focal species (e.g., through predation or competition), a yet understudied mechanism
Combinations of these three basic cases are also possible. For example:
- Combination of direct and indirect effects through an abiotic environmental variable: One or both stressors directly affect a focal species in addition to indirect effects of one or both stressors mediated by an environmental variable.
- A combination of direct and indirect effects through one or more other species. Most species in a community will experience such a net combined effect through other species.
To a great degree, the effect type of multiple stressors rules the prospects and mechanisms of recovery. This link has rarely been made and ARC was the first to offer a concept relating multiple stressor effect types and recovery options:
- if the effect of two stressors is additive, the removal of one of them will lead to a partial, but not full recovery (A)
- if one stressor dominates effects, recovery will be successful if the dominant stressor is removed (B)
- however, if the effect of the subordinate stressor increases once, it is no longer masked by the dominant stressor (C)
- if restoration targets the subordinate stressor, improvements will be small or undetectable (D)
- if two stressors are similarly important and act synergistically, removal of either of the two would already lead to a notable improvement (E)
- if stressors act antagonistically, removing only one of them may have a very limited effect or even make the situation worse (F)
- in cases of reversal, removal of one of the stressors could even increase the overall impact (G)
- if only a single stressor is present, its removal will eliminate stress completely (H)
