Box 2. Classification of multiple stressor combinations
Organisms can influence some stressors (that is, modify the strength or presence of the stressor) but not others. For example, an organism’s avoidance behaviours can modify predator behaviour. In contrast, while ectotherms typically increase their foraging rates to cope with the energetic costs of increased metabolic rates resulting from elevated temperature (Portner and Kust 2007), this behavioural response does not alter the temperature itself. Here, we classify multiple stressor combinations into three forms depending on whether: A) the focal organism cannot modify either stressor (i.e. no feedback between the focal organism and the stressors); B) the organism can modify one stressor only (bi-directional feedback between a single stressor and the organism) or C) the organism can modify both stressors (bi-directional feedbacks between both stressors and the organism) (Figure 2). Stressor interactions and impacts on organisms can hinge on these categories.
In a Type A stressor combination, an organism can employ mechanisms to cope with stressor exposure, but it is unable to directly alter the strength or presence of either stressor, except via avoidance or escape in space or time. In a Type B stressor combination, an organism can modify one stressor but not the other. Notably, the stressor that the organism can modify could also be modified (e.g., its effect amplified or reduced) by the other stressor, meaning a biotic stressor such as a predator may be differently impacted by the abiotic stressor than the prey (focal individual) (Allan et al. 2013; Allan et al.2015). For example, crayfish predators were more negatively affected by the pesticide carbaryl than were their snail prey, which resulted in lower predation rates by crayfish upon snails (Reylea 2003). Finally, Type C depicts a combination of two stressors that an organism can modify through its behavior. Additionally, the stressors may directly (or indirectly) influence one another; e.g., interactions between predators (e.g. intraguild predation) can determine whether their combined effects on prey are additive, antagonistic or synergistic (also known as independent, risk reducing or risk enhancing effects) (Sihet al. 1998; Schmitz 2007).