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).