A stressor can be defined as a variable that, as a result of human activity, exceeds its normal range of variation and affects individual species or communities (modified after Townsend et al., 2008). Most present-day ecosystems are exposed to multiple stressors acting simultaneously (Vinebrooke et al., 2004; Crain et al., 2008; Dudgeon, 2010) or sequentially (Christensen et al., 2006). Therefore, multiple stressors research is highly relevant for both fundamental and applied science, and when trying to deal with complex global problems. For example, multiple-stressor effects are believed to be responsible for the ongoing global declines of honeybees (reviews by Potts et al., 2010; Aebi & Neumann, 2011), amphibians (reviews by Sodhi et al., 2008; Mann et al., 2009; Hof et al., 2011), coral reefs (reviews by Harvey et al., 2013; McLeod et al., 2013) and freshwater biodiversity (reviews by Allan, 2004; Dudgeon, 2010; Ormerod et al., 2010; Vörösmarty et al., 2010; Woodward et al., 2010). Multiple-stressors effects are also a central concern when trying to understand and predict the all-pervasive impacts of global climate change (reviews by Lindenmayer et al., 2010; Woodward et al., 2010; Chmura et al., 2011; Hof et al., 2011; Harvey et al., 2013; McLeod et al., 2013).
The development of a general theory of multiple stressors has started relatively recently (Hay et al., 1994; Hay, 1996; Folt et al., 1999; Swanson, 2004; Vinebrooke et al., 2004) and is still ongoing (e.g. Crain et al., 2008; Downes, 2010; Statzner & Bêche, 2010; Boone et al., 2011; Verberk et al., 2013). The combined action of multiple stressors can produce either simple or complex responses. According to Folt et al. (1999), ‘simple’ additive responses are patterns where the effect of all stressors combined is equal to the sum of the effects of each individual stressor. By contrast, ‘complex’ outcomes can be synergistic or antagonistic, when the combined effect is either larger or smaller than one would expect based on the individual effects of each stressor.
From the viewpoint of a resource manager or conservation ecologist, complex outcomes of interacting multiple stressors are particularly important because they can lead to ‘ecological surprises’ (Paine et al., 1998; Lindenmayer et al., 2010). For example, due to synergistic interactions between stressors, the actual threat to a given endangered ecosystem may be far more severe than expected based on the combined knowledge from all single-stressor studies conducted to help protect this ecosystem.