Counting the costs of multiple stressors: is the whole greater than the sum of the parts?

Describing a recent drought-related forest dieback event in south-western Australia in 2011 (Matusick 2012), a colleague remarked upon the distinctive sounds of wood boring beetles feeding on weakened eucalypt trees during one of the most severe droughts on record (K. Ruthrof, personal communication). For this insect population, normally scarce and benign, drought stress had most likely triggered a surge in their abundance, thereby amplifying declines in forest health during an already stressful event. Observations of an apparent coincidence of stressors such as drought and pests are of course common across many ecosystems (Mattson and Haack 1987, Ayres and Lombardero 2000). The subsequent impacts on forest function and structure from stressors such as drought and herbivory represent complex interactions between abiotic and biotic factors (Raffa et al. 2008).