Most existing research on interactions between effects of stressors on living systems involves factorial experiments with species or systems in settings where treatments can be replicated and controlled. Factorial experiments are useful for detecting the presence of interactions but, because such systems are usually only exposed to one level of each stressor, they rarely provide sufficient information to predict responses at varying levels of stressors present in nature. Meta-analyses of results from studies of multiple stressors on various species have been conducted, but no general pattern has emerged for predicting how the effects of stressors will interact. Specific studies may find impacts as either non-interactive, synergistic or antagonistic. Beyond these generalities, the prediction of multiple stressors often assumes stressor effects to be additive and this assumption is often wrong. In spite of the inability to predict the effects of multiple stressors for environmental management, similar problems in a variety of fields have led to promising new approaches.
The National Research Council’s Ocean Studies Board has convened five highly successful panels on the subject of biological effects of manmade underwater sound and produced a progressive series of reports published in 1994, 2000, 2003, 2005, and 2017. While regulations require environmental assessments to analyze cumulative effects, the NRC2 concludes that the science is not in place to meet regulatory requirements and that understanding human impacts on highly migratory marine species, such as large whales, remains a major challenge.
Currently, environmental assessments often observe that when two or more stressors impact the same population or ecosystem, combined stressors can result in either synergistic or antagonistic interactions and little more beyond a general statement is made. Moreover, the broad scientific consensus is that adding impacts is not appropriate since cumulative effects are not consistently observed to be linearly cumulative. Therefore, in the absence of alternative approaches, most environmental assessments do not attempt to quantify cumulative effects but rather simply point out inchoate concerns about synergistic effects. Those that do attempt to make predictions to guide management actions typically rely on the predicted effects of individual stressors in spite of the consensus that such reliance is usually wrong. A similar problem also holds for estimating the effect of multiple doses of the same stressor. The current state of knowledge then is that cumulative risk from exposure to multiple stressors cannot be predicted based on existing scientific theory and data for individual marine mammals or their populations.
2 National Academies of Sciences, Engineering, and Medicine. 2017.
