Anthropogenic impacts and global changes have profound implications for
natural ecosystems and may lead to their modification, degradation or
collapse. Increases in the intensity of single stressors may lead to
ecological thresholds, which can create abrupt shifts in biotic
responses. The effects of multiple interacting stressors may create
synergistic or antagonistic interactions, leading to non-linear
responses. Here we combine both concepts -- ecological thresholds and
interactions between multiple stressors -- to understand the effects of
multiple interacting stressors along environmental gradients, and how
this can affect the occurrence of thresholds. Using an experimental
approach to investigate the effect of nutrient enrichment and saltwater
intrusion on mortality in the zebra mussel, Dreissena polymorpha, we
show that multiple stressors can create thresholds at lower levels of an
environmental gradient. Our results reveal that by treating the concepts
of ecological thresholds and multiple stressors individually we risk
underestimating widespread anthropogenic impacts. Our results reveal a
major shortcoming in how we currently investigate these two ecological
concepts, as considering them separately may be causing underestimation
of thresholds and stressor-interaction impacts.