Figure 1
From: Tipping point arises earlier under a multiple-stressor scenario
Evidence for an ecological threshold in the context of single (stable salinity in green) and multiple stressors (saltwater intrusion in yellow) for mortality levels in zebra mussel Dreissena polymorpha populations along a nutrient enrichment gradient. (a) The effect of two months of exposure to a press nutrient enrichment, both alone and combined with saltwater intrusion on the mortality of adult mussels. Points represent raw data for adult mortality observed in individual tanks. Full lines represent broken-line regression models for the stable salinity and saltwater intrusion, and shaded zones represent 95% confidence intervals. Dashed vertical lines represent significant (p < 0.05) thresholds identified for the stable salinity and saltwater intrusion. (b) Variance points and smooth curve of variance of mortality for each group of observations along the nutrient enrichment gradient for both stable salinity and saltwater intrusion. (c,d) Frequency distribution of observed mortality in the experimental study (histogram) and density of values along the gradient (curve), where the surface under the curve corresponds to one, in the absence (c) and presence (d) of saltwater intrusion.
