Figure 4. Theoretical additive effects (a, b) of temperature and copper on larval metamorphosis compared with observed interactive effects (interaction ratio, IR) of these factors (c, d).

Additivity was calculated as the sum of the effect of copper at the control temperature and the effect of temperature at the control (lowest) copper concentration and contours represent metamorphosis success. IR was calculated from Equation 3: values >1 demonstrate synergy and values <1 demonstrate antagonism. Shaded regions depict combinations of copper and temperature for which metamorphosis success was zero.