Figure 5. Gradients of toxic stress result in corresponding ecological changes. In figure (a), historical emissions from a smelter near Sudbury, Ontario, resulted in a spatial gradient of accumulated nickel, copper, and sulphate (the latter is mostly derived from the deposition of sulphur dioxide). Zinc was not emitted by the smelter, so it represents a reference (or control) gradient. Figure (b) shows a presumed ecological response to the gradient of toxic stressors, in this case represented by changes in indicators of plant biomass in stands at various distances from the smelter (the basal area of shrubs and trees [m2/ha] and the total cover [%] of all species of ground vegetation). Based on data in Freedman and Hutchinson, 1980a, 1980b.