11. Tolerance

Tolerance is the ability of organisms to cope with stress, in this case to environmental pollutants. It appears to be a widespread phenomenon, and can be achieved by physiological acclimation or genetic adaptation. It can be assessed by comparing responses (lethal or sublethal) of individuals from different populations to the same degree of stress, e.g. the same concentration of a toxicant would produce less of an effect in a tolerant population. The phenomenon is well documented for metals and organic contaminants (for example, resistance of insect populations to insecticides is well-known). There are also cases in which tolerance has been looked for but not found in chronically exposed populations; probably more cases than have been reported in the literature, as this can be viewed as “negative data” and not reported. When enhanced tolerance does not occur in polluted populations, the reasons may be difficult to ascertain; it may be because detoxification mechanisms are adequate to cope with elevated exposures, or that dispersal and mixing between contaminated and reference populations obscures any observation of tolerance, or that the fitness costs counteract the selective advantage of the tolerance, or other reasons.

Compensatory responses to pollutants at the physiological level are referred to as “acclimation.” Pre-exposure to chemicals can induce or enhance detoxification processes, discussed in the previous chapter, which reduces toxicity in pre-exposed organisms, either in the lab or at field sites. These responses (e.g. synthesis of MTs, CYPs) can mitigate effects on individuals experiencing moderate stress. Developing tolerance is energetically expensive, however, and may have deleterious effects on energy allocation. Thus exposure over extended periods may reduce fitness and increase selection pressure for genetically resistant individuals in populations that have variation and heritability for the response. The development of these populations that have adapted and become more resistant to pollution can be considered an evolutionary compensatory mechanism.