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Species may differ greatly in their ability to tolerate pollutants. Some plants (often assisted by microbial symbionts in their rhizosphere) are hyperaccumulators of heavy metals – lead, cadmium and so on – with an ability not only to tolerate but also to accumulate much higher concentrations than the norm. As a result, species such as Solanum nigrum have an important role to play in bioremediation (Sun et al., 2017), removing pollutants from the soil so that eventually other, less tolerant plants can grow there too.

Some may even be used for phytomining, where hyperaccumulator plants are used to accumulate a metal of interest from metal‐rich soils and transport them to the shoots, followed by harvesting of the shoots as a bio‐ore (Thijs et al., 2017). Thus, Alyssum bertolonii can accumulate in its aerial parts 7000–12 000 μg g^–1 dry weight of nickel, while Arabidopsis halleri and S. nigrum can accumulate and tolerate similarly high concentrations of zinc and cadmium, respectively.

Organisms with bioremediation potential also include fungi and bacteria (de Alencar et al., 2017), and remediation can be directed not only at heavy metals but also at many other pollutants, including petroleum‐ and explosives‐contaminated soil and polycyclic aromatic hydrocarbons.

Pollution can, of course, have its effects far from the original source. Toxic effluents from a mine or a factory may enter a watercourse and affect its flora and fauna for its whole length downstream. Effluents from large industrial complexes can pollute and change the flora and fauna of many rivers and lakes in a region and cause international disputes.

acid rain

A striking example of pollution at a distance is the creation of ‘acid rain’ – atmospheric deposition of acidic constituents (particularly sulphuric and nitric acid) that reach the ground as rain, snow, particulates, gases and vapour. Acid rain results predominantly from emissions of sulphur dioxide and oxides of nitrogen (Figure 2.25a, b) from the burning of fossil fuels to generate electricity, transport and industry, and increased dramatically after the Industrial Revolution in Europe and North America. Profound ecological effects, often across national boundaries at considerable distances from the polluting source, have included damage to forests and soil communities and acidification of rivers and lakes, with associated loss of biodiversity and recreational activities such as fishing.

Figure 2.25 Acid emissions have been decreasing in Europe since 1970 while they continued to increase in China. Annual emissions of (a) sulphur dioxide and (b) nitrogen dioxide in Europe from 1880 to 2005 and of (c) ammonia and oxides of nitrogen (NO_x) in China from 1980 to 2010.

Source: (a, b) After Hildrew (2018). (c) From Liu et al. (2013).

The only option to treat the causes of acidic deposition is to reduce emissions and the introduction of stringent air pollution regulations in Europe and North America, aimed at sulphur dioxide, oxides of nitrogen and ammonia, produced impressive results. In the UK alone, emissions of sulphur dioxide fell by 94% and nitrogen by 58% between 1970 and 2010. Emission reductions in Europe as a whole have been almost as good, while reductions in North America have been somewhat smaller. It should be stressed that the reductions are not entirely explained by government anti‐pollution initiatives, but are partly due to the ‘export’ of emissions to China and elsewhere where many goods destined for import to the northern hemisphere are now made. Indeed, acid rain is much less of an issue now in the north while the highest rates of deposition currently occur in parts of Asia (Figure 2.25c).

As a consequence of emission reductions, chemical recovery of northern hemisphere waterways has been evident but biological recovery has generally been rather muted. This may be partly because chemical recovery is not yet complete or there may be biological constraints, such as a lack of colonists for previously impacted habitats or biotic resistance associated with changes to food webs, such that a simple reversal of acidification does not occur and the end point might not be the same community that existed before acidification (Hildrew, 2018). More encouraging has been the recent recovery of north‐eastern US fish populations in lakes that were previously incapable of sustaining wild fish populations because of acid conditions (Warren et al., 2017).