Page 391 - Environment: The Science Behind the Stories

Toxic Substances and Their 3 At high latitudes,
Effects on Ecosystems 2 Pollutants are deposition of pollutants
exceeds evaporation
transported by 4 Pollutants enter
When toxicants concentrate in environments and harm the atmosphere and the polar food
health of many individuals, populations (p. 67) of the affected ocean currents web and
accumulate in
species become smaller. This decline in population can then biota
affect other species. For instance, species that are prey of the Atmospheric currents Evaporation Deposition
organism affected by toxicants could experience population
growth because predation levels are lower. Predators of the North Pole
poisoned species, however, would decline as their food source
became less abundant. Cascading impacts can cause changes Evaporation
in the composition of the biological community (p. 68) and Deposition
threaten ecosystem functioning. There are many ways toxi- Oceanic currents
cants can concentrate and persist in ecosystems and affect 1 At low
latitudes,
ecosystem services. evapora-
tion of
pollutants
Airborne substances can travel widely exceeds
deposition
Toxic substances are released around the world from agricul-
tural, industrial, and domestic activities and can sometimes be Equator
redistributed by air currents (Chapter 17), exerting impacts on
ecosystems far from their emission site.
Because so many substances are carried by the wind, syn-
thetic chemicals are ubiquitous worldwide, even in seemingly South Pole
pristine areas. Scientists who travel to the most remote alpine Figure 14.11 Air and water currents direct pollutants to the
lakes in the wilderness of British Columbia find them con- poles. In global distillation, pollutants that evaporate and rise high
taminated with industrial toxicants, such as polychlorinated into the atmosphere at lower latitudes 1 , or are deposited in the
biphenyls (PCBs), which are by-products of chemicals used ocean, are carried toward the poles 2 by atmospheric currents
in transformers and other electrical equipment. Earth’s polar of air and oceanic currents of water. This process concentrates
regions are particularly contaminated, because natural pat- pollutants near the poles 3 and causes elevated exposure to toxic
terns of global atmospheric circulation (p. 473) tend to move substances in polar organisms 4 .
airborne chemicals toward the poles (Figure 14.11). Thus,
although we manufacture and apply synthetic substances concentrating effect. Wastewater treatment plants also add tox-
mainly in temperate and tropical regions, contaminants are ins, pharmaceuticals, and detoxification products from humans
strikingly concentrated in the tissues of Arctic polar bears, to waterways. If chemicals persist in soil, they can leach into
Antarctic penguins, and people living in Greenland. groundwater and contaminate drinking water supplies.
Effects can also occur over relatively shorter distances. Many chemicals are soluble in water and enter organ-
Pesticides, for example, can be carried by air currents to sites isms’ tissues through drinking or absorption. For this reason,
far from agricultural fields in a process called pesticide drift. aquatic animals such as fish, frogs, and stream invertebrates
The Central Valley of California is the world’s most produc- are effective indicators of pollution. When aquatic organisms
tive agricultural region, but because it is naturally arid, food become sick, we can take it as an early warning that some-
production depends on the intensive use of irrigation, fertiliz- thing is amiss. If scientists find low concentrations of pesti-
ers, and pesticides. The region’s frequent winds often blow cides harming frogs, fish, and invertebrates, they view this as
airborne pesticide spray—and dust particles containing pes- a warning that people could be next. The contaminants that
ticide residue—for long distances. In the mountains of the wash into streams and rivers also flow and seep into the water
Sierra Nevada, research has associated pesticide drift from we drink and drift through the air we breathe. Once concen-
the Central Valley with population declines in four species trated in waters, toxic substances can move long distances and
of frogs. Families living in towns in the Central Valley suffer affect a variety of ecosystems (pp. 409–410).
health impacts, and advocates for farm workers maintain that
hundreds of thousands of the state’s residents are at risk.
Some toxicants persist in the environment
Toxic substances may concentrate in water Once a toxic substance arrives somewhere, it may degrade
quickly and become harmless, or it may remain unaltered and
Toxic substances are not evenly distributed in the environ- persist for many months, years, or decades. The rate at which
ment, and they move about in specific ways (Figure 14.12). a given substance degrades depends on its chemistry and on
Water running off from land often transports toxicants from factors such as temperature, moisture, and sun exposure. The
large areas and concentrates them in small volumes of sur- Bt toxin (pp. 274, 280) used in biocontrol and genetically
390 face water. The NAWQA findings on water quality reflect this modified crops has a very short persistence time, whereas

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