Page 393 - Environment: The Science Behind the Stories

Toxic substances that bioaccumulate in an organism’s range, remaining only in Florida, and the osprey and other
tissues may be transferred to other organisms as predators hawks saw substantial population declines. Eventually scien-
consume prey. When one organism consumes another, the tists determined that DDT was causing these birds’ eggshells
predator takes in any stored toxicants and stores them itself. to grow thinner, so that eggs were breaking in the nest and
Thus bioaccumulation takes place on all trophic levels. More- killing the embryos within.
over, each individual consumes many individuals from the In a remarkable environmental success story, populations
trophic level beneath it, so with each step up the food chain, of all these birds have rebounded (p. 314) since the United
concentrations of toxicants become magnified. This process, States banned DDT. However, biomagnification is by no means
called biomagnification, occurred throughout North America a thing of the past. DDT continues to impair wildlife in parts of
with DDT. Top predators, such as birds of prey, ended up with the world where it is still used, and mercury buildup in fish poses
high concentrations of the pesticide because concentrations risks to human health in North America (p. 485). In the Arctic,
became magnified as DDT moved from water to algae to polar bears at the top of the food chain feed on seals that contain
plankton to small fish to larger fish and finally to fish-eating biomagnified toxicants. Despite their remote location, the polar
birds (Figure 14.13). bears of Svalbard Island in Arctic Norway show extremely high
Biomagnification caused populations of many North levels of PCB contamination, as a result of biomagnification
American birds of prey to decline precipitously from the and the concentrating effect of the process of global distillation
1950s to the 1970s. The peregrine falcon was almost totally shown in Figure 14.11. Polar bear cubs suffer immune suppres-
wiped out in the eastern United States, and the bald eagle, the sion, hormone disruption, and high mortality—and because the
U.S. national bird, was virtually eliminated from the lower cubs receive PCBs in their mothers’ milk, contamination per-
48 states. The brown pelican vanished from its Atlantic Coast sists and accumulates across generations.

Toxic substances can threaten
ecosystem services

Toxicants can alter the biological composition of ecosystems
DDT concentration and the manner in which organisms interact with one another and
(parts per million) their environment. In so doing, harmful compounds can threaten
the ecosystem services (pp. 21, 134–135, 170, 308) provided
Osprey by nature. For example, pesticide exposure has been implicated
25 ppm as a factor in the recent declines in honeybee populations
(p. 272). Honeybees pollinate over 100 economically impor-
tant crops, and reduced pollination by wild bees has increased
costs for farmers by forcing them to hire professional bee-
Large fish
keepers to pollinate their crops.
Healthy, functioning ecosystems provide the service of nutri-
2 ppm ent cycling. Decomposers and detritivores in the soil (pp. 99–100)
break down organic matter and replenish soils with nutrients for
plants to utilize. When soils are exposed to pesticides or anti-
fungal agents, nutrient cycling rates are altered. This affects the
Small fish quantity of nutrients available to producers, affects their growth,
and produces adverse effects throughout the ecosystem.
0.5 ppm
Studying Effects of Hazards

Zooplankton
Determining health effects of particular environmental hazards
is a challenging job, especially because any given person or
0.04 ppm
organism has a complex history of exposure to many hazards
throughout life. Scientists rely on several different methods
with people and with wildlife, ranging from correlative sur-
veys to manipulative experiments (p. 30).
Water 0.000003 ppm

Figure 14.13 In a classic case of biomagnification, DDT Wildlife studies integrate work in the field
moves from zooplankton through various types of fish,
becoming highly concentrated in fish-eating birds such as and lab
ospreys. Organisms at the lowest trophic level take in fat-soluble
compounds such as DDT from water. As animals at higher trophic Scientists study the impacts of environmental hazards on wild
levels eat organisms lower on the food chain, each organism animals to help conserve animal populations and also to under-
passes its load of toxicants up to its consumer, such that organisms stand potential risks to people. Just as placing the proverbial
392 on all trophic levels bioaccumulate the substance in their tissues. canary in a coal mine helped miners determine whether the

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