Pollution does not always lead to immediate mortality but instead can have sublethal impacts that compromise organisms’ fitness over time, with population declines as the end result. Many pollutants take many years to biodegrade and thus continue to pose a threat to wildlife and humans long after entering the environment.
Rachel Carson’s 1962 book Silent Spring described the dangers of pollution, especially that of pesticides, with a clarity that captured public attention many years later. Carson, an American biologist, was particularly successful in drawing attention to biomagnification (also called bioaccumulation), a process by which pesticides and other toxins accumulate and concentrate more in animals at higher levels of the food chain.
The work was inspired by research that revealed that dichlorodiphenyl trichloroethane (DDT) sprayed on crops to kill insect pests and on bodies of water to kill the malaria mosquito (Anopheles spp. ) larvae, also harms non-target organisms that consume insects and fish exposed to DDT.
Note that non-target organisms at the top of the food chains, especially fish-eating birds, such as eagles, pelicans, and egrets, often had high levels of DDT concentrated in their tissues. Affected birds were generally weak and their eggshells were thin and prone to cracking during incubation.
Comprehensive conservation efforts must recognize that biodiversity faces multiple threats that need to be dealt with at different scales. As a result, bird populations dropped dramatically in areas where DDT was used, as adults died and failed to raise their young.
In the 1970s, many industrialized countries recognized the dire situation and banned the use of DDT, which eventually allowed the partial recovery of affected bird populations. Unfortunately, while some countries have chosen the safer alternatives, DDT continues to be widely used in Africa to control malaria mosquito, tsetse fly (Glossina spp.), and other disease vectors.
Researchers have recently observed the complete absence of piscivorous breeding birds in some African wetlands and some of the highest levels of DDT ever recorded in piscivorous birds. This is a cause for concern, not only for wildlife but also for long-term effects on people, especially food consumers exposed to these chemicals and workers handling these chemicals in the field.
DDT is not the only form of pollution control today, however. With the impacts of a growing human population that is gradually becoming more pervasive, pollution is compromising the quality of water, soil, and air at a faster rate than ever. Some forms of pollution can be very visible and have dramatic consequences.
More importantly, there are many less detectable forms of pollution. Although not always resulting in immediate mortality, these insidious forms of pollution have sublethal effects that compromise the fitness of organisms over time, with premature death and population decline continuing to be the end result.
Response to the silent threats of subtle and easily overlooked pollution is often delayed, especially when the negative effects are not felt until years after exposure. Taken together, pesticides and other pollutants claim 1.4–2.2 million human lives in Africa each year; globally, they claim 9 million lives, which is over three times more than the total impact of AIDS, malaria, and tuberculosis combined.
However, we are continuing to tolerate these threats, partly because the impact of pollution on our health is not always what appears, especially when pollution-related deaths result in strokes, heart disease, respiratory infections, diarrhea, or cancer, among other health problems.
Identifying the source is one of the most difficult aspects of pollution prevention. Many forms of pollution can be easily carried away from the source through the air, rivers, and even into groundwater. This transport of pollutants (called pesticide drift in the case of pesticides) involves a considerable load (perhaps up to 95%) of impacts felt by non-target species, including economically important non-target organisms.
For example, the drift of pesticides from the cotton fields of Benin caused the eradication of freshwater fish, while beneficial pollinating insects also often have a negative impact. Studies on fish in Nigeria, large mammals in South Africa, and frogs in Kenya have shown that beneficial organisms that survive this secondary pesticide exposure disrupted the reproductive and endocrine systems and thus reduced fitness.
Even humans may be exposed to secondary poisoning from pesticides, as toxic pesticide levels have been found in edible oysters and mussels in Ghana, prawns in Côte d’Ivoire, and even chickens in South Africa. To make matters worse, many pollutants take several years to biodegrade (eg. decompose in nature) and therefore continue to pose a threat to wildlife and humans even after entering the environment.
An important class of these long-lived pollutants are persistent organic pollutants (POPs). Several types of pesticides qualify as POPs, which are prone to bioaccumulation and drift. There are also many types of persistent inorganic pollutants that seep into the environment on a daily basis.
An important class of persistent inorganic pollutants that also bioaccumulate are heavy metals; these include mercury, cobalt, copper, lead and arsenic. A study from Zambia traced cobalt contamination in living trees to soil pollution from mining activities that occurred in the mid1970s.
Many products that we use daily can as well endure in the environment. For example, an aluminum can takes about 200 years to break, while a plastic bag takes 100 to 1000 years to break.
The continued use of these products should therefore alarm anyone concerned about the environment and human health. But it also offers anyone the opportunity to contribute to conservation by reducing the use of these products and the reuse/recycling of products moving up the supply chain.