Section Goals:

1. Understand examples of how ecosystem health and human health are linked.
2. Introduce the concept of biomagnification.

The Health of Earth’s Ecosystems Affects Human Health

Ecosystems have always played a role in human habitats and health. For most of human history, biological agents were a dominant factor in human health, with the distribution of the biological agents relating to ecosystem boundaries. Currently, our industrialized society dumps huge amounts of pollutants and toxic wastes into the earth’s biosphere without fully considering the consequences. Such actions seriously degrade the health of the earth’s ecosystems, and this degradation ultimately affects the health and well-being of human populations.

As just stated, biological agents were the most significant factor in health for most of human history. These included pathogenic (disease causing) organisms such as bacteria, viruses, protozoa, and internal parasites. While other causes of death (cardiovascular diseases, cancer, and accidents) have risen higher than biological causes in modern times and in industrialized countries, infectious diseases still cause about 22 million deaths a year. These diseases include: tuberculosis, malaria, pneumonia, influenza, whooping cough, dysentery and Acquired Immune Deficiency Syndrome (AIDS). Most of those affected are children in non-industrialized countries. Malnutrition, unclean water, poor sanitary conditions and lack of proper medical care all play roles in these deaths.

Compounding the problems of infectious diseases are factors such as drug-resistant pathogens, insecticide resistant carriers, and overpopulation. Overuse of antibiotics have allowed pathogens to develop a resistance to drugs. For example, tuberculosis (TB) was nearly eliminated in most parts of the world, but drug-resistant  strains have now reversed that trend. Another example is malaria. The insecticide DDT (a chemical called dichlorodiphenyltrichloroethane) was widely used to control malaria-carrying mosquito populations in tropical regions. However, after many years the mosquitoes developed a natural resistance to DDT and again spread the disease widely. Anti-malarial medicines were also over-prescribed, which allowed the malaria pathogen to become drug-resistant.

Case Study: Biomagnification of Chemicals

Chemical agents are a more modern concern which have significant effects on human health. Toxic heavy metals, dioxins, pesticides, and endocrine disrupters are examples of these chemical agents. Heavy metals (e.g., mercury, lead, & cadmium) are typically produced as by-products of mining and manufacturing processes. All of them biomagnify; they become more concentrated in species at higher trophic levels because the chemical does not cycle out of organisms, rather it builds up at each level of the food chain. For example, mercury from polluted water can accumulate in swordfish to levels toxic to humans. When toxic heavy metals get into the human body, they continue to accumulate in tissues and may  eventually cause sickness or death. Studies show that people with above-average lead levels in their bones have an increased risk of developing attention deficit disorder and aggressive behavior. Lead can also damage brain cells and affect muscular coordination.

Case Study: ENVIRONMENTAL PERSISTENCE OF DDT

The pesticide DDT was widely used for decades. It was seen as an ideal pesticide because it is inexpensive and breaks down slowly in the environment. Unfortunately, the latter characteristic allows this chemical agent to biomagnify through the food chain–meaning that the levels of DDT in animal tissue greatly increase at each level of the food chain.

Populations of bird species at the top of the food chain, e.g., eagles and pelicans, accumulate DDT in their tissue through consumption of animals that also have DDT in their tissue. Because DDT does not break down in their bodies, DDT concentrations grow through time. When these birds have sufficient levels of DDT, the shells of their eggs are so thin that they break, making reproduction impossible. After DDT was banned in the United States in 1972, affected bird populations made noticeable recoveries, including the iconic bald eagle.

In some parts of the world, the use of DDT has been approved with limitations because it is such an effective pesticide, and if the environmental spread is limited, the benefits to human health by reducing insect vectors like mosquitos outweighs the risk of ecological impact.

 

Attribution

Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0. Modified by Joni Baumgarten.