Brent Cornell

    
Biomagnification is the process in which chemical substances become more concentrated at each trophic level

• In contrast, bioaccumulation refers to the build up of a chemical substance in the tissues of a single organism

Biomagnification occurs because organisms at higher trophic levels must consume more biomass to meet requirements

• Energy transformations are only ~10% efficient, so higher order consumers must eat more to meet energy demands
• This means higher order consumers will experience increased contamination from a chemical substance

Biomagnification of a Pollutant

⇒  Click on the diagram to show representation of chemical concentration and relative toxic effects

    
An example of a chemical substance which is biomagnified is DDT (dichloro diphenyl trichloroethane)

• DDT is a chemical pesticide that is sprayed on crops and subsequently washed into waterways at low concentrations
• It is fat soluble and is selectively retained within the tissues of an organism instead of being excrete

When DDT is sprayed on water to eliminate mosquito larvae, it is taken up by algae and passed on to primary consumers

• At each subsequent trophic level the concentration of DDT stored in the body increases due to increased food intake
• Very high levels of DDT were discovered in birds that preyed on fish, and was found to interfere with eggshell formation
• Birds exposed to high levels of DDT were found to produce thinner shells, which decreased survival rates of fledglings

Relationship between DDT Exposure and Eggshell Thickness

                            Diagram:       Diagram 1         Diagram 2        Diagram 3

    
DDT was widely employed as a chemical pesticide in the 1940s and 1950s to control insect-borne diseases like malaria

• Its use as an agricultural insecticide led to the complete eradication of malaria in many high socioeconomic world regions
• It was less effective in low socioeconomic tropical regions where eradication programs could not be sustained

Eradication programs were curtailed by the World Health Organisation in 1969 over safety and environmental concerns

• DDT can bioaccumulate to toxic levels and poses a significant threat to higher trophic levels (due to biomagnification)
• Current interventions for malaria now focus on non-spraying strategies (such as the use of bednets soaked in insecticides)

Prevalance of Mosquitoes in America during the 20th Century

Malaria versus DDT

Arguments for DDT Spraying:

• DDT spraying is an affordable and effective means of killing mosquitos that carry disease (malaria, dengue fever, etc.)
• Where the use of DDT has been discontinued, the incidence of malaria and associated deaths have increased
• Health costs associated with the treatment of malaria are reduced when DDT spraying is implemented
• Alternative strategies are not as cost-effective or successful

Arguments against DDT Spraying:

• DDT spraying is associated with adverse health effects in humans (cancer, birth defects, reduced fertility, etc.)
• DDT persists in the environment for long periods of time (more than 15 years)
• DDT is biomagnified in higher order consumers, which has adverse consequences on ecosystems

Malaria Control in South America (1960 – 1993)