2.2.3 Human Impacts on Ecosystems

Learning Objectives

1. Outline the meaning of bioaccumulation and biomagnification 

2. Using case studies, discuss the impact of the uses of DDT

3. Outline how microplastics may end up in human bodies

Non-biodegradable pollutants (NBPs)

• Are pollutants that don't get digested or converted once ingested, and henced passed to the next trophic level and so on

• Example: 

  • polychlorinated biphenyls (PCBs) - a plasticiser 

  • Dichrolo-diphenyl-trichloroethane (DDT) - an insecticide

  • Mercury - a toxic heavy metal, poisoinous to human

• NBPs cause changes to ecosystems because...

  • Organisms in a trophic level can take up the pollutants by ingestion, diffusion etc. This cause an accumulation of the pollutants in the organisms within a trophic level. This process is called bioaccumulation. The pollutants often get trapped in the fatty tissues of an organism

  • Pollutants are not digested or converted to a different compounds. Therefore, when the organism in the lower trophic level get eaten by their consumers, they pass along the pollutants in their bodies. As top consumers often require more food (due to the loss of biomass cause low amount energy available in higher trophic level), they will have a magnified amount of pollutants in their bodies. This is called biomagnification

Two major concens are:

Introduction of DDT

• A synthetic pesticide or insecticide

• First extensively used in WW2 to control:

  • lice that spread typhus

  • mosquitoes that spread malaria

• After WW2, used in farming as pesticide -- this caused its production to increase with the demand in food production

Banning History of DDT

• 1970s and 1980s, agricultural use of DDT was banned in most developed countries.

• DDT was first banned in Hungary (1968) followed by Norway and Sweden (1970), the USA in 1972 and the UK in 1984.

• The use of DDT in vector control has not been banned, but it has been largely replaced by less persistent alternative insecticides.

• The Stockholm Convention banned several persistent organic pollutants (POPs) such as DDT and restricted the use of DDT to disease control.

• The Convention was signed by 98 countries and is endorsed by most environmental groups

• DDT is still used in India and North Korea

Environmental impacts of DDT

• DDT is a POP that is extremely hydrophobic and strongly absorbed by soils. DDT is not very soluble in water but is very soluble in lipids (fats). This means it can build up inside fatty tissue. Its soil half-life can range from 22 days to 30 years

• Bioaccumulation is the retention or build-up of non-biodegradable or slowly biodegradable chemicals in the body. Biomagnification or biological amplification is the process whereby the concentration of a chemical increases at each trophic level. The result is that top predators may have in their bodies concentrations of a chemical several million times higher than the same chemical’s concentration in water and primary producers.

• DDT and its breakdown products all biomagnify through the food chain 

• DDT is believed to be a major reason for the decline of the bald eagle in North America in the 1950s and 1960s

• Other species affected included the brown pelican and the peregrine falcon. Recent studies have linked the thinning of the birds’ eggshells with high levels of DDT, resulting in eggs being crushed by parents when incubating.

Use of DDT against malaria

Malaria remains a major public health challenge in many parts of the world. The WHO estimates that there are 250 million cases every year, resulting in almost 1 million deaths. About 90% of these deaths occur in Africa. In 2006, only 13 countries were still 

In Ecuador, between 1993 and 1995, the use of DDT increased and there was a 61% reduction in malaria rates

1. Energy Flow

For thousands of years, humankind’s only source of energy was radiation from the Sun. Sunlight energy, trapped by producers through photosynthesis, provided energy for food. This limited population growth as only limited amounts of food were available (i.e. that which occurred naturally). With the advent of industrial revolutions, which saw the rapid development of industry and the increased use of fossil fuels, industry was able to harness the sunlight energy trapped in coal and oil. 

Energy trapped by plants millions of years ago could be released so the amount of energy available to humans increased greatly. This enabled the use of machinery to increase, so industry and agricultural output both increased. Population growth increased rapidly due to increased food output. This change in the Earth’s energy budget has ultimately led to many of the environmental issues covered in this course – habitat destruction, climate change, the reduction of non-renewable resources, acid deposition and so on

2. Matter transfer

Timber harvesting (i.e. logging) interferes with nutrient cycling. This is especially true in tropical rainforests, where soils have low fertility and nutrients cycle between leaf litter and tree biomass. Rapid decomposition, due to warm conditions and high rainfall, leads to the breakdown of the rich leaf litter throughout the year. 

Once the trees have been removed, the canopy no longer intercepts rainfall and the soil and leaf litter is washed away, along with many of the available nutrients. In South-East Asia, large areas of tree biomass have been cleared to grow oil palm. Oil palm is used in food production, in domestic products and as a source of biofuel. Once the original forest has been removed, natural nutrient recycling is also lost. The soils are generally nutrient poor, so oil-palm trees require fertilizer to produce yields that return a reasonable profit. Fertilizers can have various negative environmental impacts. Adding fertilizers containing nitrates can cause eutrophication in nearby bodies of water due to run-off from soils causing disruption to ecosystems.

Increased agricultural land use leads to a reduction in native ecosystems, altering the nature of carbon storage. When crops are harvested, they are transported to be sold at markets away from the location where they are grown. The carbon storage present in crops is therefore transported to new locations, altering the carbon cycle on a local and global scale. Urbanization leads to increased need for energy and therefore increased use of fossil fuels, which in turns leads to greater combustion of fossil fuels. Urbanization also leads to decreased land covered by vegetation, reducing photosynthesis. The concentration of the human population in cities has increased food requirements, leading to increased land use for agriculture. Increased transport of food into cities leads to greater energy requirements and increased fossil fuel use.

Burning fossil fuels increases the amount of CO2 in the atmosphere, leading to global warming and climate change. Mining and burning of fossil fuels reduces the storages of these non-renewable energy resources and increases the storage of carbon in the atmosphere. Increased CO2 levels in the atmosphere can lead to increased vegetation growth because there is more CO2 available for photosynthesis, again altering the carbon cycle. Although burning fossil fuels may lead to increased CO2 available for photosynthesis, the other pollutants produced and the impacts of global warming will reduce primary productivity. Deforestation, urbanization and agriculture all lead to loss of ecosystem biomass, disruption of food webs and the capacity for photosynthesis