Human Impacts on Energy & Matter Flows (DP IB Environmental Systems & Societies (ESS))

Human Impacts on Energy & Matter Flows

Bioaccumulation and biomagnification

• Bioaccumulation is the build-up of persistent or non-biodegradable pollutants within an organism or trophic level because they cannot be broken down

• Biomagnification is the increase in the concentration of persistent or non-biodegradable pollutants along a food chain

  • As pollutants are passed up the food chain from one trophic level to the next, they become more concentrated

  • This means that organisms at higher trophic levels (such as top predators) accumulate higher concentrations of pollutants than those at lower trophic levels

  • This is due to the decrease in the total biodegradable biomass of organisms at higher trophic levels

• Pollutants that are persistent and non-biodegradable can accumulate along food chains

  • Examples include:

    • Polychlorinated biphenyl (PCB)

    • Dichlorodiphenyltrichloroethane (DDT)

    • Mercury

• They can cause changes to ecosystems through the processes of bioaccumulation and biomagnification

  • For example, DDT was a widely used insecticide in the mid-20th century that was found to have harmful effects on birds of prey such as eagles and falcons

  • When DDT was sprayed on crops, it would leach into waterways and eventually enter freshwater and marine ecosystems

  • DDT would then enter food chains (via plankton) and accumulate in the bodies of fish 

  • These fish would then be eaten by birds, which would accumulate higher concentrations of DDT

  • Because DDT is persistent and does not break down easily, it can continue to accumulate in the bodies of animals at higher trophic levels (such as birds of prey), leading to harmful effects such as thinning of eggshells and reduced reproductive success

• Mercury is another example of a pollutant that can accumulate along food chains

  • Mercury is released into the environment through activities such as coal-fired power plants and gold mining

  • Once in the environment, mercury can be converted into a highly toxic form called methylmercury

  • This accumulates in the bodies of fish

  • As larger fish eat smaller fish, the concentration of methylmercury within the tissues of these fish increases, leading to potential harm for humans who eat large predatory fish such as tuna or swordfish

    • In 1956, for example, a chemical factory released toxic methylmercury into waste water entering Minamata Bay in Japan

    • Mercury accumulation in fish and shellfish caused mercury poisoning in local people (who ate the fish and shellfish) and resulted in severe symptoms (paralysis, death, or birth defects in newborns)

Non-biodegradable pollutants and microplastics

• One concerning aspect of many non-biodegradable pollutants is that they can be absorbed by microplastics

  • This can increase the transmission of these pollutants within food chains (i.e. increase the level of biomagnification)

• Microplastics are tiny plastic particles, often less than 5mm in size

  • They come from various sources like plastic bottles, packaging and synthetic clothing

  • When in the environment, these microplastics act a bit like sponges, absorbing non-biodegradable pollutants such as polychlorinated biphenyls (PCBs), pesticides and heavy metals such as lead and mercury

Effect on the food chain

• Marine animals often ingest microplastics as they feed

• As smaller organisms consume microplastics containing pollutants, these toxins accumulate in their bodies

• Larger predators then consume these contaminated organisms, leading to biomagnification, where the concentration of toxins increases at higher trophic levels

• This can have negative consequences for organisms in food chains

  • For example, a study found that oysters exposed to microplastics containing pollutants experienced:

    • Lower feeding rates

    • Altered growth patterns

    • Reduced reproductive success

  • This was found to negatively impact the fitness of individual oysters and the success of the population as a whole

Human activities and ecosystem impacts

• Human activities can significantly change the natural flows of energy and matter within ecosystems

• Burning fossil fuels:

  • Releases carbon dioxide into the atmosphere, contributing to global warming

  • Increased CO₂ availability can increase photosynthesis rates

    • However, other pollutants and climate change effects (e.g. temperature rise and changing rainfall patterns) can outweigh this benefit, reducing primary productivity

  • For example, burning coal to generate electricity emits CO₂ but also releases sulfur dioxide (SO₂)

  • This pollutant contributes to acid rain and affects soil pH, which in turn impacts plant health and nutrient availability

  • This further reduces photosynthesis rates

• Deforestation:

  • Clearing forests for agriculture, urbanisation, or logging disrupts ecosystems

    • As well as causing habitat loss and disruption of food webs, deforestation reduces the carbon sink capacity of forests

  • This contributes to climate change

• Urbanisation:

  • Urban development replaces natural habitats with impervious surfaces like concrete, leading to increased runoff and reduced infiltration

  • Urban areas generate "heat islands", increasing local temperatures

• Agriculture:

  • Intensive agriculture involves the use of fertilisers, pesticides and monoculture practices

  • This can lead to soil degradation, water pollution and loss of biodiversity