The Effects of Pollution

Most things get more dilute as they spread out. Pollutants that resist being broken down do the opposite: they get more concentrated at every step up a food chain, until the animal at the top is carrying a dose thousands of times higher than anything in the water around it. That is why eagles, not plankton, were the first casualties of DDT.

Biomagnification and bioaccumulation

These two words appear together and are constantly swapped. Keep them apart with one question: within one organism, or up a chain?

The reason biomagnification happens is a consequence of energy transfer. Total biomass decreases at higher trophic levels, so a predator has to eat many prey animals to survive. Each of the small organisms at the bottom consumes only a small volume of pollutant. But the organism at the top consumes many of those smaller organisms and so receives a much larger dose. Because the pollutant is persistent — it does not break down and is not excreted — that dose stays in its body.

DDT: the case that started it

DDT (dichlorodiphenyltrichloroethane) is a persistent pollutant and was a widely used insecticide in the mid-twentieth century. Sprayed on crops, it leached into waterways and eventually entered freshwater and marine ecosystems. From there it entered food chains via plankton, accumulated in the bodies of fish, and reached its highest concentrations in the birds of prey that ate those fish.

The damage was not simple poisoning. Because DDT is persistent and does not break down easily, it built up in the tissues of animals at higher trophic levels and caused thinning of eggshells. The thin shells could not withstand the weight of the parent bird during incubation, so eggs broke, fewer chicks hatched, and over time bird populations fell.

DDT has now been banned worldwide, except where it remains essential for controlling mosquitoes that transmit malaria.

Mercury

Mercury behaves the same way. It is released by activities such as coal-fired power generation and gold mining. Once in the environment, microorganisms convert it into a highly toxic form called methyl mercury, which accumulates in the bodies of fish. As larger fish eat smaller fish, the concentration of methyl mercury in their tissues increases — which is why health advice exists around eating large predatory fish such as tuna and swordfish.

Plastic pollution

Plastics damage both land and water habitats because they are non-biodegradable — they do not break down, they only break up.

• Macroplastics are plastic items more than 5 mm in length: bags, bottles, food packaging, fishing nets.
• Microplastics are pieces less than 5 mm. They come from macroplastics broken into smaller pieces by wave action or UV rays.

What plastic does in marine habitats

• Animals try to eat it. Turtles may attempt to eat a plastic bag that resembles a jellyfish. Albatrosses accidentally consume plastic while fishing and give it to their chicks when they regurgitate food; the chicks may later starve because their stomachs are full of plastic.
• Animals become caught in it, in fishing lines and nets, leading to injury and death.
• Toxins are released as the plastic breaks down, which can then lead to biomagnification in the food chain.
• Very small particles are ingested by animals at the base of food chains, so the plastic itself enters the food chain.

NOS: scientists can change what people do

Public understanding of plastic pollution has shifted enormously in recent years, and the reason is effective communication of scientific findings — popular wildlife documentaries showing footage of seabirds feeding plastic to their chicks, for instance.

Clear science communication can change public behaviour in measurable ways. People have petitioned food companies and supermarkets to reduce plastic packaging, chosen to shop where less plastic is used, improved household recycling and taken their rubbish home, and switched to non-plastic items such as bamboo toothbrushes and paper straws.

Worked examples