14.1 Essential ideas

14.1.3 Impacts of humans on ecosystems

There are many ways in which humans have an impact on ecosystems. The following are discussed in this chapter:

  • Introduction of alien species and subsequent loss of endemic species
  • Chemical pollution and biomagnification
  • Macro and microplastic and its effect on marine species

Invasive alien species

  • Any species that is not endemic, or native, to an ecosystem, is considered an alien species.
  • Alien species are often introduced unintentionally, for example in the ballast tanks of cargo ships. Sometimes they are introduced intentionally, as agricultural crops or livestock.
  • Introduced species require continuous control by human intervention, or they can escape into local ecosystems and become invasive.
  • Invasive species may be able to exploit habitats more efficiently than endemic species because they have no natural predators.
  • If an invasive species occupies a niche similar to an endemic species, it can cause a decrease in numbers due to competitive exclusion. For example, grey squirrels, were introduced into England in 1875, and have now excluded the endemic red squirrel from most of its native habitat.

red squirrel distributionFigure 14.1.3a – Distribution of red and grey squirrels in Britain

Chemical pollution and biomagnification

Pest control is especially important in regions where malaria is prevalent. The very first chemical pesticides, including DDT, were extremely effective at killing mosquitoes, but they are also persistent chemicals that do not biodegrade.

  • Persistent chemicals – synthetic pesticides like DDT and heavy metals like mercury – are pollutants and toxins which exist in the abiotic environment at very low concentrations, and are ingested by small consumers. (Some plants also accumulate pollutants in spongy tissues.)
  • The pollutants accumulate in the tissues of organisms through the subsequent trophic levels. Since these chemicals cannot be broken down by organic processes, the concentration of persistent chemicals increases at each trophic level. This process is called biomagnification.

DDT values in food chainFigure 14.1.3b – Biomagnification of DDT in a food chain
The numbers are representative values of the concentration in the tissues of DDT and its derivatives (in parts per million, ppm)

The species most at risk are the top predators, since they tend to have the largest biomass, and can accumulate lethal concentrations of toxins in their tissues over long lifetimes. Even when the concentration of toxin in the abiotic environment is low, biomagnification puts ecosystems at risk.

Plastic debris in marine environments

About 70% of the plastic from consumer products ends up in the oceans. Some of it sinks to the ocean floor, but most is carried out by ocean currents and accumulates on surfaces, or in shallow water as macroplastic or microplastic debris. Macroplastic debris can be harmful when it is ingested directly by sea life.

laysan albatrossFigure 14.1.3c – Laysan albatross, Phoebastria immutabilis
The specimen shown here has ingested plastic it mistook for food. A large bird – this species has a wingspan up to 6m – the albatross often kills its own offspring by feeding them plastic.

A more insidious problem is microplastic debris. This forms when large plastic items are mechanically broken into tiny pieces and chemically ‘bleached’ by sunlight, to become invisible particles suspended in seawater. Microplastic debris is carried by ocean currents to massive areas of accumulation. These are known as the five gyres.

5 gyres

Figure 14.1.3d – The five gyres

Microplastic debris is very dangerous because it can accumulate in the lungs and bloodstream of animals. It can also absorb organic pollutants like CFCs and PCBs from the environment, thereby contributing to biomagnification.

Nature of Science

Biological control programmes, in which non-native species are used to control agricultural pests, have associated risks. An important part of the scientific process is to discuss ethical implications and perform risk-benefit analyses in order to determine if the benefits can be verified. Controlled experimentation is necessary before any control programme is approved.

ballast waterFigure 14.1.3e – Ballast water explained
Container ships need to load seawater to maintain buoyancy after unloading cargo. The water contains marine life that is carried to other parts of the world and introduced to new ecosystems. It is thought that the invasive zebra mussel (below), Dreissena polymorpha, was introduced to the Great Lakes of North America in this way.

zebra musselsFigure 14.1.3f – Zebra mussels 

red squirrel and grey squirrel

Figure 14.1.3g – Red squirrel and grey squirrel
In the UK, the invasive Eastern grey squirrel, Sciurus carolensis, outnumbered the endemic Eurasian red squirrel, Sciurus vulgaris, by six times in 2006! Culling programmes are in place, and in 2013 the population of red squirrels increased for the first time.

International mindedness

Many developed countries export toxic waste to less developed countries. Is financial compensation a fair exchange for hazardous waste?

malaria mosquitoFigure 14.1.3h – Malaria mosquito drawing blood 
Malaria is caused by a parasitic plasmodium that is transmitted through the saliva of mosquitoes. 


Discuss the trade-off between control of the malarial parasite and DDT pollution.

macroplastic debrisFigure 14.1.3i – Macroplastic debris
Plastic bags that look like jellyfish strangle predators that mistake them for prey.

Food for thought

The Great Pacific Gyre is conservatively estimated at 700#000km2! That’s about the same size as Texas.

Science and social responsibility (Aim 8)

  • A worldwide ban on DDT (dichlorodiphenyltrichloroethane) was put in place in 2001. The same year a production ban was placed on PCBs (polychlorinated biphenyls), a class of compounds used to soften plastics and extend the efficacy of pesticides.
  • CFCs (chlorofluorocarbons) are a class of organic compounds used in coolants and aerosol sprays that contribute greatly to the depletion of the ozone layer. Production of all CFCs was recently banned by over 100 countries.
  • Could a ban on non-biodegradable plastics be next?