Different organisms react in different ways to pollution. Some are extremely sensitive to the environmental conditions and may die once the environment is polluted. Yet some thrive in severely polluted habitats. By observing the existence or loss of certain species, we may gain an initial assessment of the environmental quality. We may not need equipment or to take samples to the laboratory to know whether a place is polluted or not. The organisms in there are secretly telling us!
Lichens are the best clean air indicators. They are symbionts of algae and fungi, with fungi outside and algae inside. While the algae contain chlorophyll to carry out photosynthesis to provide nutrients for themselves and the fungi, the fungi absorb moisture in the air through the hyphae, for both to use. Lichens can be classified according to the form: fruticose, foliose and crustose. Fruticose lichens are most sensitive to pollution, while foliose lichens and crustose lichens rank second and third.
Air pollutants such as sulphur dioxide dissolve in rainwater and mist, turning them acidic. When acid rain or mist fall on lichens, the chlorophyll structure of the lichens will be altered, changing their colour and stopping the algae from photosynthesising. Hence, from observing the change in lichen species at a site or analysing the chlorophyll level in the lichens, we can learn about the air pollution.
Water skaters, commonly called “water scissors” in Cantonese, are frequently seen on Hong Kong streams. They glide on the water surface, while preying on aquatic insects, by making use of the surface tension. Pollutants such as detergents and laundry powder damage the surface tension of water and make it impossible for water skaters to move on the surface, and so they eventually drown. Other insect larvae living on stream beds – including water penny (a kind of beetle nymph) and caddisfly larvae – are all very sensitive to water quality and can be used as indicators for water quality monitoring.
Pollution levels can also be predicted from the physical state of animals. Tributyltin (TBT) is an organo-tin compound that was used extensively in the 1960s as additive in ship paint to prevent algae and shellfish from attaching to ships. Numerous studies later found that even at very low concentrations of TBT, over 150 species of sea snails showed abnormal growth in their reproductive systems. The female would grow with male reproductive system and hence become infertile and even die, lowering the reproduction capacity of the whole population. Therefore, many countries have banned TBT and adopted sea snails as biological indicators to monitor levels of TBT in seawater.
Not all organisms can be used as biological indicators. In general, the indicators are easily observed common species. They have to react swiftly to the change in pollutants. From their role in the food web, we can predict the impacts of pollution on the local ecosystem. When you are out in the wild, do pay more attention to nearby organisms and you may discover pollution in time and help stop it at source!
Text | Helen Yau