National 5 Biology Course Notes Unit 3 : Life on Earth Part 5 : Human impact on the environment

Increasing human population The graph shows the human population explosion that has taken place in recent centuries. This increase in population leads to a need to produce more and more food The use of fertilisers to supply necessary plant nutrients and of pesticides to destroy pests that damage crops helps to produce high crop yields. However, these chemicals can cause damage to ecosystems.

How fertilisers can damage ecosystems Fertilisers can be washed by rain from farm fields into lakes where they increase the growth of surface microscopic plants called algae. The increase in growth of these algae leads to large numbers of them coating the water surface in what are called algal blooms When the algae die, they provide food for bacteria which rapidly multiply and use up oxygen leading to the death of aquatic animals, like fish Algal bloom on a lake surface The flow chart outlines this process: Fertiliser is leached into the water Fertiliser causes increased growth of algae (algal bloom) Bacteria feed on dead algae and use up oxygen in the water Water animals such as fish die due to lack of oxygen

Accumulation of pesticide (DDT) in a food chain Accumulation of pesticides in food chains Pesticides are sprayed on crops to kill pests like insects that damage the crops. Predators of these pests eat a lot of them and so accumulate pesticides in their bodies. This trend continues along a food chain with predators at each level accumulating more pesticides in their bodies. The problem is made worse by the fact that organisms often cannot get rid of the pesticide and store it in their bodies. Predators at the top of the food chain can have levels of pesticide that are damaging or lethal. The diagram shows the accumulation of the now banned insecticide DDT in an aquatic food chain in concentrations of parts per million (ppm) after it had been washed into water from farmland,

Indicator species Indicator species are organisms whose presence or absence from an area indicates the level of pollution. For some organisms, e.g. bloodworms in water, the fact that they are present indicates pollution while for others, e.g. lichens, the fact that they are absent from trees indicates air pollution. Rivers can be polluted with organic waste, e.g. sewage. Bacteria feed on this and use up oxygen causing death of other organisms such as fish. Some of the invertebrate indicator species whose presence indicates polluted or clean water are shown below: Indicator species present Stonefly or mayfly Freshwater shrimp Water louse Bloodworms Rat-tailed maggot Level of pollution Very low Low Moderate High Very high Increasing level of pollution Absence from water indicates pollution Presence in water indicates pollution

Indicators of Air Pollution Lichens are plants that grow in exposed places such as tree bark. Tree trunk covered in lichens Air pollutants like sulphur dioxide from burning fossil fuels dissolves in rainwater and damages lichens preventing them from growing. This makes lichens natural indicators of air pollution. In places where no lichens are growing, this is often a sign that the air contains pollutants such as sulphur dioxide. Tree trunk stained with soot and having no lichens due to pollution

Biological control Biological control is a way of controlling pests that doesn’t use pesticides. Instead, natural predators of the pest are used to kill them. For example aphids (also called greenfly) feed on crops and garden plants damaging them and killing them. Ladybirds are natural predators of aphids and can be put on infected plants to control the aphid population Ladybirds feeding on aphids Myxomatosis in rabbits Myxomatosis is a fatal virus disease of rabbits. It has been used to control the rabbit population in countries such as Australia. However those rabbits surviving the disease had genetic resistance and resistance in rabbit populations has been growing GM crops GM crops can be genetically modified to contain genes that produce chemicals lethal to pests so that pesticides do not have to be used or to produce crops that don’t need as much fertiliser.