Human Influences on Ecosystems

Increasing Food Production

Humans have increased food production in several ways. The methods you need are limited to agricultural machinery, chemical fertilisers, insecticides, herbicides and selective breeding.

Worked example (explaining a method): Explain how using a herbicide can increase the yield of a crop. Step 1. A herbicide kills weeds. Step 2. This reduces competition between the weeds and the crop plants for light, water and mineral ions, so the crop grows better. Answer: less competition from weeds.

Worked example (explaining a method): Explain how using an insecticide can improve a crop. Step 1. An insecticide kills insects that eat or damage the crop. Step 2. So the quality and the yield of the crop are improved. Answer: improves quality and yield.

Worked example (explaining a method): Explain how using a chemical fertiliser can improve the yield of a crop. Step 1. Chemical fertilisers provide mineral ions (such as nitrate) that the crop plants need. Step 2. With more mineral ions available, the crop grows better and the yield increases. Answer: provides minerals.

Monocultures and Intensive Livestock

A monoculture is the large-scale growing of a single crop plant. Intensive livestock production keeps many animals in a small space to maximise production.

Worked example (advantages and disadvantages): State one advantage and one disadvantage of growing a large-scale monoculture. Step 1. Advantage: the crop is easier to plant and harvest by machine. Step 2. Disadvantage: a pest or disease can spread quickly through the whole crop. Answer: easy to harvest; pests spread quickly.

Worked example (intensive livestock): State one advantage and one disadvantage of intensive livestock production. Step 1. Advantage: more meat, milk or eggs are produced from a small area. Step 2. Disadvantage: disease can spread quickly between animals kept close together. Answer: more food; disease spreads.

Habitat Destruction

Biodiversity is the number of different species that live in an area. Humans destroy habitats for several reasons, and this reduces biodiversity.

By altering food webs and food chains, humans can have a negative impact on habitats. For example, if a producer or a prey species is lost, the consumers that depend on it also decrease in number.

Worked example (reasons for habitat destruction): State two reasons why humans destroy habitats. Step 1. Humans clear land for housing, crop production or livestock production. Step 2. Humans also destroy habitats to extract natural resources. Answer: housing/farming; extracting resources.

Worked example (food webs and habitats): Explain how removing a producer from a habitat can have a negative impact on the other organisms. Step 1. The producer is the start of the food chains, so the primary consumers lose their food. Step 2. Their numbers fall, and the animals that eat them also decrease. Answer: consumers lose their food.

Deforestation

Deforestation is an example of habitat destruction. It has several undesirable effects.

Worked example (explaining an effect): Explain why deforestation can cause flooding. Step 1. Tree roots take up water from the soil, and trees slow the flow of rain to the ground. Step 2. With fewer trees, less water is taken up and more runs off quickly into rivers, so flooding is more likely. Answer: less water absorbed.

Worked example (explaining an effect): Explain why deforestation can increase the amount of carbon dioxide in the air. Step 1. Fewer trees are left to take in carbon dioxide for photosynthesis. Step 2. Burning or decay of the trees also releases carbon dioxide. Answer: less photosynthesis; burning and decay.

Water Pollution: Sewage and Fertiliser

Water in rivers and lakes can be polluted by untreated sewage and by excess fertiliser. Both have damaging effects on aquatic ecosystems.

Worked example (effects of sewage): Describe the effect of untreated sewage on the fish in a river. Step 1. Microorganisms decompose the sewage and use up oxygen in the water. Step 2. With less oxygen, many fish cannot respire and they die. Answer: less oxygen; fish die.

Worked example (effects of fertiliser): Describe one effect of excess fertiliser on a pond. Step 1. Fertiliser washes into the pond and adds nutrients. Step 2. Algae and water plants grow rapidly. Answer: rapid growth of algae and plants.

Worked example (pollution and disease): State one other way in which untreated sewage can harm humans. Step 1. Untreated sewage contains microorganisms. Step 2. Some of these cause diseases, which can spread if people use the water. Answer: microorganisms cause disease.

Eutrophication

ExtendedThe process of eutrophication

Eutrophication happens when excess nitrate and other ions enter water, for example from fertiliser. For your exam, the process is described in six steps:

1. Increased availability of nitrate and other ions.

2. Increased growth of producers, such as algae.

3. Increased decomposition after the producers die.

4. Increased aerobic respiration by decomposers.

5. Reduction in dissolved oxygen.

6. Death of organisms that need dissolved oxygen.

Worked example (explaining eutrophication): Explain why fish die in a river with a lot of fertiliser runoff. Step 1. Extra nitrate makes algae grow rapidly. When the algae die, decomposers increase, and their aerobic respiration uses up dissolved oxygen. Step 2. With a low concentration of dissolved oxygen, fish cannot respire and they die. Answer: decomposers use up oxygen.

Worked example (putting steps in order): Put these in order: (a) dissolved oxygen falls, (b) algae grow rapidly, (c) decomposers respire more, (d) producers die. Step 1. Extra nitrate makes algae grow rapidly (b). Step 2. The producers die (d), decomposers respire more (c), and dissolved oxygen falls (a). Answer: b, d, c, a.

Non-biodegradable Plastics

Many plastics are non-biodegradable: decomposers cannot break them down, so they stay in the environment for a very long time.

Worked example (effects of plastics): Describe one effect of non-biodegradable plastic on animals in the sea. Step 1. Plastic is not broken down, so it builds up in the sea. Step 2. Animals can eat the plastic or become trapped in it, which can harm them. Answer: animals eat or are trapped by plastic.

Worked example (plastics on land): Describe one effect of non-biodegradable plastic in a terrestrial ecosystem. Step 1. Plastic is not broken down, so it builds up in the soil and as litter. Step 2. Animals can eat it or become trapped in it. Answer: builds up; harms animals.

Air Pollution and Climate

Pollution of the air by methane and carbon dioxide causes the enhanced greenhouse effect and climate change.

Worked example (air pollution): State one source of carbon dioxide and one source of methane released by human activity. Step 1. Carbon dioxide: burning fossil fuels. Step 2. Methane: digestion in cattle and other livestock. Answer: fossil fuels; livestock.

Worked example (climate change): Explain how an increase in carbon dioxide in the air can cause climate change. Step 1. Carbon dioxide traps heat from the Earth's surface (the enhanced greenhouse effect). Step 2. So the average temperature of the Earth rises, which changes climates, for example rainfall patterns and sea level. Answer: traps heat; climate change.