Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 798–813 of the course textbook. Image credits are on slide 16.
Forests, deserts and other ecosystems have continued over long periods, and some may have lasted for millions of years: rainforests in South East Asia are thought to be at least 70 million years old. This is stability.
Ecosystems need a supply of energy, recycling of nutrients, genetic diversity, and climate within tolerance limits. Sealed mesocosms can be used to investigate this, as energy can pass through, but matter cannot.
A large area of rainforest is needed to produce atmospheric water vapour by transpiration, which brings cooling, air flows and rainfall. Removing trees reduces rain, which can kill more trees. A tipping point may be reached, beyond which the forest changes to another state. Some estimates put it at 20–25% deforestation, and about 17% of the original forest had gone by 2018.
These have a disproportionate effect on community structure, and the ecosystem may collapse if they are removed. Beavers and wolves are examples. So is the agouti, a rodent that buries Brazil nuts and so spreads the seeds of the Brazil nut tree.
Harvesting is sustainable if its rate is lower than the rate of replacement. Timber in Finland is managed with a legal duty to regrow, whereas Atlantic cod stocks have collapsed from over-fishing.
Consider soil erosion, leaching of nutrients, the supply of fertilizers and other inputs, pollution from agrochemicals, and carbon footprint.
Nitrate and phosphate from fertilizers leach into water, and algae bloom. Plants die in the shade, and decomposers use up oxygen, raising the biochemical oxygen demand. BOD is measured by keeping a water sample in the dark at 20 °C for five days and recording the fall in oxygen. Fish and other organisms can die as a result.
Toxins that build up in tissues reach higher concentrations at each trophic level, since each consumer eats many organisms. DDT and mercury are examples.
Watch out: bioaccumulation is build-up within one organism. Biomagnification is the rise from level to level.
Plastics persist, because they are not biodegradable. Macroplastics entangle animals or fill their stomachs, as in Laysan albatross chicks and sea turtles. Microplastics, smaller than 5 mm, enter food chains.
Methods include returning apex predators and other keystone species, reconnecting habitats over large areas, and reducing human impact. Natural processes can then run again.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all ten statements, from quick recall to data questions and a longer answer. It has 42 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 24 marks in total. Your teacher has the mark scheme.
The Food and Agriculture Organization (FAO) records the mass of fish caught from the sea each year. The graph shows catches from three oceans and the whole world.
A 21-year trial in Switzerland compared four farming methods. The activity of three soil enzymes, made by soil microorganisms, was measured as a percentage of the activity in soil given mineral fertiliser only.
Chinese brake fern (Pteris vittata) can take up large amounts of arsenic, a poison. Ferns were grown for 20 weeks in soils with different arsenic concentrations, and arsenic in the fronds (leaves) was measured.
Air pollution was measured in New York City and in nearby rural areas over one summer.
| City | Rural | |
|---|---|---|
| pH of rain | 4.2 | 4.5 |
| Nitrogen dioxide / ppb | 32 | 9 |
| Sulfur dioxide / ppb | 6 | 2 |
| Ozone / ppb | 28 | 44 |
Cottonwood saplings were grown at eight sites in and around the city, and their shoot growth was measured.
You will need: Duckweed, clear containers, pond or aged tap water, diluted plant fertiliser
Safety: Wash hands after handling. Dispose of plants in general waste, not in drains or natural water.
Carry this out with your teacher's supervision and follow your school's lab rules.
In 1966 Robert Paine removed a sea star from an area of shore and saw diversity fall, leading to the idea of keystone species.
Manipulating nature can reveal hidden links, but it also interferes. Consider when knowledge justifies that interference.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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