Ch 4 · Conservation · O Level Biology 5090
Topic 4 of 4 in Human Influences on Ecosystems — O Level Biology 5090 — 48 lessons.
Introduction
The previous two lessons described how humans damage ecosystems. This one is about the remedies: using resources at a rate they can replace, protecting species before they vanish, and treating the waste we make. Conservation questions are often explain or discuss, and the credit goes to answers that pair a method with the reason it works.
Key ideas — Sustainable resources: forests and fish stocks
A sustainable resource is one which is produced as rapidly as it is removed from the environment so that it does not run out. Some resources can be maintained indefinitely if managed properly; the syllabus examples are forests and fish stocks.
🧑🏫 Teacher's explanation
Imagine you have a bank account where you earn \100 in interest every month. If you only spend \80 a month, your balance grows or stays steady; this is sustainability. If you spend \120, you are depleting your capital until it hits zero. In biology, the 'interest' is the natural reproduction rate of a species, and the 'spending' is how much we harvest.
📖 Definitions
🔍 Worked example — Easy
A forest is harvested for timber. If 500 trees are removed annually, calculate the minimum number of trees that must be replanted each year to ensure the resource is sustainable.
🔍 Worked example — Standard
A fishery has a population of 10,000 fish. Each year, the population grows by 15\% through natural reproduction. If the current fishing quota is 1,200 fish per year, determine if the fish stock is being managed sustainably.
🔍 Worked example — Exam-style
A conservationist observes that a fish population is declining despite a quota being in place. Suggest two specific changes to the fishing regulations that could help stabilize the population.
⚠️ Common errors
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🎯 Exam question
(a) Define the term 'sustainable resource'. [1] (b) Describe two ways in which fishing regulations can be used to maintain fish stocks. [2] [3 marks]
Key ideas — Endangered species and how to conserve them
Species become endangered (at risk of extinction) or extinct because of climate change (habitats become too hot, dry or wet), habitat destruction, hunting (for meat, ivory, horn or fur), overharvesting, pollution and introduced species that prey on or outcompete them.
🧑🏫 Teacher's explanation
Imagine a bank account. If you withdraw money faster than the interest adds it, your balance eventually hits zero. In biology, a species is like that balance. If the 'withdrawal rate' (death/hunting/habitat loss) is higher than the 'deposit rate' (reproduction/growth), the species goes extinct.
📖 Definitions
🔍 Worked example — Easy
A population of sea turtles decreases from 5,000 individuals to 1,250 individuals due to overharvesting. Calculate the percentage decrease in the population.
🔍 Worked example — Standard
Compare the effectiveness of using a seed bank versus captive breeding for the conservation of a specific plant species and a large mammal species respectively.
🔍 Worked example — Exam-style
An ecosystem is suffering from a decline in a keystone predator. Explain how 'ecotourism' could act as a conservation method to prevent extinction.
⚠️ Common errors
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🎯 Exam question
(a) State two ways the orchid population could be conserved. [2] (b) Explain why maintaining genetic variation is important for the survival of the orchid species. [2] [4 marks]
Key ideas — Why conserve, and the problems of small populations
Reasons for conservation programmes, and the tools of captive breeding. Conservation programmes aim to reduce extinction, protect vulnerable ecosystems such as coral reefs and rainforests, and maintain ecosystem functions on which humans depend: nutrient cycling by decomposers and nitrogen-fixing bacteria, and resource provision of food, drugs (many medicines come from plants), fuel and genes (wild relatives of crops carry alleles for disease resistance that breeders may need).
🧑🏫 Teacher's explanation
Think of a species like a giant library of books. Each book is a different 'allele' (a version of a gene). If the library is huge, you have many different stories to choose from. If the library burns down and you only have 5 books left, you lose most of your stories. If a new disease arrives that requires a specific 'story' to survive, and you've lost that book, the whole library is in trouble. This is why small populations are dangerous: they lose the 'instruction manual' for survival.
📖 Definitions
🔍 Worked example — Easy
A population of cheetahs has decreased from 500 individuals to 50 individuals. Calculate the percentage decrease in the population size.
🔍 Worked example — Standard
Explain why a decrease in population size leads to an increased risk of extinction for a species.
🔍 Worked example — Exam-style
A conservationist uses IVF to increase the offspring of a rare bird. In one cycle, a female produces 4 embryos, and 3 survive to hatching. If the goal is to increase the population by 20 individuals, how many successful IVF cycles are required if each successful cycle results in 3 surviving offspring?
⚠️ Common errors
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🎯 Exam question
(a) State two reasons why a small population size increases the risk of extinction. [2] (b) Describe how captive breeding using In Vitro Fertilisation (IVF) can help to increase the population size of this species. [2] [4 marks]
Key ideas — Sewage treatment and recycling
Sewage treatment protects health and ecosystems: it removes pathogens that cause cholera and typhoid, and prevents eutrophication (lesson 24.20.3). In a treatment works, screening removes large objects; sedimentation lets solids (sludge) settle; the liquid is treated aerobically in trickling filters or aeration tanks, where bacteria break down organic matter; sludge is digested anaerobically, producing methane used as fuel; and the cleaned water is returned to rivers.
🧑🏫 Teacher's explanation
Think of sewage treatment like a massive water purification factory that cleans up the mess humans make so we don't poison our rivers. If we just dumped raw sewage into a river, it would be like dumping a bucket of rotting food into a clean fish tank; the bacteria in the water would go crazy, use up all the oxygen, and kill everything.
📖 Definitions
🔍 Worked example — Easy
Identify the stage in sewage treatment where large, visible objects like plastic bottles or rags are removed.
🔍 Worked example — Standard
Explain why the aeration of sewage in a treatment plant is necessary for the removal of organic matter.
🔍 Worked example — Exam-style
A factory accidentally leaks high levels of nitrates into a nearby lake. Describe the sequence of events that leads to the death of fish in this lake.
⚠️ Common errors
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🎯 Exam question
(a) State the type of respiration used by the bacteria in this stage. [1] (b) Explain how this process helps to prevent the death of fish in the river where the treated water is eventually released. [3] [4 marks]
Worked example
Question. Explain how fish stocks in a sea area can be conserved so that fishing can continue in the future. [4]
🔍 Worked example — Easy
A fishery removes 5,000 tonnes of tuna per year. The natural reproduction rate of the tuna population in that area is 4,500 tonnes per year. Calculate the percentage change in the total tuna biomass after one year.
🔍 Worked example — Standard
A conservationist uses a captive breeding programme to increase the population of a rare frog. The initial population is 20 individuals. After using artificial insemination (AI), the population increases to 120 individuals. Calculate the factor by which the population has increased.
🔍 Worked example — Exam-style
Explain why a decrease in population size leads to a higher risk of extinction for a species. [3]
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Practice
Answers