Population Viability Analysis - IELTS Reading Answers & Explanations
From Cambridge IELTS 01 Academic Reading Test 4 · Part 3 · Questions 28–39
Reading Passage
You should spend about 20 minutes on Questions 28-39 which are based on Reading Passage 3 below.
Population viability analysis
Part A
To make political decisions about the extent and type of forestry in a region it is important to understand the consequences of those decisions. One tool for assessing the impact of forestry on the ecosystem is population viability analysis (PVA). This is a tool for predicting the probability that a species will become extinct in a particular region over a specific period. It has been successfully used in the United States to provide input into resource exploitation decisions and assist wildlife managers and there is now enormous potential for using population viability to assist wildlife management in Australia’s forests.
A species becomes extinct when the last individual dies. This observation is a useful starting point for any discussion of extinction as it highlights the role of luck and chance in the extinction process. To make a prediction about extinction we need to understand the processes that can contribute to it and these fall into four broad categories which are discussed below.
Part B
A Early attempts to predict population viability were based on demographic uncertainty Whether an individual survives from one year to the next will largely be a matter of chance. Some pairs may produce several young in a single year while others may produce none in that same year. Small populations will fluctuate enormously because of the random nature of birth and death and these chance fluctuations can cause species extinctions even if, on average, the population size should increase. Taking only this uncertainty of ability to reproduce into account, extinction is unlikely if the number of individuals in a population is above about 50 and the population is growing.
B Small populations cannot avoid a certain amount of inbreeding. This is particularly true if there is a very small number of one sex. For example, if there are only 20 individuals of a species and only one is a male, all future individuals in the species must be descended from that one male. For most animal species such individuals are less likely to survive and reproduce. Inbreeding increases the chance of extinction.
C Variation within a species is the raw material upon which natural selection acts. Without genetic variability a species lacks the capacity to evolve and cannot adapt to changes in its environment or to new predators and new diseases. The loss of genetic diversity associated with reductions in population size will contribute to the likelihood of extinction.
D Recent research has shown that other factors need to be considered. Australia’s environment fluctuates enormously from year to year. These fluctuations add yet another degree of uncertainty to the survival of many species. Catastrophes such as fire, flood, drought or epidemic may reduce population sizes to a small fraction of their average level. When allowance is made for these two additional elements of uncertainty the population size necessary to be confident of persistence for a few hundred years may increase to several thousand.
Part C
Beside these processes we need to bear in mind the distribution of a population. A species that occurs in five isolated places each containing 20 individuals will not have the same probability of extinction as a species with a single population of 100 individuals in a single locality.
Where logging occurs (that is, the cutting down of forests for timber) forest-dependent creatures in that area will be forced to leave. Ground-dwelling herbivores may return within a decade. However, arboreal marsupials (that is animals which live in trees) may not recover to pre-logging densities for over a century. As more forests are logged, animal population sizes will be reduced further. Regardless of the theory or model that we choose, a reduction in population size decreases the genetic diversity of a population and increases the probability of extinction because of any or all of the processes listed above. It is therefore a scientific fact that increasing the area that is loaded in any region will increase the probability that forest-dependent animals will become extinct.
Questions
Questions 28–31 Yes / No / Not Given
Do the following statements agree with the views of the writer in Part A of Reading Passage 3?
Questions 32–35 Matching Headings
These questions are based on Part B of Reading Passage 3.
In paragraphs A to D the author describes four processes which may contribute to the extinction of a species. Match the list of processes (i-vi) to the paragraphs.
NB There are more processes than paragraphs so you will not use all of them.
i. Loss of ability to adapt
ii. Natural disasters
iii. An imblance of the sexes
iv. Human disasters
v. Evolution
vi. The haphazard nature of reproduction
Questions 36–38 Sentence Completion
Based on your reading of Part C, complete the sentences below with words taken from the passage. Use NO MORE THAN THREE WORDS for each answer.
- While the population of a species may be on the increase, there is always a chance that small isolated groups are susceptible to 36
- Survival of a species depends on a balance between the size of a population and its 37
- The likelihood that animals which live in forests will become extinct is increased when 38 takes place
Questions 39–39 Multiple Choice (One Answer)
Choose the appropriate letter A-D.
Answers & Explanations Summary
| # | Answer | Evidence | Explanation |
|---|---|---|---|
| Q28 | YES | One tool for assessing the impact of forestry on the ecosystem is population viability analysis (PVA) | Excerpt/Passage Explanation: The passage says that there is a special method used to measure how the work of cutting down trees changes the natural world and the animals in it. Answer Explanation: The answer means that experts who study nature want to find out how cutting down forests affects the animals that live there naturally. Reason For Correctness: The correct answer is YES because the writer describes a specific scientific tool used to look at the 'impact' (the effect) of 'forestry' (the business of cutting down trees) on the 'ecosystem' (the environment including plants and animals). This tool helps calculate if a 'species' (a type of animal) is at risk of dying out, which shows that researchers are studying these effects. |
| Q29 | NO | It has been successfully used in the United States to provide input into resource exploitation decisions and assist wildlife managers and there is now enormous potential for using population viability to assist wildlife management in Australia’s forests | Excerpt/Passage Explanation: The passage says that while the United States has already used this system, in Australia, there is a big opportunity to use it to help take care of forests and animals, meaning it is not yet a common old method there. Answer Explanation: The answer is 'NO' because the statement 'PVA has been used in Australia for many years' is the opposite of what the text says. Reason For Correctness: The correct answer is 'NO' because the text distinguishes between where PVA has already been used and where it could be used. It mentions that the United States has successfully used PVA. In contrast, it states that there is 'enormous potential' for using it in Australia. The word 'potential' refers to something that is possible in the future or has the ability to be developed; it does not mean it has already been used for a long time. Therefore, the statement contradicts the passage's claim that its use in Australia is a future opportunity rather than a past history. |
| Q30 | NO | A species becomes extinct when the last individual dies | Excerpt/Passage Explanation: The passage explains that for a group of animals to be called extinct, the very last animal in that group must no longer be alive. Answer Explanation: The answer means the statement is incorrect because a species is not considered extinct as long as one member of it is still living. Reason For Correctness: The correct answer is NO because the writer defines extinction based on the death of the final member of a species. If there is still one individual alive, the species has not reached the state of extinction yet. In the text, the author specifically links the moment of extinction to when the 'last individual dies', meaning zero members are left. |
| Q31 | NOT GIVEN | A species becomes extinct when the last individual dies. This observation is a useful starting point for any discussion of extinction as it highlights the role of luck and chance in the extinction process | Excerpt/Passage Explanation: The passage defines the moment of extinction and mentions that luck plays a part in it, but it does not say whether the process of extinction is considered a natural occurrence. Answer Explanation: The answer 'NOT GIVEN' means that the passage does not provide enough information to know if the writer believes extinction is a natural event that happens on its own. Reason For Correctness: The correct answer is 'NOT GIVEN' because, in Part A, the author focuses on defining what extinction is (the death of the last individual) and explaining how humans can use a tool called PVA to predict it. While the text mentions 'luck and chance' and the impact of human activities like 'forestry' and 'resource exploitation,' it never explicitly states that extinction is a 'naturally occurring phenomenon.' Because the writer's specific view on whether extinction is natural is missing from this section, we cannot confirm or deny the statement. |
| Q32 | vi | Whether an individual survives from one year to the next will largely be a matter of chance. Some pairs may produce several young in a single year while others may produce none in that same year. Small populations will fluctuate enormously because of the random nature of birth and death and these chance fluctuations can cause species extinctions even if, on average, the population size should increase | Excerpt/Passage Explanation: The passage says that living or having babies is not certain and depends on luck. It mentions that birth and death have a 'random nature,' meaning they do not follow a strict schedule. This random way of having babies is what the phrase 'haphazard nature of reproduction' means. Answer Explanation: The answer 'vi' matches Paragraph A with the idea that having babies and surviving happens mostly by luck or chance. Reason For Correctness: The correct answer is 'vi' because Paragraph A explains that the survival of animals and the number of babies they have are a 'matter of chance.' The text uses words like 'random nature' and 'chance fluctuations' to describe birth and death. The word 'haphazard' means something that happens by chance or without a clear plan, which is exactly how the paragraph describes 'reproduction' (the process of having babies/birth) in small populations. |
| Q33 | iii | Small populations cannot avoid a certain amount of inbreeding. This is particularly true if there is a very small number of one sex. For example, if there are only 20 individuals of a species and only one is a male, all future individuals in the species must be descended from that one male | Excerpt/Passage Explanation: The passage explains that if a group has very few of one gender, like having only one male, the species has trouble staying healthy and growing. Answer Explanation: The answer refers to a situation where the number of males and females in a group is not equal or even. Reason For Correctness: The correct answer is iii because Paragraph B discusses factors that make it harder for a species to survive. It specifically points out that having a 'very small number of one sex' causes problems. For example, if there is only one male in a group of twenty, it leads to inbreeding. This 'imbalance' (not having enough of one gender) makes the animals less healthy and more likely to go extinct. Keywords such as 'one sex' and 'one is a male' directly relate to the concept of sex imbalance. |
| Q34 | i | Without genetic variability a species lacks the capacity to evolve and cannot adapt to changes in its environment or to new predators and new diseases | Excerpt/Passage Explanation: The passage states that if a group of animals does not have enough genetic differences, it loses the power to evolve and cannot change itself to stay safe when its world changes or when new sicknesses appear. Answer Explanation: The answer 'i' means that Paragraph C explains how a species might die out if it loses its ability to change and adjust to new situations. Reason For Correctness: The correct answer is 'i' because Paragraph C discusses how a lack of genetic variety prevents a species from evolving. Without the ability to evolve, the animals cannot survive changes in their surroundings, new hunters, or new illnesses. The paragraph specifically uses the word 'adapt' when describing what the species can no longer do when it loses its genetic diversity. |
| Q35 | ii | Catastrophes such as fire, flood, drought or epidemic may reduce population sizes to a small fraction of their average level | Excerpt/Passage Explanation: The passage states that major natural events like fires, floods, or diseases can happen and suddenly make the number of animals in a group very small. Answer Explanation: The answer means that the fourth paragraph (D) discusses how big, sudden events in nature can cause species to die out. Reason For Correctness: The correct answer is linked to Paragraph D because this section mentions 'catastrophes', which is a synonym for 'natural disasters'. The text specifically lists examples like fire, flood, and drought. These environment-related events add a level of uncertainty to survival because they can quickly decrease the number of animals in a population. |
| Q36 | extinction / extinctions | Small populations will fluctuate enormously because of the random nature of birth and death and these chance fluctuations can cause species extinctions even if, on average, the population size should increase | Excerpt/Passage Explanation: The passage explains that small groups of animals can change in number very quickly because of luck in births and deaths. These random changes can cause a whole group to die out (extinction), even if the total population is usually growing. Answer Explanation: The answer 'extinction/extinctions' refers to the event where every member of a specific animal group dies, meaning the group no longer exists. Reason For Correctness: The correct answer is extinction because the passage states that 'chance fluctuations' (random changes) can cause 'extinctions' in small populations. Even when the population size 'should increase' (which means it is 'on the increase'), these small groups are still 'susceptible to' or at risk of dying out completely. The word 'susceptible' in the question matches the idea that these random events can 'cause' the species to disappear. |
| Q37 | locality / distribution | Beside these processes we need to bear in mind the distribution of a population | Excerpt/Passage Explanation: The passage says that along with other factors, we must think about how the total group of animals is divided or spread across different locations. Answer Explanation: The answer means the specific area or way a group of animals is spread out across a region. Reason For Correctness: The correct answer is found in the first paragraph of Part C. The text explains that knowing the total number of animals (the size) is not enough to understand if they will survive. It states that we also need to consider how those animals are spread out across different areas. For example, the risk of dying out is different if a group is all in one place versus being split into several smaller, separate groups. Therefore, the survival of a species depends on both its total length or size and how it is spread out, which the text calls its distribution or locality. |
| Q38 | logging | It is therefore a scientific fact that increasing the area that is loaded in any region will increase the probability that forest-dependent animals will become extinct | Excerpt/Passage Explanation: The passage states as a fact that when more forest land is used for logging (here written as 'loaded' in the text), it becomes more likely that the animals living in those forests will disappear forever. Answer Explanation: The answer means that the danger of animals dying out completely grows higher when humans cut down trees for wood in certain areas. Reason For Correctness: The correct answer is found in Part C, which explains the relationship between human activities and animal survival. The passage notes that cutting down trees (logging) pushes animals out of their homes. It concludes that as we cut down more trees in any given area, the mathematical chance (or 'probability') that forest-dwelling animals will die out forever (extinction) goes up. |
| Q39 | B | This is a tool for predicting the probability that a species will become extinct in a particular region over a specific period To make a prediction about extinction we need to understand the processes that can contribute to it and these fall into four broad categories which are discussed below |
Excerpt/Passage Explanation: The passage says that there is a way to guess how likely it is for animals to stop existing. It explains that to do this, we must look at four different groups of reasons that can lead to extinction. Answer Explanation: The answer means the text is about the different things that change the chance of a group of animals living or dying out. Reason For Correctness: The correct answer is B because the whole passage focuses on how to calculate if a species will survive or go extinct. Part A introduces a tool used to check this probability. Part B then explains four main categories of factors, or 'influential factors' (such as births, breeding issues, and the environment), that change this chance. The words 'predicting the probability' in the text match 'assessing survival probability' in the answer, while 'processes' and 'categories' match the idea of 'influential factors'. |