Read the following passage. Lake Malawi in East Africa has more species of fish than any other lake in the world. Many of these species have evolved from a common ancestor. Lake Malawi is one of the largest lakes in the world and was formed several million years ago. Since then, the water level has fluctuated greatly. As a result, what is now a large lake was at one time many smaller, separate lakes. The country of Malawi has a total area of 118,000 km². The actual land area is only 94,800 km², because approximately one-fifth of the country is Lake Malawi. In December 1990, forests covered 41.4% of the actual land area of Malawi. In December 2016, forests covered 26.4% of the actual land area of Malawi. Deforestation and farming along the shores of Lake Malawi have caused increased soil erosion and loss of nutrients into the lake. This has resulted in a decrease in some populations. The mark-release-recapture method can be used to estimate the size of a fish population. However, this method can produce unreliable results in very large lakes.

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Read the following passage - AQA - A-Level Biology - Question 10 - 2021 - Paper 1

Question 10

Read the following passage. Lake Malawi in East Africa has more species of fish than any other lake in the world. Many of these species have evolved from a common ... show full transcript

Worked Solution & Example Answer:Read the following passage - AQA - A-Level Biology - Question 10 - 2021 - Paper 1

Step 1

Suggest and explain how this speciation may have occurred.

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Answer

Speciation in Lake Malawi may have occurred through the isolation of fish populations in smaller lakes during periods of lower water levels. As these populations became separated, they underwent different selective pressures, leading to adaptations unique to each population.

Due to limited gene flow between the isolated pools, genetic divergence increased, eventually resulting in different alleles being passed on and selected in each population. Over time, these differences could have accumulated to the point where the groups could no longer interbreed, thus forming distinct species.

Step 2

Calculate the mean loss of forest cover in km² per week during this time period.

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Answer

To calculate the mean loss of forest cover, we first determine the forest cover in December 1990 and December 2016.

In December 1990, forest cover was 41.4% of 94,800 km²: $ext{Forest Cover}_{1990} = 0.414 imes 94800 = 39,228 ext{ km}^2$

In December 2016, forest cover was 26.4% of 94,800 km²: $ext{Forest Cover}_{2016} = 0.264 imes 94800 = 25,068.8 ext{ km}^2$

Next, we find the total loss of forest cover: $ext{Loss} = ext{Forest Cover}_{1990} - ext{Forest Cover}_{2016} = 39,228 - 25,068.8 = 14,159.2 ext{ km}^2$

The time period from December 1990 to December 2016 is 26 years, which is equal to 26 x 52 = 1,352 weeks.

Hence, the mean loss per week is:

ext{Mean Loss} \ ext{(in km}^2 ext{/week)} \\approx 10.47 ext{ km}^2/ ext{week}$$

Step 3

Explain why loss of nutrients into Lake Malawi has resulted in a decrease in some fish populations.

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Answer

The loss of nutrients into Lake Malawi can lead to an increase in algal blooms due to excess nutrients (eutrophication). These blooms can block sunlight from penetrating the water, affecting the growth of submerged plants which are essential for oxygen production and habitat stability.

Moreover, when the algae die and decompose, they consume a significant amount of oxygen in the water due to the activity of decomposing microorganisms, leading to hypoxic conditions. Low oxygen levels can negatively affect fish survival rates, particularly for species that require higher oxygen levels for respiration and overall health.

Step 4

Explain how the mark-release-recapture method can be used to estimate the size of a fish population.

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Answer

The mark-release-recapture method involves several steps:

Capture: A sample of fish is captured and marked in a way that does not harm them.
Release: The marked fish are then released back into the water.
Wait: After some time, researchers take a second sample of fish from the same area.
Recapture: The number of marked fish in the second sample is noted.

The population size can be estimated using the formula: $N = rac{(M imes C)}{R}$ where:

$N$ = population size,
$M$ = number of marked fish in the first sample,
$C$ = total number of fish in the second sample,
$R$ = number of marked fish recaptured in the second sample.

Step 5

Suggest why the mark-release-recapture method can produce unreliable results in very large lakes.

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Answer

In large lakes, the mark-release-recapture method may produce unreliable results due to several factors:

Low Recapture Probabilities: The large area may result in marked fish being widely dispersed, making it unlikely that they will be recaptured during the second sampling.
Movement Patterns: Fish may migrate to different areas within the lake, reducing the number of marked fish in the sampled area.
Sampling Errors: If the sampling is not random, or if certain habitats are over-represented, the results may not accurately reflect the population.
Population Dynamics: Changes in the population due to reproduction, death, immigration, or emigration during the study period can lead to discrepancies, further complicating accurate estimations.

Read the following passage - AQA - A-Level Biology - Question 10 - 2021 - Paper 1

Worked Solution & Example Answer:Read the following passage - AQA - A-Level Biology - Question 10 - 2021 - Paper 1

Suggest and explain how this speciation may have occurred.

Calculate the mean loss of forest cover in km² per week during this time period.

Explain why loss of nutrients into Lake Malawi has resulted in a decrease in some fish populations.

Explain how the mark-release-recapture method can be used to estimate the size of a fish population.

Suggest why the mark-release-recapture method can produce unreliable results in very large lakes.

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