Photo AI
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 a... show full transcript
Worked Solution & Example Answer:Read the following passage - AQA - A-Level Biology - Question 10 - 2021 - Paper 1
Step 1
Lake Malawi in East Africa has more species of fish than any other lake in the world (line 1). Suggest and explain how this speciation may have occurred.
Answer
Speciation in Lake Malawi likely occurred due to the geographical isolation of fish populations resulting from the fluctuating water levels over millions of years. When the lake was smaller, subpopulations were separated, allowing for distinct evolutionary paths due to environmental pressures and ecological niches. Over time, these separate populations adapted to their unique habitats, leading to the emergence of new species. The concept of adaptive radiation explains this process, where species diversify rapidly to fill different ecological roles. Genetic divergence occurred, reinforcing the differences between populations.
Step 2
The percentage of forest cover in Malawi decreased between December 1990 and December 2016 (lines 9–10). Calculate the mean loss of forest cover in km² per week during this time period.
Answer
First, we need to calculate the area of forest cover in both years:
- In December 1990: 41.4% of 94,800 km² =
- In December 2016: 26.4% of 94,800 km² =
Next, we find the difference in forest cover:
The time period from 1990 to 2016 is 26 years, or weeks.
Thus, the mean loss of forest cover per week is:
Step 3
Loss of nutrients into Lake Malawi has resulted in a decrease in some fish populations (lines 12–13). Explain why.
Answer
Nutrient loss into Lake Malawi leads to a decrease in fish populations primarily due to its impact on the ecosystem's food web. When soil erosion increases, it causes sedimentation and the loss of essential nutrients that algae and aquatic plants need for photosynthesis. A decrease in these primary producers results in less food availability for herbivorous fish, disrupting the entire food chain. Additionally, when nutrients are washed into the lake in high quantities, it can lead to algal blooms, which reduce oxygen levels in the water, creating a hypoxic environment where fish cannot thrive. Hence, decreased nutrient availability directly affects fish health and population dynamics.
Step 4
The mark-release-recapture method can be used to estimate the size of a fish population (lines 13–14). Explain how.
Answer
The mark-release-recapture method involves three key steps: First, a sample of fish is captured from the population, marked (using a non-toxic method), and then released back into the water. After allowing time for the marked fish to mix with the population, a second sample is captured. The number of marked fish in this second sample is counted. The population size can then be estimated using the formula:
where N is the estimated population size, M is the number of marked fish in the first sample, C is the total number of fish captured in the second sample, and R is the number of marked fish recaptured. This method is effective as it gives an estimate of the population by using proportions, assuming random mixing of the marked and unmarked fish.
Step 5
Suggest why the mark-release-recapture method can produce unreliable results in very large lakes (lines 14–15).
Answer
The mark-release-recapture method can produce unreliable results in large lakes primarily due to the difficulty in ensuring that the marked fish distribute evenly throughout the population. In expansive areas, fish may cluster or remain in isolated pockets, leading to biases in sampling. Factors such as varying environmental conditions, heterogeneous habitats, and human activity can also affect fish behavior post-release. Additionally, if the capture technique is harmful or affects the survival of marked fish, this can lead to an underrepresentation of marked individuals in subsequent samplings, skewing results and leading to inaccurate population estimates.