Ecosystems (OCR A Level Biology): Exam Questions

Algae are photosynthetic organisms that live in water.

A rapid increase in the population of algae is known as an algal bloom.

Scientists studied the population of algae in a river in the UK at different times of year. Their results are shown in Fig. 21.1.

Fig. 21.1

(i) Calculate the percentage decrease in the population of algae between the peak population and 1 November.

percentage decrease = .......................................................... [2]

(ii) The river in which the study was conducted is described as a dynamic ecosystem.

Use Fig. 21.1 to explain why this ecosystem could be described as dynamic.

 [2]

(iii) A student concluded that the increase in population of algae was due to higher temperatures and higher light intensity in the summer months.

Considering Fig. 21.1 as the student’s data source, discuss the weaknesses in this conclusion.

 [3]

Accurate analysis of an ecosystem’s biodiversity requires a detailed classification of organisms.

The spruce pine plant is given the binomial name Pinus glabra.

(i) Place a tick (✓) in the box next to the species most closely related to Pinus glabra.

[1]

(ii) Explain why Pinus glabra and humans, Homo sapiens, are classified in the same domain but in different kingdoms.

[2]

A scientist sampled the species of trees present in two different habitats containing Pinus glabra.

The results of the sampling are shown in Table 5.

Table 5

Using Simpson’s Index of Diversity, the scientist calculated the biodiversity (D) of Habitat A as 0.71.

Use the formula given to calculate the biodiversity of Habitat B.

Show your working.

State which habitat, A or B, has the greater biodiversity.

D = 1 - 

D (Habitat A) = 0.71
D (Habitat B) = .........
Habitat with the greater biodiversity = ..........

Habitat B was situated beside a lake and showed evidence of ecological succession.

The scientist planned to investigate how the biodiversity changed from the edge of the lake to the other side of habitat B.

(i) State the collective name of the animal and plant populations that are present at the end of primary succession.

[1]

(ii) Suggest how the scientist could achieve the following during their investigation:

Sample all stages of succession in the habitat ..................... Minimise sampling bias ...................
Sample insect biodiversity ..................

[3]

(iii) The scientist also measured primary production in both the woodland and lake habitats.
Suggest the units the scientist should use to measure primary production in the two habitats.

Woodland  ...................
Lake  ...................

[1]

Nitrogen cycling within ecosystems is controlled by various bacterial species. The table below lists four groups of bacterium that are involved in the nitrogen cycle. 

Complete the table to show the locations of each type of bacterium in the cycle and the reactions they perform.

Nitrogen fixation is an important part of the nitrogen cycle.

The rate of nitrogen fixation is reduced by the presence of oxygen.

Rhizobium uses the enzyme nitrogenase to fix atmospheric nitrogen.

Fig. 4 shows a simplified representation of the structure of nitrogenase and the reaction that it catalyses.

Fig. 4

(i) What can you conclude from Fig. 4 about the molecules or ions that affect the functioning of the nitrogenase enzyme?

[4]

(ii) Leghaemoglobin is a molecule that improves the performance of nitrogenase. It has very similar properties to mammalian haemoglobin.

Suggest two ways in which leghaemoglobin improves the performance of the nitrogenase enzyme.

[2]

Many species of bacteria act as decomposers within ecosystems by breaking down organic material.

Scientists analysed the energy flow within a grassland ecosystem.

They estimated that the energy in the decomposers’ trophic level was 950 000 Jm^-2 yr^-1.

The energy within the producers’ trophic level was 800% greater than that of the decomposers.

(i) Calculate the energy in the producers’ trophic level in kJm^-2 yr ^-1.

Answer: ................. kJm^-2 yr ^-1 [2]

(ii) Calculate the percentage efficiency of the energy transfer from producers to decomposers.

Give your answer to two significant figures.

Answer: ................% [1]

This Question is about ecosystems in the Southern (Antarctic) Ocean.

Observe the food chain:

phytoplankton (producers) → krill (shrimps etc.) → small fish → large fish → seals

Table 21.1 shows the transfers of energy and the quantities of energy stored as biomasses for the food chain. Magnitudes are given in kilojoules per square metre of sea surface per year.

Table 21.1

(i) For larger and less numerous organisms, such as the seal, it is more appropriate to record energy flows per square kilometre.

Calculate the energy input to the seal population from large fish. Record your answer in kilojoules per square kilometre of sea surface per year.

[2]

(ii) Calculate the percentage of energy stored in large fish biomass converted to energy in seal biomass.

[2]

(i) The biomass of large fish in the Southern Ocean is a food resource for humans. It is increasingly harvested by powerful, long-distance trawlers. If over-exploited, the Southern Ocean ecosystem may be permanently altered.

• Suggest two measures that an international treaty might impose, to prevent fishing from causing permanent damage to the Southern Ocean.

• Identify the practical difficulties that might prevent your two measures from being effective.

First measure .............................................................

Difficulty.........................................................

Second measure................................................

Difficulty..................................................

[4]

(ii) Krill can also be harvested as a human food source.
The fishing industry aims to harvest large fish.
Some environmentalists say that krill harvesting should be increased.

Use this information and Table 21.1 to put forward arguments for and against harvesting krill instead of large fish as a human food source.

[2]

A small, permanent pond is the habitat for a climax community of producers (aquatic plants and algae) and consumers (bacteria, protoctista, worms, snails, arthropods and small vertebrates like newts and fish).

Why might ecologists call this a ‘climax community’?

The protoctist Paramecium caudatum is usually between 200 and 300 μm in length. An accurate measurement would help in the correct identification of a specimen from this pond.

What laboratory equipment would you select to make an accurate measurement of the length of Paramecium caudatum?

An animal fell into the pond. It drowned and decayed. Within a year the biological compounds in its body had been completely recycled.

(i) What nitrogenous excretory molecule from the decomposers would pass to the next stage of the nitrogen cycle?

[1]

(ii) Complete the flow chart to show what happens to this nitrogenous compound, and name the groups of bacteria involved at steps 1 and 2, as it is converted to a form that plants can take up and use.

[4]