Populations in ecosystems

In tropical rainforests, there is a high biodiversity of animal species (lines 1–3).

Explain why.

Nutrient recycling occurs rapidly on the ground of tropical rainforests (lines 4–5).

Explain how two abiotic factors in these rainforests enable the rapid rate of recycling.

The recycling of nitrogen-containing compounds from dead organic matter sustains the growth of plants (lines 5–7).

Describe the roles of bacteria in this recycling.

Natural regeneration of rainforests occurs by secondary succession (lines 8–10).

Describe the process of secondary succession.

Farming practices have prevented the natural regeneration of rainforests by secondary succession (lines 12–14).

Suggest two farming practices that prevent the natural regeneration of rainforests by secondary succession.

Tansy, Tanacetum vulgare, is a plant that grows in dense patches. The tansy beetle, Chrysolina graminis, feeds on, and completes its life cycle on T. vulgare.

C. graminis is 8–10 mm long and green in colour.

Two other green beetles can also be found in the same habitat.

• The dock beetle, Gastrophysa viridula, which is 4–7 mm long.

• The mint leaf beetle, Chrysolina herbacea, which is 6–10 mm long.

Figure 2 shows the life cycle of C. graminis.

How many different genera of beetles are there in the information provided?

Ecologists investigated the population size of C. graminis using the

mark-release-recapture method.

Table 2 shows the ecologists’ results.

The ecologists captured and marked the first sample of beetles during the second week of August. They identified the species of beetle before marking them.

Use the information provided to suggest:

two reasons why the first sample of beetles was captured during the second week of August

one reason why the beetles had to be identified before being marked.

Use the data provided in Table 2 to estimate the population size of C. graminis.

Show your working.

The ecologists repeated the investigation over a five–year period to monitor the population size of C. graminis.

State two precautions they would have taken each year to obtain comparable data.

A weed is a plant growing where it is not wanted. Fat hen is a weed which grows in some crop fields.

Describe how you could estimate the number of fat hen plants in a field.

In potato plant fields, fat hen plants can grow up to a height of 2 m and absorb large quantities of nutrients from the soil.

Fat hen has a negative effect on the growth of potato plants.

Use the information provided to explain why.

In your answer, name the type of competition occurring between fat hen plants and potato plants.

Table 1 gives some features of fat hen seeds.

During a growing season, it was estimated that a total number of 550 fat hen plants grew on the fields of a farm.

Calculate the maximum mass, in kg, of viable fat hen seeds on this farm after a growing season.

Show your working.

Assume that all the seeds produced during the growing season remain on the farm.

Read the following passage.

The wall gecko is a medium-sized lizard. In an isolated habitat of southern Italy, the wall gecko shows phenotypic diversity. Scientists investigated whether disruptive selection was leading to sympatric speciation in the wall gecko.

Pale geckos live only on walls and are nocturnal (active at night). Dark geckos 5 live mainly on the dark trunks of olive trees and are diurnal (active during the day). These diurnal geckos can change skin colour when occupying different surfaces during the day.

Comparison of mitochondrial genes indicated that the diurnal geckos formed a distinct genetic group. This comparison also confirmed that all the geckos in 10 the habitat were of the same species.

The scientists used the mark-release-recapture method to estimate the size of the population of geckos in the habitat.

Use the information in the passage and your own knowledge to answer the following questions.

The wall gecko shows phenotypic diversity (lines 1–2).

Suggest two factors that have resulted in this phenotypic diversity.

The ability of diurnal geckos to change skin colour (lines 7–8) is advantageous.

Explain why.

The scientists concluded that it was probable that disruptive selection was leading to sympatric speciation in the wall gecko.

Use the information in the passage to evaluate this conclusion.

Explain how comparison of mitochondrial genes could indicate that the nocturnal geckos formed a distinct genetic group (lines 9–10).

In your answer, explain how new techniques enable the comparison of genes to be completed rapidly.

Describe and explain two precautions required to ensure that the estimate of the size of the population of geckos was valid (lines 12–13).

Do not include sample size as one of the required precautions.

In your answer, include the formula to estimate the size of the population using the mark-release-recapture method.

Define the biological term population.

Ecologists monitored the biodiversity of birds in a protected woodland.

They recorded the number of bird species breeding in the woodland on the same day, every year for a 30-year period.

Figure 4 shows their results.

Figure 4

Figure 4 not reproduced here due to third-party copyright restrictions

The figure is taken from the following website:

Figure 1a. Anne E. Magurran, Stephen R. Baillie, Stephen T. Buckland, Jan McP. Dick, David A. Elston, E. Marian Scott, Rognvald I. Smith, Paul J. Somerfield, Allan D. Watt, Long-term datasets in biodiversity research and monitoring: assessing change in ecological communities through time, Trends in Ecology & Evolution, Volume 25, Issue 10, 2010, Pages 574-582, ISSN 0169-5347, https://doi.org/10.1016/j.tree.2010.06.016.

Which statement correctly represents the species richness of the woodland?

Tick () one box.

A student studying the information in this question concluded that all woodlands should be protected to increase the biodiversity of birds.

Evaluate the student’s conclusion.

It is not possible to extrapolate the data for the number of bird species recorded breeding each year beyond 1979.

Explain why.

In this woodland, the ecologists measured the similarity in the bird community by comparing each year to 1949 using an index of similarity.

This index ranges from 1.0 for total similarity to 0.0 for total dissimilarity.

Figure 5 shows their results.

Figure 5

Figure 5 not reproduced here due to third-party copyright restrictions

The figure is taken from the following website:

Figure 1b, Anne E. Magurran, Stephen R. Baillie, Stephen T. Buckland, Jan McP. Dick, David A. Elston, E. Marian Scott, Rognvald I. Smith, Paul J. Somerfield, Allan D. Watt, Long-term datasets in biodiversity research and monitoring: assessing change in ecological communities through time, Trends in Ecology & Evolution, Volume 25, Issue 10, 2010, Pages 574-582, ISSN 0169-5347, https://doi.org/10.1016/j.tree.2010.06.016.

Suggest how the changes in the index of similarity in the bird community provide evidence for the process of succession.

In Figure 5, the index of similarity for the last 10 years remains fairly constant.

Name the stage of a succession this represents.

Suggest one reason why the index of similarity is not absolutely constant.

The Living Planet Index (LPI) is an index designed to monitor the state of the world’s biodiversity.

The LPI is arbitrarily scaled to be 1.0 in 1970, the baseline year.

Figure 6 shows the LPI from the Living Planet Report, 2008. The dotted lines represent ± 2 standard deviations from the mean, which includes over 95% of the data.

Figure 7 shows an alternative version of Figure 6 published on a news website.

Figure 6 Figure 7

Figures 6 and 7 not reproduced here due to third-party copyright restrictions

The figures are taken from the following website:

Figure 2a and 2b, Anne E. Magurran, Stephen R. Baillie, Stephen T. Buckland, Jan McP. Dick, David A. Elston, E. Marian Scott, Rognvald I. Smith, Paul J. Somerfield, Allan D. Watt, Long-term datasets in biodiversity research and monitoring: assessing change in ecological communities through time, Trends in Ecology & Evolution, Volume 25, Issue 10, 2010, Pages 574-582, ISSN 0169-5347, https://doi.org/10.1016/j.tree.2010.06.016.

The news website published the headline:

The LPI shows human activities cause significant decrease in biodiversity.

Suggest three reasons why this headline is not valid.

Use all the information provided.

Turn over for Section B

Section B

one question.

You are advised to spend no more than 45 minutes on this section.

In the following passage, the numbered spaces can be filled with biological terms.

An ecosystem supports a certain size of population of a species, called

Write the correct biological term beside each number below that matches the space in the passage.

Suggest two reasons for conserving rainforests.

Give three reasons for the low efficiency of energy transfer from secondary consumers to tertiary consumers in an ecosystem.

Lemurs are small mammals. Lemurs live in trees and feed on leaves and fruit. Scientists used a computer program to predict the expected distribution of two species of lemur, Eulemur rufus and Eulemur rufifrons, on the island of Madagascar. These predictions were based on the environmental needs of each species. Then, the scientists determined the actual distribution of these two species of lemur on the island of Madagascar.

Figure 2 shows the scientists’ results.

Using Figure 2, give three conclusions you can make about the distribution of these lemur species.

Using all the information, suggest how speciation happened to produce two species of lemur.

The scientists used the mark-release-recapture method to determine the number of lemurs in one area of forest. They captured, marked and released a first sample of 30 lemurs. A week later, they captured a second sample of 25 lemurs from the same area of forest. The scientists calculated that there were 250 lemurs in that area of forest.

Suggest one precaution needed when marking the lemurs to make sure the estimate of the number of lemurs is valid.

Using the information provided, calculate how many lemurs in the second sample were marked.

A coral reef is an underwater ecosystem formed as a ridge of mainly calcium carbonate deposits. Algae are photosynthesising organisms. Some algae grow on coral reefs. Succession results in a wide variety of fish living on coral reefs.

Describe a method that could be used to determine the mean percentage cover of algae on a coral reef.

Do not include information on the difficulties of using your method underwater.

Explain how succession results in a wide variety of fish living on coral reefs.

Do not describe the process of succession in your answer.

Ecologists investigated the effect of two fish species, the redband parrotfish and the ocean surgeonfish, on algal growth on an artificial reef. They made this artificial reef by submerging many large concrete blocks at a depth of 16–18 metres off the coast of Florida. They attached four large wire cages, A, B, C and D, to each block and populated the cages as shown.

A – No fish

B – Two redband parrotfish

C – Two ocean surgeonfish

D – One redband parrotfish and one ocean surgeonfish

After 34 weeks, the ecologists measured the mean percentage cover of all algae within each set of wire cages. The ecologists used a statistical test to find out whether the mean for each set of cages was significantly lower than the mean for set A.

Table 3 shows the probability (P) values that the ecologists obtained using this statistical test.

Using all the information, evaluate the effect of the two fish species on algal growth on coral reefs.

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. 5

The country of Malawi has a total area of 118 000 km². The actual land area is only 94 080 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. 10

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 fish 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. 15

Use the information in the passage and your own knowledge to answer the following questions.

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.

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.

Loss of nutrients into Lake Malawi has resulted in a decrease in some fish populations (lines 12–13).

Explain why.

The mark-release-recapture method can be used to estimate the size of a fish population (lines 13–14).

Explain how.

Suggest why the mark-release-recapture method can produce unreliable results in very large lakes (lines 14–15).

In northern India, there is a conflict of interests between farmers of livestock (eg cows) and people trying to conserve ibex (a type of wild goat).

When livestock are given extra food, their populations can grow too large and compete with ibex.

Name the type of competition between livestock and ibex.

Livestock will outcompete ibex if they:

• are in the same habitat

• eat a similar diet.

Scientists investigated this conflict of interests.

Table 1 summarises some of the scientists’ findings.

* A score of 0.0 indicates that the food or habitat is the same.

There must be a balance between the need for conservation of the ibex and the need for farmers to keep livestock.

Using all the information, suggest and explain three actions that the farmers could take to achieve this balance.

Succession occurs in natural ecosystems. Describe and explain how succession occurs.

Figure 1 shows percentages of energy transferred from sunlight to a zebra in a grassland ecosystem.

Use Figure 1 to calculate the percentage of sunlight energy that would be transferred into the faeces and urine of a zebra. Give your answer to 3 significant figures.

In this ecosystem the net productivity of the vegetation is 24 525kJ m² year¹

Use this information and Figure 1 to calculate the energy stored in new tissues of the

The sundew is a small flowering plant, growing in wet habitats such as bogs and marshes. The soil in bogs and marshes is acidic and has very low concentrations of some nutrients. The sundew can trap and digest insects.

Describe how you could estimate the size of a population of sundews in a small marsh.

Suggest and explain how digesting insects helps the sundew to grow in soil with very low concentrations of some nutrients.

Yeast cells can respire aerobically or anaerobically. A student used the apparatus shown in Figure 3 to measure the rate of respiration in yeast.

She:

• positioned the flask in a water bath so that the yeast culture reached a constant temperature

• then left the apparatus for one hour before starting her investigation.

Suggest one reason why it was important that the student left the apparatus for one hour after the yeast culture reached a constant temperature.

During her investigation, the coloured liquid moved to the right.

Explain why it moved to the right.

The student found that the coloured liquid moved 1.5 cm in 24 hours. The diameter of the lumen (hole) of the capillary tubing was 1 mm.

πr 2 π The volume of a capillary tubing is given by , where is 3.14 and = length.

Calculate the volume of gas produced in cm³ hour¹. Show your working.

Figure 4 shows a typical population growth curve for yeast under laboratory conditions.

Explain why a log scale is used to record the number of cells.

Many yeast cells die during the death phase.

Suggest one reason why.

The following equation can be used to make predictions of the growth in the population of yeast cells under ideal laboratory conditions.

X_t the population after a certain time

t time period in hours over which r applies

A population of 2000 yeast cells was left for 10 hours. The value for the growth rate was 0.5

Assuming no yeast cells died, calculate the predicted size of the population after 10 hours. Show your working.

Scientists investigated the process of succession on sand dunes. They measured the percentage cover of different species of plants on sand dunes of different ages. Some of the results the scientists obtained are shown in Figure 5.

Describe how you would determine the mean percentage cover for beach grass on a sand dune.

The scientists concluded that the results shown in Figure 5 were due to succession taking place.

Use Figure 5 to explain why the scientists reached this conclusion.

The scientists also investigated how the proportion of sunlight reaching the ground changed during succession. Some of the results the scientists obtained are shown in Figure 6.

Use Figure 5 to explain the results in Figure 6.

Using evidence from Figure 6, what can you conclude about the net primary productivity (NPP) in the sand dunes that are older than 1000 years?

Explain your answer.

Dengue is a serious disease that is caused by a virus. The virus is carried from one person to another by a mosquito, Aedes aegypti. One method used to try to reduce transmission of this disease is the Sterile Insect Technique (SIT). This involves releasing large numbers of sterile (infertile) male A. aegypti into the habitat. These males have been made infertile by using radiation.

Explain how using the SIT could reduce transmission of dengue.

Describe how the mark-release-recapture method could be used to determine the population of A. aegypti at the start of the investigation.

The release of radiation-sterilised A. aegypti has not been very successful in controlling the transmission of dengue.

Suggest one reason why.

Recently a new method was developed to control A. aegypti. Scientists produced transgenic males carrying a ‘lethal gene’ which kills their offspring before they can reproduce.

The scientists released transgenic males every week in one area of a city in Brazil. At regular intervals they determined the number of A. aegypti per km² in the area where transgenic males were released and in a control area where no transgenic males were released.

Figure 2 shows their results.

Suggest why the scientists released more transgenic males every week.

The release of transgenic males proved successful in reducing the number of A. aegypti.

Describe how the results in Figure 2 support this conclusion.