Populations in Ecosystems (A Level only) (AQA A Level Biology)

A group of biologists wanted to study succession that occurred on a small island of bare rock that formed off the coast of Iceland 200 years ago. Using historical records and more recent scientific research, they estimated how the percentage cover of different lichen and plant species found on the island had changed since the island was formed. The graph in Figure 1 shows their results.

Figure 1

The island is small, with a surface area of about 100 square metres. Describe how the group of biologists could determine the present-day mean percentage cover of grass on the island.

The biologists hypothesized that the results shown in the graph in Figure 1 were due to succession taking place on the island.

Explain why the biologists think this. Use the graph in Figure 1 to support your answer.

If the biologists just wanted to study the abundance of broad-leaved trees on the island in Figure 1, they could have used frequency rather than percentage cover. Suggest why this would be possible.

The biologists in part (a) are trying to decide if the grasses and broad-leaved trees that are currently present on the island (see Figure 1) represent the climax community.

Help the biologists to decide by stating two features of a climax community.

The Frégate Island giant tenebrionid beetle or Frégate beetle (Polposipus herculeanus), is a flightless species of beetle in the family Tenebrionidae. It is endemic to Frégate Island in the Seychelles. A PhD student wanted to estimate the number of Frégate beetles on the island.

Explain how they could use the mark-release-recapture method to do this.

The PhD student in part (a) used the mark-release-recapture technique to estimate the size of the population of Frégate beetles on the island. They collected 176 beetles and marked them before releasing them back into the same area. Later, they collected 198 beetles, 22 of which were marked.

Calculate an estimate of the number of beetles on the island. Show your working.

The PhD student made sure that their study met the conditions required for the results from the mark-release-recapture method to be valid.

State three of these conditions.

Ecologists, such as the PhD student, can also use a method called proportional sampling to estimate the population size of an animal species, such as the Frégate beetle. This method is also based on assumptions that must be met in order for the results to be considered valid. Two of these assumptions are:

         1) The size of the area where the animal population lives is known

         2) The animals are uniformly distributed (evenly spread out) in this area

i) State one way the assumptions made in proportional sampling differ from those made in mark-release-recapture.

ii) Define the term ‘population’.

A biologist wanted to investigate the biodiversity of insects living in farmed fields in Somerset. The biologist used sweep-netting along transects through the middle of fields on four different farms and counted the number of different insect species present in the sweep-net samples from each farm. Table 1 below shows the results of this study.

Table 1

What can you conclude about the number of insect species present on Farm B compared with the number of insect species present on the other three farms? Give evidence from Table 1 to support your answer.

The biologist used the data in Table 1 to report that the difference between the mean number of insect species present on Farm B and the number present on the other three farms differed significantly (p < 0.05).

Explain what the biologist meant by the term “differed significantly (p < 0.05)”. Use the words probability and chance in your answer.

The biologist in parts (a) and (b) wanted to know many fields they should sample at each farm. They carried out a preliminary investigation and recorded the total number of different insect species present in an increasing number of fields. Figure 1 shows the results.

Figure 1

Use the results in Figure 1 to explain why 5 would not be an appropriate number of fields to sample and why 25 would also not be an appropriate number of fields to sample.

i) What is species richness?

ii) State two ways that farming reduces species richness.

Elodea canadensis (Canadian pondweed) is a species of aquatic plant from North America that has become invasive in the UK. A student grew Elodea canadensis and a similar species (that is native to the UK) in water tanks. The student grew the two species separately and together. The graphs in Figure 1 show their results.

Figure 1

The student kept the abiotic factors the same throughout their experiment in Figure 1 to ensure their results were reliable.

i) Explain what is meant by the term abiotic factor.

ii) State two abiotic factors the student should have kept the same.

Explain the results shown in Figure 1 for when both species of pondweed are grown together.

Evaluate the benefit of laboratory experiments, like the one in part (a) and Figure 1, in understanding interactions between organisms in their natural environment.

Canadian pondweed (Elodea canadensis) has been introduced into many garden ponds in the UK and has spread to some natural ponds. Here, it competes with many naturally occurring pond plant species. Suggest how the introduction of Canadian pondweed may lead to a reduction in the diversity of the whole community in a natural pond.

A group of ecologists investigated the relationship between a population of stoats (predators) and a population of mice (prey). The number of individuals in each population was measured at regular intervals over three years. The results of the study are shown in the graph in Figure 1.

Figure 1

Describe and explain the results shown by the graph in Figure 1.

Using the information in Figure 1, calculate the rate at which the stoat population decreased over the final 6 months. Show your working.

The ecologists could use their results in Figure 1 to calculate the percentage increase in the population of mice between months 12 and 18.

i) Explain how they could do this.

ii) Calculate the percentage increase in the population of mice between months 12 and 18.

Define intraspecific competition.

Scientists conducted an experiment where two species of bacteria, Pseudomonas fluorescens and Pseudomonas aeruginosa were grown separately in different petri dishes which contained yeast as a source of food. The two species were then grown together in the same petri dish also containing yeast as a source of food. The scientists' results are shown in Figure 1. 

Figure 1

Use Figure 1 to describe and explain the population growth curve of P. aeruginosa when grown alone. 

Use Figure 1 suggest an explanation for the difference in the final population size of aeruginosa when grown with P. fluorescens compared with when it is grown alone. 

Suggest why the growth rate of P. fluorescens is slower in the presence of P. aeruginosa compared with when it is grown alone. 

P. fluorescens and P. aeruginosa are aerobic organisms, which means they use oxygen for aerobic respiration. 

Suggest how the population growth of P. aeruginosa when grown with P. fluorescens would differ if it were an anaerobic organism. 

Scientists wanted to investigate the dispersal of seeds as the distance from a tall 90m tree increases in an open field. They found that the seeds were mainly dispersed by wind. Figure 1 shows the area of land and the position of the tree. 

Figure 1

Use Figure 1 to explain how scientists could obtain data on seed dispersal by this tree in the open field. 

The data collected by the scientists is shown in Figure 2. 

Figure 2

Use Figure 2 to explain the changes in the mean number of seeds as the distance from the base of the trunk increases. 

The distance reached by the seeds would have been different if the tree were of a different heights. Suggest and explain why. 

The tree represents a climax community. Scientists investigated the growth of the seeds from this tree. They looked at how tall the seeds grew and also the biomass of any growth produced. The scientists found that seeds produced by this species of tree did not grow successfully. Suggest why.

Feather mites are a type of parasite that lives on or in host organisms. The populations of many organisms may be reduced by the effects of parasites. Feather mites are small parasites found on the wing feathers of many birds. The mites feed on the oil that the birds produce. This oil keeps the feathers in good condition. Birds unable to oil their feathers properly use more energy in maintaining their body temperature. This results in less energy being available for other processes.

Scientists investigated the relationship between the numbers of feather mites and the breeding success of chickens. They randomly sampled a number of different chicken coops. They collected data on the following:

• The total number of eggs laid

• The number of chicks that hatched from the eggs

• The number of chicks that survived

• The total number of feather mites on the two parent birds

Explain why the scientists sampled the chicken coops at random. 

The scientists calculated the percentage of each pair’s eggs from which chicks survived and called this ‘breeding success per pair’. Table 1 shows their results. 

Table 1          

Does the data in Table 1 support the hypothesis that the presence of feather mites reduces the ability of chickens to reproduce successfully?

The scientists wanted to test the significance of the data they collected. State the choice of statistical test and explain why the test is appropriate. 

Feather mites eat pathogenic bacteria and fungi as well as the oil on the bird feathers. Explain how this may affect the breeding success of the birds.

Ecologists investigated the effect of the predation of leeches on a population of different tadpoles belonging to three different species of toads. They set up four different pond  communities each containing the following:

• 200 American toad tadpoles

• 300 Greenhouse toad tadpoles

• 300 Oak toad tadpoles

The ecologists then added a different number of leeches to each of the four communities. Figure 1 shows their results. 

Use Figure 1 to describe the percentage survival of the tadpoles as the number of leeches increase for each toad species. 

Using Figure 1, explain the changes in the percentage survival of the Greenhouse toad.

The ecologists concluded that leeches cause significant reduction in the population of tadpoles. Evaluate this statement. 

The ecologists collected further data from pond communities containing 100, 150, 200 and 250 leeches. They wanted to calculate if there was a significant difference in the tadpole populations between the pond communities. State and explain which statistical test they should use. 

A group of scientists investigated the formation of a community of land plants from a small lake over many years. They measured the abundance of different species, the soil pH, temperature, the number and type of predators and number of hours of sunlight monthly over 10 years. Figure 1 shows a series of diagrams made by one of the scientists to illustrate the changes observed. 

Figure 1

Use Figure 1 to describe how a community of land plants formed in stage 5.

Four small rivers flow into this lake. These rivers flow through forested areas. Suggest how deforestation might affect the process of succession in the lake.

Stage 5 illustrates the climax community. During this stage the number of different species in the habitat and the size of each population remain fairly constant. Explain what limits the size of populations in a climax community

Explain why it would be more appropriate to use a transect rather than random quadrats when investigating succession in this way.