Populations in ecosystems (A-level only)
Charlock is a weed which grows in some barley fields.
Describe how you could estimate the number of charlock plants in a field.
In barley fields, charlock plants can grow up to a height of 1.5 m and absorb large quantities of nutrients and water from the soil. Charlock has a negative effect on the growth of barley plants.
Use the information provided to explain why. In your answer, name the type of competition occurring between charlock plants and barley plants.
The table below gives some features of charlock seeds.
| Feature of seed | Value |
|---|---|
| Mean mass / mg | 1.4 |
| Maximum number produced per charlock plant during a growing season | 12,000 |
| Percentage viable (able to develop) after a growing season | 85 |
During a growing season, it was estimated that a total number of 650 charlock plants grew on the fields of a farm.
Calculate the maximum mass, in kg, of viable charlock 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.
Redroot pigweed is a vigorous annual weed that grows in some soybean fields.
Describe how you could estimate the total population of redroot pigweed plants in a field.
In soybean fields, redroot pigweed plants can grow up to a height of 2.1 m and rapidly absorb large quantities of nutrients and water from the soil. This has a highly negative effect on the growth of the soybean plants.
Use the information provided to explain why. In your answer, name the type of competition occurring between redroot pigweed plants and soybean plants.
The table below gives some biological features of redroot pigweed seeds.
| Feature of seed | Value |
|---|---|
| Mean mass / mg | 0.45 |
| Maximum number produced per redroot pigweed plant during a growing season | 115,000 |
| Percentage viable (able to develop) after a growing season | 72 |
During a growing season, it was estimated that a total of 1,400 redroot pigweed plants grew on the fields of a farm.
Calculate the maximum mass, in kg, of viable redroot pigweed 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.
A conservation biology team monitored the population density of the desert ironclad beetle (Asbolus verrucosus) in a protected arid dune reserve. They employed the mark-release-recapture method.
In the primary sampling session, they captured, marked, and released 848484 beetles.
After allowing a period of six days for the marked individuals to mix thoroughly with the rest of the population, a secondary sample of 757575 beetles was collected from the same dune area. The researchers calculated the total population of desert ironclad beetles in this reserve to be 350350350.
Determine the number of beetles in the second sample that were found to be marked.
A marine biologist is monitoring the population of the spiny lobster (Palinurus elephas\textit{Palinurus elephas}Palinurus elephas) around a rocky reef using the mark-release-recapture technique.
In the initial survey, the biologist captures, tags, and releases 969696 lobsters.
Three weeks later, a second survey is conducted in which 808080 lobsters are captured from the same reef area. The biologist calculates that the estimated total population size of spiny lobsters in this reef area is 640640640.
Calculate the number of tagged lobsters that were recaptured in the second survey.
A group of ecologists used the mark-release-recapture method to estimate the population size of red squirrels in a woodlands nature reserve.
In the first sample, they captured, marked, and released 454545 squirrels.
Two weeks later, they captured a second sample of 404040 squirrels from the same area of the reserve. The ecologists calculated that the total population of red squirrels in this reserve was 360360360.
Using this information, calculate how many squirrels in the second sample were marked.
A salt marsh is an ecosystem with diverse communities of halophytes (salt-tolerant plants) and salt marsh invertebrates. Some herbivorous organisms feed on these plants. Succession on a salt marsh eventually results in a wide variety of animal species inhabiting the marsh.
Describe a method that could be used to determine the mean percentage cover of sea lavender (Limonium vulgare) in a salt marsh. Do not include information on the difficulties of working in a salt marsh.
Explain how succession results in a wide variety of animal species living in a salt marsh. Do not describe the process of succession in your answer.
Ecologists investigated the effect of two invertebrate herbivore species, the saltmarsh snail (Melampus bidentatus) and the meadow grasshopper (Chorthippus paralellus), on sea lavender growth in a salt marsh. They cleared experimental plots of vegetation in a salt marsh in East Anglia at an elevation of 1.2–1.6 metres above mean high water spring tides. They secured four fine mesh cages, W, X, Y and Z, over the soil surface in each plot and populated the cages as follows:
- W – No herbivores
- X – Four saltmarsh snails
- Y – Four meadow grasshoppers
- Z – Two saltmarsh snails and two meadow grasshoppers
After 18 weeks, the ecologists measured the mean percentage cover of sea lavender within each set of 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 W.
Table 3 shows the probability (PPP) values that the ecologists obtained using this statistical test.
Table 3
| Set of cages | P P\,P value |
|---|---|
| X | = 0.008 |
| Y | = 0.721 |
| Z | = 0.035 |
Using all the information, evaluate the effect of the two herbivore species on sea lavender growth on salt marshes.
A rocky shore is an ecosystem with diverse communities of seaweeds (macroalgae) and marine invertebrates. Some herbivorous organisms feed on the seaweeds. Succession on a rocky shore eventually results in a wide variety of animal species inhabiting the shore.
Describe a method that could be used to determine the mean percentage cover of seaweed on a rocky shore. Do not include information on the difficulties of working on a rocky shore.
Explain how succession results in a wide variety of animal species living on a rocky shore. Do not describe the process of succession in your answer.
Marine biologists investigated the effect of two grazer species, the common limpet (Patella vulgata) and the common periwinkle (Littorina littorea), on seaweed growth on a rocky shore. They cleared experimental plots of rock surface on a rocky shore in South West England at a tidal height of 1.5–2.0 metres above chart datum. They attached four large mesh cages, A, B, C and D, to the rock surface in each plot and populated the cages as follows:
- A – No grazers
- B – Two common limpets
- C – Two common periwinkles
- D – One common limpet and one common periwinkle
After 26 weeks, the biologists measured the mean percentage cover of all seaweed within each set of cages. The biologists 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 (PPP) values that the biologists obtained using this statistical test.
Table 3
| Set of cages | PPP value |
|---|---|
| B | =0.002= 0.002=0.002 |
| C | =0.684= 0.684=0.684 |
| D | =0.352= 0.352=0.352 |
Using all the information, evaluate the effect of the two grazer species on seaweed growth on rocky shores.
Glyphosate is a widely used herbicide. Scientists investigated the rate of glyphosate breakdown by soil microorganisms at two different temperature treatments: 15∘C15^\circ\text{C}15∘C and 25∘C25^\circ\text{C}25∘C.
They measured the concentration of glyphosate in soil samples over 10 days. Each sample started with the same concentration of glyphosate and had the exact same soil mass. Their results are shown in the graph below:
Suggest why, in this investigation, the scientists expressed the concentration of glyphosate in units of μg g−1\mu\text{g }\text{g}^{-1}μg g−1.
Calculate the difference in the rate of breakdown of glyphosate per day between day 0 and day 3 at 15∘C15^\circ\text{C}15∘C and 25∘C25^\circ\text{C}25∘C. Show your working and state the units for your answer.
Sulfamethoxazole is a widely used antibiotic that often contaminates aquatic environments. Scientists investigated the rate at which sulfamethoxazole is broken down by populations of sediment-dwelling microorganisms at two different pH treatments: pH 5.5 and pH 7.5.
They measured the concentration of sulfamethoxazole in sediment samples over 8 days. Each sample started with the same initial concentration of sulfamethoxazole and had the exact same dry mass of sediment. Their results are shown in the graph below:
Suggest why, in this investigation, the scientists expressed the concentration of sulfamethoxazole in units of μg kg−1\mu\text{g }\text{kg}^{-1}μg kg−1.
Calculate the difference in the rate of breakdown of sulfamethoxazole per day between day 0 and day 4 at pH 5.5 and pH 7.5. Show your working and state the units for your answer.
In the coastal salt marshes of Northern Europe, there is a conflict of conservation interest between commercial keepers managing domestic pollinators and conservationists protecting the wild Sea Aster Bee (Colletes halophilus), a threatened solitary bee species.
When managed bees are provided with supplementary sugar-water feeds by keepers, their populations can grow artificially large and outcompete the wild Sea Aster Bee.
Name the type of competition that occurs between managed domestic pollinators and the wild Sea Aster Bee.
Managed pollinators will outcompete the Sea Aster Bee if they:
- forage at the same vegetation heights (strata)
- visit a similar range of flowering plant species for pollen and nectar.
Ecosystem managers investigated this ecological conflict. Table 1 summarizes some of their findings.
Table 1
| Managed pollinator | Difference in floral diet compared to Sea Aster Bee* | Difference in foraging height compared to Sea Aster Bee* |
|---|---|---|
| Red Mason Bee | 0.9 | 1.4 |
| Honeybee | 0.2 | 0.0 |
| Buff-tailed Bumblebee | 0.0 | 2.6 |
* A score of 0.0 indicates that the floral diet or foraging height is identical to that of the Sea Aster Bee.
There must be a balance between the need for conservation of the threatened Sea Aster Bee and the economic or pollination needs of local agricultural managers.
Using all the information provided, suggest and explain three actions that the keepers or ecosystem managers could take to achieve this balance.
Populations in ecosystems (A-level only) Questions
Practise AQA A Level Biology Populations in ecosystems (A-level only) with exam-style questions for A Level Biology. 14 questions, matched to the AQA A Level Biology (7402) specification and written in Paper 1, Paper 2 and Paper 3 style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.