Aim for FiveBiology

Interpreting and evaluating experimental results with graphing

Competition between duckweed species

  • Unit 8
  • 9 points
  • About 25 minutes

You can use a calculator on this question, just like on exam day.

A long question with an experiment and a data table that you turn into a graph. On this site you describe the graph you'd draw (type, axes, units, scale, points and error bars), then analyze the data and make and justify a prediction. On the exam: Question 2 of 6 in Section II (90 minutes, 50% of the exam score); about 25 minutes suggested.

The question and its sources

Duckweeds are tiny floating plants that reproduce mainly by budding off new leaflike fronds. Two species, A and B, are often found on the same ponds, where they compete for space on the water surface, light and nutrients.

Experiment

Researchers filled identical containers with the same volume of pond-water nutrient solution and kept them in the same growth chamber. They started three treatments, with eight containers each: 10 fronds of species A alone; 10 fronds of species B alone; and 10 fronds of species A plus 10 fronds of species B together. After 21 days, they counted the fronds of each species.

Source: Hypothetical study

Table 1. Number of fronds after 21 days, mean ± 2SE

SpeciesGrown aloneGrown with the other species
Species A412 ± 30305 ± 28
Species B268 ± 2296 ± 15

Source: Hypothetical data

Suggested time: 25 minutes

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Part (a)

1 point

Describe how a density-dependent factor affects the growth rate of a population as the population's density increases.

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Part (b)

4 points

(i) Describe the graph you would construct to represent the data in Table 1. In your description, name the type of graph, give the label and units for each axis, describe the scale, describe how you would show both species and both treatments, list the values you would plot, and describe the error bars. (ii) Based on the data, determine whether growing with species A had a significant effect on the number of species B fronds.

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Part (c)

2 points

(i) Calculate the percent decrease in the number of species B fronds when grown with species A compared with species B grown alone. (ii) State the null hypothesis for the effect of species A on species B in this experiment.

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Part (d)

2 points

The researchers continue growing species A alone in one container with no added nutrients for 90 days. (i) Predict how the number of species A fronds will change over the 90 days. (ii) Justify your prediction.

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