Volume 26, Number 3, February 2014
Extension
Looking at evidence
Bill Indge
The graphs and questions in this document are reproduced from the Powerpoint presentation. They
rely on information given in the presentation, including the notes section, and are not intended to be
used as a stand-alone worksheet.
Competition from invasive species: cane toads
The cane toad was deliberately introduced into Australia in 1935 to control insect pests that fed on
sugar cane. Since then it has spread rapidly over much of the tropical north of Australia. The secretion
of the large glands behind the eyes is known to be very toxic and is believed to have caused the
deaths of native mammals and reptiles.
Research workers investigated tadpole deaths shortly after cane toads colonised the area. The graph
shows the results for one pool.
1 The research workers suggested that the tadpoles had died because they had eaten the eggs of the
cane toad, which are also toxic. Use the graph to evaluate this suggestion.
2 Other than eating the eggs of the cane toad, give two other suggestions for the death of the
tadpoles.
3 Suggest what measurements you could use to test the hypothesis that the tadpole deaths were due
to abiotic factors affecting the water.
Philip Allan Updates © 2014
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4 Suggest the advantage of

using the same species of tadpole in each container in a particular trial

assigning the tadpoles to the containers randomly

carrying out a total of 80 trials
5 One out of 80 tadpoles died in container A and one out of 80 tadpoles died in container B. None died
in container C. Could the tadpole deaths have been due to pollutants in the water?
This graph shows the number of adult tree frogs at one site, before and after the arrival of cane toads.
Many of the tadpoles that died were tree frogs.
Philip Allan Updates © 2014
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6 In his column ‘Where have all the amphibians gone’, Roger Downie says that ‘there is little evidence
that cane toads are a cause of native amphibian declines in Australia’. Does the information from this
research support this statement?
Philip Allan Updates © 2014
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Exposure to short-wavelength ultraviolet light: the
common frog
Stage 1 The effect of UV-B on egg hatching and development
Philip Allan Updates © 2014
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The bars on these graphs show 2 × S.E.
When the bars overlap there is a probability of greater than 0.05 that any difference in the values is
due to chance.
1 In the introduction to the paper from which the data above are taken, the scientists write that:
‘A number of studies have failed to find evidence for negative effects of UV-B radiation on the early
stages in amphibian development. This has led to the conclusion that the eggs of amphibians are
tolerant to UV-B radiation. It has also led to the suggestion that the increase in UV-B radiation as a
result of depleted ozone is not likely to have any direct negative effects on the populations of many
amphibians.’
Do these data support this suggestion? Give the evidence for your answer.
Stage 2 The effect of UV-B on older tadpoles
Philip Allan Updates © 2014
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2 Using all the data from this investigation, what conclusions can you draw about the effects of UV-B
radiation on the development of the common frog?
Philip Allan Updates © 2014
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Habitat loss and the common toad
Some toad breeding sites in the study area
Urban sites:
Site of breeding pond or ponds
Estimated population size
Garden
20
Garden
10
Garden
14
Public park
13
Rural sites:
Site of breeding pond or ponds
Estimated population size
Wooded heathland
500–5000
River valley marsh
>5000
Old parkland
2000–4000
1 The estimated population sizes of the adult toads in the urban sites was smaller than the population
sizes in the rural sites. Suggest a reason for this.
2 Do the data from this investigation support either of the following conclusions? Give an explanation
in each case.
A Large populations of toads have a higher genetic diversity.
B Fewer tadpoles survive in urban populations because these populations have a lower genetic
diversity.
Philip Allan Updates © 2014
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