Looking at evidence Bill Indge. Looking at evidence Introduction The material in this presentation is designed to encourage students to think critically.

Looking at evidenceBill Indge

Looking at evidence

Introduction

The material in this presentation is designed to encourage

students to think critically about what they read. It is designed as

a teaching package for discussion so no formal mark scheme is

given. The graphs and questions are available as a separate

Word document.

The presentation draws on three different sources of information.

Students should begin by reading the article ‘Where have all the

amphibians gone’ by Roger Downie in the February 2014 issue of

Biological Sciences Review.

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Adults lay eggs. A clump of eggs is known as spawn.

Eggs hatch to produce tadpoles. The tadpoles form

the larval stage.

Adult frog

The amphibian life-cycle

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• Competition from invasive species• Short-wavelength ultraviolet light• Habitat loss• Exploitation by humans• Pollution• Disease• Climate changeHere we will concentrate on the first three factors.

Some of the factors that may be involved in reducing amphibian numbers

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Competition from invasive species: cane toads

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• 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.

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160

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80

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0 1 2 3 4 5 6 7 8 9 10Time after cane toad eggs laid/days

• Research workers found 11 similar patterns of tadpole deaths in five other pools shortly after cane toads colonised the area.

• A total of more than 1300 tadpoles of ten species died.

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.

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0 1 2 3 4 5 6 7 8 9 10Time after cane toad eggs laid/days

Other than eating the eggs of the cane toad, give two other suggestions for the death of the tadpoles.

• Research workers found 11 similar patterns of tadpole deaths in five other pools shortly after cane toads colonised the area.

• A total of more than 1300 tadpoles of ten species died.

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Could the tadpole deaths be due to abiotic factors affecting the water?

Suggest what measurements you could use to test the hypothesis that the tadpole deaths were due to abiotic factors affecting the water

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• The researchers compared water from pools where tadpole deaths occurred with water from pools in which there were no deaths.

• They measured dissolved oxygen, temperature, salinity and pH from samples collected in the morning and in the afternoon.

• They found no significant differences between the readings from the pools where tadpole deaths occurred and the pools where there were no deaths.

Could the tadpole deaths be due to abiotic factors affecting the water?

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Container with filtered tap water

Container with water from pool where tadpole

deaths occurred

Container with water from pool

where no tadpole deaths occurred

• A single tadpole was added to each container.• The tadpoles were assigned randomly to the

containers.• The same species was used for each trial.• There was a total of 80 trials.

A B C

Could the tadpole deaths be due to pollutants in the water?

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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 trialsOne 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?

Could the tadpole deaths be due to pollutants in the water?

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4

3

2

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0 1 2 3 4 5 6 7 8

Years from start of study

Before cane toads arrived

After cane toads arrived

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.

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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?

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The researcher’s conclusions

‘A causal link between toad breeding and tadpole mortality is

supported by observations that:

•in at least 9 of the 11 water bodies involved, toads bred

immediately prior to mortality events

•water quality was indistinguishable from that of control

ponds, and tadpoles placed in that water remained healthy

•dead tadpoles showed no sign of disease

•laboratory trials showed rapid, 100% mortality in native

tadpoles exposed to freshly laid toad eggs’

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Exposure to short-wavelength ultraviolet light:the common frog

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No UV-B Normal UV-B

Enhanced UV-B

Stage 1 The effect of UV-B on egg hatching and development

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.

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No UV-B Normal UV-B

Enhanced UV-B

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No UV-B Normal UV-B

Enhanced UV-B

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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.

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28

26

No UV-B Normal UV-B

Enhanced UV-B

Stage 2 The effect of UV-B on older tadpoles

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.

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No UV-B Normal UV-B

Enhanced UV-B

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No UV-B Normal UV-B

Enhanced UV-B

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No UV-B Normal UV-B

Enhanced UV-B

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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?

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Habitat loss:common toad

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Site of breeding pond or ponds Estimated population size

Garden 20

Garden 10

Garden 14

Public park 13

Site of breeding pond or ponds

Estimated population size

Wooded heathland 500–5000

River valley marsh >5000

Old parkland 2000–4000

Urban sites

Rural sites

Some toad breeding sites in the study area

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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.

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Large population Many different alleles

Small population Few different alleles

Population size and genetic diversity

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0 10 20 30Percentage of loci with more than one allele

100

80

60

Small urban populations

Large rural populations

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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.