Looking at evidence
Bill 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.
Looking at evidence
The amphibian life-cycle
Adult frog
Eggs hatch to
produce tadpoles.
The tadpoles form
the larval stage.
Adults lay eggs. A
clump of eggs is
known as spawn.
Looking at evidence
Some of the factors that may be involved
in reducing amphibian numbers
• Competition from invasive species
• Short-wavelength ultraviolet light
• Habitat loss
• Exploitation by humans
• Pollution
• Disease
• Climate change
Here we will concentrate on the first three factors.
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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.
Number of dead tadpoles of
species of frogs and toads
other than cane toads
Looking at evidence
• 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.
200
160
120
80
40
0
0
1
2
3
4
5
6
7
8
9
Time after cane toad eggs laid/days
10
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.
Number of dead tadpoles of
species of frogs and toads
other than cane toads
Looking at evidence
• 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.
200
160
120
80
40
0
0
1
2
3
4
5
6
7
8
9
Time after cane toad eggs laid/days
10
Other than eating the eggs of the cane toad, give two other
suggestions for the death of the tadpoles.
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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
Looking at evidence
Could the tadpole deaths be due to abiotic
factors affecting the water?
• 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.
Looking at evidence
Could the tadpole deaths be due to pollutants
in the water?
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
B
C
Food added
after
48 hours
• 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.
Looking at evidence
Could the tadpole deaths be due to pollutants
in the water?
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
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?
Looking at evidence
Number of adult tree frogs
counted per night
Mean ± standard deviation
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.
4
Before cane toads arrived
After cane toads arrived
3
2
1
0
1
2
3
4
5
6
7
Years from start of study
8
Looking at evidence
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?
Looking at evidence
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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Number of eggs surviving to
produce tadpoles
Mean ± 2 × S.E.
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.
30
28
26
No UV-B
Normal
UV-B
Enhanced
UV-B
Number of newly hatched
tadpoles showing abnormal
development
Mean ± 2 x S.E
Looking at evidence
2
1
0
No UV-B
Normal
UV-B
Enhanced
UV-B
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Mean length of newly
hatched tadpoles/mm
Mean ± 2 x S.E
14
13
12
No UV-B
Normal
UV-B
Enhanced
UV-B
Looking at evidence
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.
Looking at evidence
Number of tadpoles surviving
for length of investigation
Mean ± 2 × S.E.
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.
30
28
26
No UV-B
Normal
UV-B
Enhanced
UV-B
Looking at evidence
Percentage of tadpoles
showing abnormal
development
40
20
0
No UV-B
Normal
UV-B
Enhanced
UV-B
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Time spent as tadpole/days
Mean ± 2 x S.E
80
78
76
74
72
70
No UV-B
Normal
UV-B
Enhanced
UV-B
Looking at evidence
Mean mass of tadpoles at
maturity/g
Mean ± 2 x S.E
0.7
0.6
0.5
0.4
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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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
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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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Population size and genetic diversity
Large population
Many different alleles
Small population
Few different alleles
Percentage of tadpoles surviving
for 70 days
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100
80
Small urban populations
Large rural populations
60
0
10
20
30
Percentage of loci with more than one allele
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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.