Looking at evidence Bill Indge Looking at evidence

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.

Looking at evidence Competition from invasive species: cane toads

Looking at evidence • 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.

Looking at evidence 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 p. H 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’

Looking at evidence Exposure to short-wavelength ultraviolet light: the common frog

Looking at evidence 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

Looking at evidence 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

Looking at evidence 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

Looking at evidence 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?

Looking at evidence Habitat loss: common toad

Looking at evidence Some toad breeding sites in the study area Urban sites Site of breeding pond or ponds Estimated population size Garden 20 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

Looking at evidence 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.

Looking at evidence Population size and genetic diversity Large population Many different alleles Small population Few different alleles

Percentage of tadpoles surviving for 70 days Looking at evidence 100 80 Small urban populations Large rural populations 60 0 10 20 30 Percentage of loci with more than one allele

Looking at evidence 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.