Cane toads and Australian snakes

Transcription

1 Cane toads and Australian snakes This activity was adapted from an activity developed by Dr Thomas Artiss (Lakeside School, Seattle, USA) and Ben Phillips (University of Sydney). Cane toads (Bufo marinus) were introduced into Australia in The cane toad was originally introduced to deal with another introduced pest, the cane weevil, which was decimating sugar cane plants, an important cash crop. However, the cane toad ate nearly everything else but the cane weevil. u Source: Barista 2005 Archives. Available Internet. URL: < archives/2005_01.html> (Accessed 20 October 2005) More problematic is the fact that cane toads secrete a toxin through pores in their skin when they are threatened. Most animals attempting to eat a cane toad will die from the toxin. Thus, in addition to the impact this introduced species has had on the animals it eats, it has had a significant impact on the predators that would normally keep its numbers in check. Consequently, the cane toad has increased its numbers and range, and currently it occupies a range of about a million square kilometres. Two biologists, Ben Phillips and Richard Shine, set out to examine the impact that cane toads had on snakes, an animal that normally preys on toads in Australia. In Australia, it is estimated that 49 species of snake are potentially impacted by the cane toad, and the majority of these species will be unable to survive the toxin produced by cane toads. Ben Phillips Source: Senior Biology 1 of 11

2 Unit 2 Professor Richard Shine u Source: Phillips and Shine compared the body sizes of two species of snake. One species was suspected to be unaffected by cane toads since it has a high physiological resistance to the toxin. The second species was suspected to have been impacted by the cane toad because: it is large enough to ingest the toads it has very low physiological resistance to toad toxin its range overlaps with the cane toads range. Phillips and Shine examined the body sizes of snakes from a museum, and compared the changes in the size of the snakes over time. They suspected that in the susceptible species (redbellied black snakes) the snakes should gradually increase in size over time in response to the introduction of the cane toads. They reasoned that larger members of the red-bellied black snake populations would be better able to cope with the toxin because of their larger body size. Smaller members of the population would need to ingest far less of the toxin before succumbing to the effects, and ultimately dying. In other words, a smaller snake would be at far greater risk, and even a little toxin would kill it. The larger members of the population would be better able to deal with toxins if they ingested a cane toad. Therefore, the larger snakes would tend to survive and pass on their large body size genes to their offspring. The body size of red-bellied black snake populations would tend to increase over time. In contrast, the species which was resistant to toad toxin (keelback snakes) are not likely to experience any change in body size since there was nothing favouring larger or smaller body size. 2 of 11 Senior Biology

3 Analysing the data Answer the following questions in your Learning Log. Prediction Scan the data provided in the table below. It shows the body size of each snake specimen and the number of years since that population of snakes was exposed to cane toads. 1. How do you think the body size of keelback snakes has changed over 60 years? 2. How do you think the body size of red-bellied black snakes has changed over 60 years? Hypotheses 3. Write each of your predictions in the form of a hypothesis. For example, The body size of keelback snakes (increased or decreased or hasn t changed) in response to the introduction of cane toads. 4. Explain the reasons for each of your predictions. Graphing the data To assess the accuracy of your prediction you will plot your data in Microsoft Excel. To do this, you will create a two-line graph that depicts the average change in body sizes of each of the snake species from the introduction of the cane toad (time zero) to the present (roughly 60 years later). The following data table shows the average body sizes of keelback and red-bellied black snakes. The time represents the number of years since exposure. Senior Biology 3 of 11

4 Unit 2 Data table Time (years) Keelback snake Body size (cm) Red-bellied black snake Time (years) Body size (cm) of 11 Senior Biology

5 Note that Phillips and Shine were not able to gather data for every year. Drawing the graph using Microsoft Excel Step 1: Select the Cane toads and snakes Excel spreadsheet. Step 2: You should have four labelled columns of data: the first column is the number of years since the introduction of the cane toad; the second is the average body size of the keelback snake; the third is the number of years since introduction of the cane toad; the fourth column is the change in the average body size of the red-bellied black snake. Step 3: Now you are ready to graph your data. Highlight the second column of data (not the heading). Go to the Insert dropdown menu and select Chart. The Chart Wizard box should appear. In the Chart type box, select the XY Scatter graph. In the Chart sub-type box, select the second option. Senior Biology 5 of 11

6 Unit 2 Step 4: Select Next. The data you highlighted will appear in the Data range box. These values will appear on the y-axis of the graph. Step 5: Next you need to assign the values for the x-axis. Select the Series tab above the graph and then select the button on the right-hand side of the X Values box. The graphing box will minimise. 6 of 11 Senior Biology

7 Step 6: Highlight the data in the first column of the table (again, click and drag to select the data). If you have done this right, the Chart Source Data box should read =Sheet1!$A$3:$A$46 (or something similar) and there should be a flashing box around the values you selected in the table. If you make a mistake, simply highlight the information in the Chart Source Data box, delete it and select the data you want. Step 7: Select the button on the right-hand side of the Chart Source Data box again to return to the wizard. Step 8: Click in the Name box and type in a name for the data in the series (e.g. Average size of keelback snakes ). Senior Biology 7 of 11

8 Unit 2 Step 9: Select the Next button. Add titles to the Value (X) axis box and the Value (Y) axis box. Step 10: Select Next. Save the graph as a new sheet (this allows you to print a full page graph) and then select Finish. If you accidentally select Finish before selecting new sheet, the graph will be placed on the same sheet as your data table. You will then be able to select the table and position it beside the data table. Congratulations! You have produced a graph showing the changes in the average body size of keelback snakes since cane toads were introduced into Australia. Analysing the graph 5. Has the average body size of the keelback snakes changed during the 60-year period? 6. Is this the result that you predicted for keelback snakes? Comparing the red-bellied black snake data The easiest way to compare two sets of comparable data is to graph them on the same set of axes. You will now repeat the steps you just completed to produce a second graph that shows the data for both snakes. This also gives you an opportunity to practise your Excel skills. 8 of 11 Senior Biology

9 Step 11: Select the Sheet 1 tab (in the bottom left-hand corner of the screen) to return to the data table. Select the second column of data (without the heading) and open the Chart Wizard (see Step 3, above). Select an XY Scatter graph and the second option in Chart sub-type. Step 12: Select Next. The data you highlighted will appear in the Data range box. These values will appear on the x-axis of the graph. Step 13: Select the Series tab. Make sure Series 1 is highlighted in blue in the Series box. Click in the Name box and type in a name for the data in the series (e.g. Average size of keelback snakes ) again. Step 14: Select the Add button below the Series box to add your second set of red-bellied black snake data (Series 2). Click in the Name box and type in a name for the data in the series (e.g. Average size of red-bellied black snakes ). Step 15: Click on the button on the right-hand side of the X Values box. The graphing box will minimise. Select the data in the third column of the table to add the years since exposure for the redbellied black snakes on the x-axis. Select the X values button again to return to the wizard. Step 16: Click on the button on the right-hand side of the Y Values box. Select the data in the fourth column of the table to add the body size data for the red-bellied black snakes on the y-axis. Select the Y Values button again. Notice that both graphs are displayed on the same set of axes. Senior Biology 9 of 11

10 Unit 2 Step 17: Select the Next box. Enter titles for your chart and the x- and y-axes. Step 18: Select Next. Save the graph as a new sheet and select Finish. Analysing the data The data analysis section of a report highlights what you would like your reader to notice in your graph. Think about what data or trends are most important. The following questions and hints will be useful when you are writing your report for the send-in task. Notice that on the basis of the graphs, it may be possible to argue for or against change occurring in the body size of the snakes. Consider the following questions: (a) Were your predictions supported? If not, why not? (b) Explain any changes that have occurred in the size of the snakes since the introduction of the cane toad, making specific references to your graphs. Do you think that either of the snake populations has changed? Explain why or why not. If the population has changed, what has it changed in response to (i.e. what is the selective force that has acted, and what has changed in the snake populations in response to this force?)? (c) What do you think you might find if you were able to collect data on the snakes and the cane toads in another 100 years? Would the snakes change? Would the cane toads change? Challenge question: (d) The data was also analysed using mathematical statistics to determine if there was a real change in body size of the snakes after time. Suggest some simple mathematical calculations that could be used to further analyse the data. 10 of 11 Senior Biology

11 Source: Phillips, B & Shine, R 2004, Adapting to an invasive species: Toxic cane toads induce morphological change in Australian snakes, Proceedings of the National Academy of Sciences, 101: Available Internet: abstract/101/49/ Senior Biology 11 of 11