Suppose that researchers wanted to examine the combined effects of an introduced predator a

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Online Analyzing Data to accompany Ecology, Fifth Edition Bowman • Hacker

1.1 What Are the Combined Effects of Introduced Predators and Parasites on Amphibian Populations?

Suppose that researchers wanted to examine the combined effects of an introduced predator (a trout) and the trematode parasite Ribeiroia on amphibian populations. To do this, they established frog populations in each of 40 artificial ponds. Each pond was assigned at random to one of four treatments (10 ponds per treatment): 1) neither trout or parasites were added to the pond (the “No trout, no parasite” treatment); 2) no trout were added but parasites were added (“No trout, parasite added”); 3) trout were added but parasites were not added (“Trout added, no parasite”); and 4) both trout and parasites were added (“Trout added, parasite added”). Each pond contained refugia where tadpoles could avoid attack by trout, to avoid fish predators driving frog populations to extinction in an artificial pond, unlike what typically occurs in a natural pond. After two breeding seasons, the researchers estimated the density of frogs in each pond. The results are shown in the table.

Treatment Average frog density (per 10 m 2

of pond surface area) No trout, no parasite 180.2 No trout, parasite added 111.4 Trout added, no parasite 125.8 Trout added, parasite added 14.3

Question 1. Construct a bar graph showing the average density of frogs for each of the four treatments (see Web Stats Review 1.1.2 for a description of bar graphs).

Answer:

(Solution Manual) 1 / 4

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Question 2. Independent of any possible effects of trout, estimate the reduction in frog density caused by the addition of parasites. Justify the calculations used to answer this question.Answer: To remove any possible effects of trout and estimate how parasites alone affected frog density, the average frog density in the “No Trout, no parasite” treatment is compared to the average frog density in the “No Trout, parasite added” treatment. Based on the results in the table, parasites alone reduced frog density by 180.2 – 111.4 = 68.8 frogs per 10 m 2 of pond surface area.Question 3. Independent of any possible effects of parasites, estimate the reduction in frog density caused by the addition of trout. Justify the calculations used to answer this question and compare the relative effects of trout and parasites on amphibian populations.Answer: To remove any possible effects of parasites and estimate how trout alone affected frog density, the average frog density in the “No Trout, no parasite” treatment is compared to the average frog density in the “Trout added, no parasite” treatment. Based on the results in the table, trout alone reduced frog density by 180.2 – 125.8 = 54.4 frogs per 10 m 2 . In this experiment, parasites reduced frog densities by a slightly greater amount (68.8 frogs per 10 m 2 ) than did trout (54.4 frogs per 10 m 2 ).Question 4. Describe the combined effects of parasites and trout on frog densities. Interpret this result and suggest a hypothesis for why this may have occurred.Answer: When both parasites and trout were added, frog densities dropped by an average of 180.2 – 14.3 = 165.9 frogs per 10 m 2 . To interpret this result, recall that the reduction in frog density caused by parasites alone was 68.8 frogs per 10 m 2 , while the reduction in frog density caused by trout alone was 54.4 frogs per 10 m 2 . Thus, when both parasites and trout were added, we might expect that frog densities would have dropped by a total of 68.8 + 54.4 = 123.2 2 / 4

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frogs per 10 m 2 . However, the addition of both parasites and trout caused frog densities to drop by 165.9 frogs per 10 m 2 —a value that is about 35% higher than our expected value of 123.2 frogs per 10 m 2 . One hypothesis that could explain this result is that infection by parasites made frogs more vulnerable to trout, thereby reducing frog densities more greatly than would occur from the combined (additive) effects due to parasites alone plus those due to trout alone. 3 / 4

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Online Analyzing Data to accompany Ecology, Fifth Edition Bowman • Hacker

2.1 How Do Changes in Soil Crust Cover Influence Climate?

Dryland ecosystems, including deserts and some grasslands, shrublands, and woodlands, cover around 12% of the terrestrial surface. As described in the Case Study for Chapter 22, soil crusts, composed of cyanobacteria, mosses, and lichens, are an important feature in many of these drylands. Well-developed soil crusts are dark in color and have a rough surface texture.Climate change models indicate that dryland ecosystems will experience significant changes in temperature and precipitation. In 2005 researchers started an experiment to assess the influence of predicted climate change on soil crusts in a piñon-juniper woodland. All plots received ambient sunlight and rainfall. Treatment plots received additional warming (+2°C/ 3.6 ˚F from 2005 to 2008 and +4°C/ 7.2˚F from 2008 to 2016) using infrared heat lamps and twice weekly watering to simulate projected climate change for the Colorado Plateau (Reed et al. 2012).Relative to control plots, all treatment plots had losses of moss and lichen species and increases in cyanobacteria. In turn these changes influenced the soil surface color and roughness (Figure 1).

Figure 1 Effects of Climate Change on Soil Crusts Photos of (A) control plots and plots subjected to (B) watering, (C) warming, and (D) watering plus warming treatments. Note the variation in surface color and

roughness. (After W. A. Rutherford et al. 2017. Sci Rep 7: 44188.)

Treatment Cyanobacteria cover (proportion of total) Roughness a

Soil moisture at

• cm (%) Albedo

Control 0.68 8.24 0.10 0.23 Control 0.71 5.37 0.06 0.32 Control 0.28 12.59 0.11 0.24 Control 0.41 12.31 0.15 0.18 Control 0.06 6.39 0.13 0.20

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