Does clear-cutting of trees in an area cause local extinction of the tailed frog? We don’t really know, but an article that begins to address that issue attempts to quantify aspects of the habitat and microhabitat of streams with and without tailed frogs. The following data are from “Distribution and Habitat of Ascaphus truei in Streams on Managed, Young Growth Forests in North Coastal California” (Journal of Herpetology [1999]: 71–79).

Assume that the habitats and microhabitats examined were selected independently and at random from those that have tailed frogs and those that do not have tailed frogs. Use a significance level of .01 for any hypothesis tests required to answer the following questions.

a. Is there evidence of a difference in mean stream gradient between streams with and streams without tailed frogs? (Assume that it is reasonable to think that the distribution of stream gradients is approximately normal for both types of streams. It is possible for gradient percentage to be either positive or negative, so the fact that the mean −2 standard deviations is negative is not, by itself, an indication of nonnormality in this case.)

b. Is there evidence of a difference in the mean water temperature between streams with and streams without tailed frogs?

c. The article reported that the two depth distributions are both quite skewed. Does this imply that it would be unreasonable to use the independent samples t test to compare the mean depth for the two types of streams? If not, carry out a test to determine whether the sample data support the claim that the mean depth of streams without tailed frogs is greater than the mean depth of streams with tailed frogs.

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Habitat Characteristics Stream gradient (%) Water temp. (°C) Microhabitat Characteristics Depth (cm) Habitat Characteristics Stream gradient (%) Water temp. (°C) Microhabitat Characteristics Depth (cm) n 18 18 82 n 31 31 267 Sites With Tailed Frogs X 9.1 12.2 5.32 Sites Without Tailed Frogs X 5.9 12.8 8.46 $ 6.00 1.71 2.27 S 6.29 1.33 5.95