1. The mountain yellow-legged frog (Rana muscosa) was once a common inhabitant of the Sierra Nevada (California, USA). It has declined during the past century due in part to the introduction of non-native fi sh, such as trout, into naturally fi sh-free habitats. The bar chart shows the average number per lake of tadpoles (aquatic larval stage) and frogs in lakes with and without trout in 1996.

(a) State the number of tadpoles per lake with and without trout.

With trout:

Without trout:

(b) Compare results for lakes with and without trout.

(c) The trout might affect the number of frogs or tadpoles by competing for resources.Suggest one other way in which trout might affect the number of tadpoles or frogs in lakes.


In order to restore the frog population, introduced trout were removed from the lakes. The map of the LeConte Basin study area shows the distribution of mountain yellow-legged frogs and trout populations just prior to the removal of the trout in 2001. The graphs show the population of tadpoles and frogs in the lakes before, during, and after the removal of the trout.

(d) State the tadpole density in each lake in 2004.

Upper LeConte Lake: . . . . . . tadpoles 10 m–1 shoreline.

Lower LeConte Lake:. . . . . . . tadpoles 10 m–1 shoreline.

(e) Suggest one possible reason for the difference in tadpole density between Upper and Lower LeConte lakes.

(f) Describe the effect of removing trout on frog density in Upper and Lower LeConte Lakes.

(g) Using the map and graph, predict whether the removal of the trout from Upper and Lower LeConte Lakes will lead to a permanent recovery in the number of frogs and tadpoles.




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