Ecology 5th Edition by Manuel Molles

Exercise 20

The rivers of central Portugal have been invaded, and densely populated, by the Louisiana crayfish, Procambarus clarki, which looks like a freshwater lobster about 12 to 14 cm long. The otters of these rivers, which were studied by Graga and Ferrand de Almeida (1983), can easily catch and subdue these crayfish. Using the model for prey choice: explain why the diets of the otters of central Portugal would shift from the highly diverse menu shown in figure 7.17, which included fish, frogs, water snakes, birds, and insects, to a diet dominated by crayfish. For the crayfish, assume low handling time, very high encounter rates, and high energy content.
Figure 7.17 Like many other widespread carnivores, the diets of river otters show great geographic variation (data from Graça and Ferrand de Almeida 1983).