A new book, called "Time to Eat the Dog: The Real Guide to Sustainable Living" suggests that man's best friend might not be best for the environment. The article states the following:

The Vales, specialists in sustainable living at Victoria University of Wellington, analysed popular brands of pet food and calculated that a medium-sized dog eats around 164 kilos (360 pounds) of meat and 95 kilos of cereal a year.
Combine the land required to generate its food and a "medium" sized dog has an annual footprint of 0.84 hectares (2.07 acres) -- around twice the 0.41 hectares required by a 4x4 driving 10,000 kilometres (6,200 miles) a year, including energy to build the car.

To be honest, I was not buying into this at first, but they later said that the researchers sent there results to the Stockholm Environment Institute and there results were essentially the same. The same results said that:

Cats have an eco-footprint of about 0.15 hectares, slightly less than driving a Volkswagen Golf for a year, while two hamsters equates to a plasma television and even the humble goldfish burns energy equivalent to two mobile telephones.