According to pet columnist Gina Spadafori, keeping rabbits as pets is more environmentally friendly than keeping cats and dogs. You can grow their food in a garden, and then you can throw the recycled paper litter and droppings into a compost pile to fertilize the garden. If you buy locally grown hay, you reduce the need for shipping food across the country, thereby cutting down on carbon emissions.

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http://www.sfgate.com/cgi-bin/article.cgi?f=/g/a/2007/11/27/petscol.DTL