Ecology of rabbits in New Zealand compared with California rodents

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Even though there are some introduced mammals in New Zealand, the country is still relatively free of pests in comparison with California. For example, in New Zealand there is one kind of rabbit, one hare, one mouse and several rats, whereas there are about 100 species of rabbits and rodents in California.
During the past ten years I have had the opportunity of studying the changes in environmental conditions that have caused some species of rodents in California to become unusually numerous, hence also to become a pest to agriculture The basic factor responsible for this situation is land use; when man alters the environment either intentionally to suit his needs or as a result of his method of using the land, many kinds of rodents then all but disappear, but a few other species are often favoured by the new conditions, and build up into dense populations.