Outdoor cats are voracious predators, and they have been linked to the decline of many bird, reptile, and amphibian populations. In the United States alone, cats kill an estimated 2.4 billion birds and 12.3 billion mammals each year. This includes many threatened and endangered species.
In addition to killing wildlife, cats also contribute to the spread of disease. They can carry and transmit diseases such as toxoplasmosis, salmonella, and rabies to humans and other animals.
For these reasons, some people argue that it is irresponsible to keep cats as pets. They believe that cats should be kept indoors at all times, or that they should be spayed and neutered to prevent them from reproducing and adding to the population of feral cats.