http://www.pawsoftulsa.org/spaytosave/just1litter.htm

http://www.woodhavenlabs.com/justonelitter.html

From http://www.fmhs.org/sn/why.asp

Simple arithmetic illustrates how "just one litter" contributes to the mass killing: Two dogs breed. Six offspring are born. The six offspring reproduce within one year, and are responsible for six offspring each. In one year a litter of six can become 36. And unfortunately, it doesn't stop there. At the end of ten years just one unaltered dog can be responsible for 4,372 births. One unaltered cat can be responsible for 420,000 kittens in just seven years!