Are Owned Cats Causing an Overpopulation Crisis?

Statistics
Cats vs. dogs at the shelter:
At the third largest shelter in the country, Santa Clara County, California, 36% of the animals handled are dogs, vs. 64% cats.
Euthanasia totals are 80% cats and 20% dogs.
Litters produced by owned cats:
A Massachusetts SPCA (MSPCA) study in 1991 shows 20% of cats have a litter prior to being altered.
A National Pet Alliance (NPA) study of Santa Clara County in 1993 concluded 16% of female cats have a litter prior to being altered.
A 1981 study of Las Vegas, out of Kansas State University, found that 16% of the unspayed females reproduced.
Stray cats fed, but not claimed:
A HSUS national survey in 1992 found nearly 25% of households feed stray cats.
Are owned cats altered?
The MSPCA study reported 87% of owned cats were already altered.
The Las Vegas study from KSU found 86% of owned female cats were spayed.
A report from Tufts University showed 91.5% of owned female cats were spayed.
The Santa Clara County survey found 86% of owned cats were altered.
Death rates:
Dr. Roger Nassar, who conducted the Las Vegas study, concluded the average life span of the owned cats, based on respondents' answers, was 7.02 years.
Ellen Perry Berkeley, in her book Maverick Cats, from various sources indicates the life span of feral cats to be 2-3 years.
Also from Maverick Cats, we find only 33% of feral kittens are alive at age 1 year.
More interesting statistics