I would think that if it were strong enough to get the arrow to the animal, that would be more than enough to kill it.  They are rather small and fragile after all.  10-15# should be plenty to pierce them sufficiently.

strictly speaking, 15 is probably sufficient.

Of course, you'll have no accuracy and won't be able to hit anything. I'd use a bow suitable for target practice maybe ~35#min.

Many states have minimum bow weight regulations for hunting.   For deer and such, it's usually in the 45-pound range.

BTW, I'm learning more than I ever thought I'd want to about bows and bowmaking.   I started out to make a "board" bow following directions off the internet.  First try was a failure, bad wood...

I'm now at about the same progress with #2, using a very nice piece of red oak.  I'm taking my time....remove a little wood, test the "tiller", remove a little more...   
Slow but sure.   Like they say, "the bow is in there somewhere."
I've been reading up on various "traditional" websites, and I just got the first copy of the 3-volume "Traditional Bowyer's Bible."
I'll take pics when I get closer to "done".

I considered doing just this, but the resources I am working from already exist in a form much better than I could manage.
I strongly reccomend the following sites:

http://residents.bowhunting.net/sticknstring/brdbows.html

and:

http://www.angelfire.com/magic/jawge/boardbowbuildalong.html

Both of these have a wealth of information.   There is also an excellent forum associated with the second site:

http://www.bowsite.com/bowsite/tf/lw/threadsx2.cfm

really,huh,but how does a bumper hit a squirrel

i think there bloody tough when i shoot them with my caterpult the skin doesnt break but a hole appers under the skin and the ribs on the otherside are broken its the skin thats strong but it has to be ours would rip if we did the same things as a sqirrel

I'm sure squirrels have a thick hide and are very resilient, but they are rather small animals. An arrow sticking 3 centimeters into a deer probably only hits musscle, a squirrel would be pierced.