Survival! Eating cactus as an alternative to water on Snake Island

There is a type of barrel cactus that is very poisonous. As you know, nopales, a popular paddle cactus used in Mexican dishes, is very edible and very nutritious. I don't know that there is a general "rule of thumb" in determining whether a cactus is safe to eat. I took my chances with this one and found it to be good. I have eaten it many times with absolutely no problems.