One is poisonous and one is not poisonous.
cactus
Yes, holiday cactus plants [Schlumbergera and Zygocactus spp] are poisonous. So not one of their body parts are meant to be chewed, eaten or swallowed. Touching or smelling the body parts isn't a problem. Problems result only with the mistaken notion of the plant and its parts as a source of drink or food.
camels for starters...
i think yes
Cactus
Many types of cactus are edible and commonly eaten by people. Prickly pear cactus is one example.
Voles are not "poisonous," and are eaten by birds of prey worldwide.
Yes, the part of the cactus that's eaten dies. But no, what's left of the cactus plant doesn't die necessarily. It depends upon the size of the piece that's eaten, the manner in which the piece is removed, and the condition of the remaining body parts.
no there not because if they were they wouldn't sell them at the store.
the not poisonous one
It does get eaten. By humans and camels and such... But normal animals don't because they have little pointy things that jab your face. It hurts a lot!