I've had issues with mice/rats that have eaten entire plants however they avoid the really spiny plants for obvious reasons. You will know it if is rodents because the damage will only occur over night and they will leave behind the skin (they don't eat it) and droppings depending on if they are mice or rats (two completely different animals by the way) the droppings will be much bigger for rats but are both the shape of an ellipse shape.
You can get the small red pellets by Tomcat which WILL kill any mice or rat, the problem is it tastes so good anything else will eat it too, be very very careful with that stuff I can't stress it enough, I.E. the neighbours cat, possums, birds, dogs and I have even heard of people loosing sheep from it, it will kill the lot. That being said probably the only thing that will kill off a large population of rats.
I have have a mate who lives on a property who had problems with rabbits and let me say they caused all sorts of devastation to his cacti and other plants.
I would buy two traps a small mouse trap $1 and a larger rat trap $3 put peanut butter on them and see what turn up.
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