question led me to the wikipedia entry for poisonous plants.
For the moment, let's focus on berries.
The purpose of berries, from the plant's point of view, is to transplant seeds. here I'm using purpose in the general sense; obviously plants don't consciously do anything.
So wouldn't poisonous berries defeat the purpose? A sick animal is not going to spread the seeds.
A couple ideas...
1. Some berries are poisonous to humans but not birds. Okay, solid.
2. Maybe if the berries were delicious to humans, squirrels, whoever- as well as birds, then we would eat them all, leaving none for the birds. This would have an adverse effect on the seed distribution. plausible enough. any evidence of this?
3. In fact the purpose is not to spread the seeds; sometimes the berries only serve to ward off predators. kind of doubtful.
4. There must be some fascinating chemistry stuff going on here, with the poisons. I don't even know what to ask....is the poison a necessary byproduct of some other process that has some other purpose?
I'm interested to hear the hivemind's take on this!