system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some plants have defense mechanisms like poison ivy and gympie gympie (called the suicide plant because it's so painful) and others don't?
gpt
Plants develop all manner of elaborate defensive mechanisms to thwart their predators.

Some grow tall and encase themselves in armored bark.  Some are covered with nasty thorns.  Some pack horrible irritating oil.  Some fill their leaves with noxious alkaloids.

All these cost the plant time and energy though, which gives plants that pour all their effort into growth an advantage sometimes.  Yes they're defenseless against a wandering cow, but if you and 10,000 of your seedlings can overwhelm the slower-growing competition it's still a good strategy.

Basically, nasty thorny poisony plants are optimized to fight animals, fast growing leafy plants are optimized to fight other plants.