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WHAT MAKES PLANTS GO BAD?

Why do plants become carnivorous?

They do so only when it's very hard to get nutrients like nitrogen. They still get their energy from photosynthesis, and it costs a lot of energy for them to be carnivorous. So they only arise in unusual environments like acidic bogs that are nitrogen-poor but sunny. And some cease to be carnivorous in the winter, when there's less light.

There's a continuum of plants ranging from noncarnivorous to so-called "protocarnivorous" plants to fully carnivorous plants. Protocarnivorous plants trap and kill insects or other animals but don't have special enzymes to digest their prey. Some of these evolve to become carnivorous. But don't think of evolution as goal-directed: depending on changes in their environment, some evolve *away* from being carnivorous. It's mainly a matter of how easy it is to get nitrogen.

There's an interesting conflict in this game. Plants have many clever ways of trapping insects and forcing the insects to pollinate them. This is a perfect first step in becoming protocarnivorous. But this sets up a battle of competing forces: will evolution optimize the trap for pollination or for killing insects and feeding the plant?

This is called the 'pollinator-prey conflict'. Many carnivorous plants try to have it both ways! And some are good at releasing insects that pollinate the plant, while killing others.

The game gets more complicated, too. One species of tree frog in Borneo specializes in eating insects caught in pitcher plants. And there's an ant that dives into pitcher plants to eat insects caught there - and also lubricates the top of the plant to make it easier for insects to fall in!