Carnivorous plants can photosynthesise, so why eat flies?
why do carnivorous plants eat insects if they are capable of photosynthesis and can make their own food?
Chris - They've got leaves, they can photosynthesise, which means they can use energy in sunlight to drive a reaction between carbon dioxide and water, which they get from the air and from the ground to make glucose - sugar (that's food). So, why on earth do they need to catch flies to supplement that diet? What's wrong with that? Well, the point is that plants don't just rely on glucose alone. It's a bit like that poster, "Men can't live on bread alone" (that picture of a beer bottle on a woman's chest). Plants also need other micronutrients and minerals, and things which they normally obtain through their roots. In other words, they would put down roots into the soil, those minerals will be drawn up with the water they take in, and they would then be used. Other things like proteins and amino acids will be brought in that way and other macromolecules produced by fungi that plants make associations with in the soil. Those are called their hartig nets, their mycorrhizal relationship.
But carnivorous plants often live in "awful", nutrient-poor places. In other words, very nutrient scarce environments where they're very boggy, so most of the nutrients have probably been leached away by water. As a result, the soil is so poor that many of those trace elements that keep plants growing normally, just aren't available in appreciable amounts. So the plants need to look to the air to obtain that nutrition and they do it by catching insects because if they catch an insect, insects have got lots of iron, they've got lots of proteins, they've got lots of other micronutrients in them that the plants have adapted and evolved to make use of, and to supplement the poor source of things that are coming in through the soil.
What that means is that the plant can now exploit a niche in the environment that other plants can't. They can grow in places where the competition from other plants for light is really quite weak because other plants can't grow very well. So despite being able to photosynthesise, they've made up for the short fall in the general, other nutritional requirements of the plant by looking to the air in the form of insects that they grab and eat to supplement their diet.
Dave - So eating not for energy, but for fertiliser essentially?
Chris - Yes. They're finding their fertiliser from the sky!