Why Do Carnivorous Plants Grow Well in Nitrogen Poor Soil?
You or a friend of yours probably had a Venus flytrap plant as a kid (or perhaps as an adult, too). The plant would sit, waiting for a fly to amble along into its green jaws. Aside from some water, though, you never had to fertilize the flytrap. This is because carnivorous plants get their nutrition from insects, and it's why these plants do so well in nitrogen-poor soil. Does this Spark an idea?
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Mechanisms
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Carnivorous plants trap their prey through a number of structures. Flytraps have two-sided traps; sundews and pitcher plants have sweet nectar that attracts and traps bugs, for example. The plants release digestive acids that destroy the insects, while the plant absorbs the resulting nutrients. In contrast, the root systems of the plants aren't very strong (or present, in some cases), because they do not need to dig nutrients such as nitrogen out of the soil.
Opportunity Abounds
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Regular, soil-nitrogen-dependent plants are, of course, not going to do well in an area where the soil has little or no nitrogen. They're going to die there, leaving an open patch of ground. This is perfect for carnivorous plants, because they don't need anything in the soil, and there's no competition for space from other plants. They can grow there in relative peace.
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Better Soil, Worse Luck
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If the soil improves nutritionally, that can spell trouble for the carnivorous plants. Regular plants that get their nitrogen from soil would be able to grow in the area, potentially taking over if the insects don't or can't travel down to the carnivorous plants.
Which Came First?
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Whether carnivorous plants became carnivorous in response to tough conditions over time, or if they have the ability to evolve into noncarnivorous plants in the right nutritional conditions is unknown. Some carnivorous plants can definitely switch between the two states; the International Carnivorous Plant Society (ICPS) notes plants like Triphyophyllum won't grow the leaves necessary to trap insects if they're already receiving enough nutrition. So, some carnivorous plants can adapt to new soil conditions and survive.
Optional Carnivores?
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The Independent newspaper reported in December 2009 that researchers at the Royal Botanic Gardens, Kew had realized that plants such as tomatoes and potatoes may in fact be carnivorous. Kew itself reported that petunias may fall in this group as well. The plants can't actively trap and eat insects, but they do have patches of sticky hairs that catch insects. The insects then decay and parts drop to the ground, blending into the soil. The plants take up the nutrients from the decayed insects. According to the Independent article, our fertilizing these plants may give them enough nutrition that the carnivorous aspects aren't really apparent, although they still exist even on garden-grown plants. Take away that convenient fertilizer and compost, though, and it's possible the plants could switch to relying more on insects for food. Kew researcher Professor Mark Chase notes in the article, "They do trap small insects on a regular basis. They do kill insects. We suspect in the domesticated varieties they are getting plenty of food through the roots from us so don't get much benefit from trapping insects. In the wild they could be functioning in the way that could properly be considered carnivorous."
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