Like the man-eating plant in Little Shop of Horrors, carnivorous plants rely on animal prey for sustenance. Fortunately for humans, carnivorous plants found in nature are not dependent on a diet of human blood but rather are satisfied with the occasional fly or other insect. The existence of carnivorous plants has fascinated botanists and non-botanists alike for centuries and raises the question, "Why are some plants carnivorous?"
A recent article by Drs. Jim Karagatzides and Aaron Ellison in the September issue of the American Journal of Botany (www.amjbot.org/cgi/content/full/96/9/1612) addresses this question. As Ellison stated, "The general answer to this is that in environments that have few nutrients (such as bogs, where we study carnivorous plants), carnivory allows these plants to capture nutrients 'on the wing'. But if it's so good to be a carnivorous plant in these kinds of environments, why aren't there more carnivorous plants? Knowing how much it 'costs' a carnivorous plant to make a trap is a key piece of information needed to understand why there aren't more carnivorous plants."
Elllison and Karagatzides simultaneously measured both costs and benefits for traps, leaves, roots, and rhizomes of 15 different carnivorous plant species, including pitcher plants and the Venus fly trap. By measuring the construction cost of carbon needed to create these plant structures and comparing it to the payback time—the amount of time the structure takes to photosynthesize to recoup the carbon used in its construction—Ellison and Karagatzides were able to determine how beneficial a trap might be to a plant.
Contrary to expectations, the average cost to create a trap was actually significantly lower than the cost to create a leaf. Ellison said, "The most interesting result is that carnivorous traps are 'cheap' to make (at least compared with leaves). Models of the evolution of carnivory in plants have suggested that traps should be 'expensive' structures—they take a lot of carbon and nutrients to make, and so only when they can't recover these costs in any other way should carnivory be adaptive (or evolutionarily favored). But because carnivorous plants have very low rates of photosynthesis, it still takes a very long time for the plants to 'pay' for them (by accumulating new carbon through photosynthesis)."
Understanding how carbon and mineral nutrients are allocated among different plant organs, different species, and vegetation of different biomes is one of the major goals of the field of plant ecology. Carnivorous plants are a model organism to study these carbon and mineral nutrient tradeoffs because the plants inhabit open environments where light and water are not limiting but nutrients are in extremely short supply, and therefore it is relatively easy to separate out experimentally the effects of nutrient limitation from effects of limitation of light or water.
Ellison and Karagatzides's findings advance our understanding of how complete food webs function. Ellison stated, "Nicholas Gotelli [from the University of Vermont] and I, along with our students and colleagues, have spent more than 10 years developing this micro-ecosystem as a model for how complete food webs—including the plant as both producer and habitat, and the aquatic food web that lives in the pitchers as both detritivore and mutualist—and aquatic ecosystems actually work. These studies have provided new insights into population dynamics and extinction, the importance of food webs for persistence of top predators, and now how organisms allocate nutrients to better control and modulate energy and nutrient fluxes across ecosystem boundaries."
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