“Murderous plants.” This is how many botanists (and others) described Venus flytraps, pitcher plants, and other carnivorous plants in the 19th century. But the attention carnivorous plants received wasn’t all sensational and negative. Linnaeus called them “miracles of nature.” And Charles Darwin spent 15 years writing an entire book (Insectivorous Plants) about them.

What sets carnivorous plants apart from other plants is their ability to trap, digest, and absorb nutritionally beneficial substances from animal prey. In essence, they have a second energy-acquisition system in addition to photosynthesis.

Often, the best way to approach a question in biology—such as why carnivorous plants are confined to nutrient-poor habitats—is with a simple experimental manipulation. This can be one of the most important steps in trying to understand how things work.

Carnivorous plants occur on every continent except Antarctica. This doesn’t seem surprising. What is surprising is that they compete successfully only in nutrient-poor environments. Why don’t they dominate in all habitats? After all, they can make their own food by capturing energy from the sun and converting it into a usable chemical form. Animals can’t do this. And they can capture and kill many types of animals, primarily insects, making use of the nutrients in these organisms. Most plants can’t do this.

Researchers have used some logic, along with a clever experimental manipulation, to address this question. Here’s their approach.

First: the logic. Noting that carnivorous plants are almost exclusively found in nutrient-poor habitats, researchers concluded that the less adequate the soil nutrition—particularly with respect to nitrogen, which is necessary for all growth—the less non-carnivorous plants can survive and grow. Carnivorous plants “win” in these environments because their carnivory provides them with the nitrogen necessary to compensate for the poor soil.

But why don’t carnivorous plants also outcompete other plants everywhere else? The researchers proposed that the structures necessary for carnivory—flytraps or pitchers, for example—are costly to produce and maintain. This would put carnivorous plants at a competitive disadvantage anywhere the soil is sufficiently nourishing.

Is there experimental evidence to support this speculation? Yes.

Second: the manipulation. Give some plants extra nitrogen.

The setup. Researchers worked with pitcher plants growing in bogs in the northeastern United States. Every two weeks during the growing season (June through September), for three consecutive years, they added 5 milliliters of a solution directly to each open pitcher on each plant, using one of three types of solution:

Solution 1: distilled water only

Solution 2: low concentration of nitrogen

Solution 3: high concentration of nitrogen

The researchers measured (1) the number, size, and shape of pitchers (which are necessary for catching insects) and (2) the photosynthetic rate (as micromoles of CO₂ used per square meter per second) of the largest leaf on all plants.

The results

• Plants that received either distilled water or a low concentration of nitrogen produced normal carnivorous pitchers with large tubes.
• Plants that received high concentrations of nitrogen produced leaves with small carnivorous tubes (less than half the size of typical pitchers). In some cases, no tubes were produced at all, and all of the leaves were entirely non-carnivorous.
• Photosynthetic rate increased as leaves became more flattened and less carnivorous.
• Photosynthetic rate decreased when plants invested more in making pitchers.

Conclusions. The results lead to two general conclusions.

• 1. With low availability of nitrogen—as in bogs and other nutrient-poor habitats—carnivorous plants invest more in the structures necessary for carnivory. These plants reduce their investment in carnivory when there is an excess supply of nitrogen.

• 2. There is a photosynthetic cost to producing carnivorous structures. Although pitchers are good for capturing prey, they are less efficient at photosynthesis than other leaves. Consequently, unless a habitat is nutrient-poor, carnivorous plants are less efficient than (and thus outcompeted by) non-carnivorous plants.