Why Be A Carnivorous Plant?

Do plants eat? The answer is no… and yes. We are taught in biology class that plants make their own food through a process called photosynthesis. This is when plants take in sunlight and convert it to energy. Then a special chemical reaction is initiated that allows the plant to make sugar and other carbohydrates (aka food) from carbon dioxide (CO2) and water. On the other hand, you can go to your local garden center or hardware store and find products on the shelf that contain the words “Plant Food” on the label. While plants can make carbohydrates from photosynthesis, they must also make proteins. These proteins are important to make enzymes, which bring about chemical reactions, including … you guessed it … photosynthesis. These enzymes require an important element, nitrogen, in a form that plants cannot make themselves. In addition, plants must produce a chemical that stores energy and can be used quickly to power these same chemical reactions. This chemical is called adenosine triphosphate, or ATP, which, as the name indicates, contains phosphorus. So, the “Plant Food” you see at the garden center contains nutrients that plants do not make themselves but that are needed to make enzymes and ATP.

In the wild, there is nobody around to feed plants nitrogen and phosphorus. Many plants find themselves growing in soils lacking these important nutrients. So, what do plants do? Plants have evolved many ways to solve this problem. I will focus on just one fascinating and unusual solution – carnivory. Carnivory is the eating of an animal. Carnivorous plants (like the famous venus flytrap) have leaves that can do two things: photosynthesis (which is typical for most plants) and the capture and digestion of insects (which is not). By capturing and digesting insects, carnivorous plants can supplement the meager supply of nitrogen and phosphorus (and other nutrients) they obtain from the soil with nitrogen and/or phosphorus obtained from the insects they capture.

Many carnivorous plants, including the venus flytrap and pitcher plants, often find themselves in habitats that not only have nutrient-poor soils but that experience frequent fires. It turns out that many carnivorous plants can survive such fires and grow back quickly after. However, one thing that has puzzled scientists is whether and why carnivorous plants would benefit from fires. Many fires increase the availability of some nutrients to the soil. If there is an increased availability of soil nutrients, one would expect the need for carnivory to diminish. So, why do we often see carnivorous plants in ecosystems that are frequently “fertilized” by fires? 

One possible benefit of carnivory in fire-prone ecosystems is that it enables plants to take advantage of soils lacking in nitrogen after a fire. Yes, I know. I just told you that fires often increase the availability of some nutrients to the soil, but nitrogen often is NOT one of those nutrients. In certain fire-prone areas, fire can reduce nitrogen availability in the soil by burning organic matter. This converts the nitrogen in the organic matter to a gas, which is then lost to the atmosphere. Therefore, carnivory could allow plants to tolerate low soil nitrogen further reduced by fire.

Another possible benefit of carnivory in fire-prone ecosystems is that it enables plants to obtain phosphorus from insect prey before a fire. Before a fire, soil phosphorus is in short supply. Carnivorous plants may encounter competition from other plants for phosphorus in the soil. Unlike nitrogen, phosphorus is an abundant element in the ash that is deposited after a fire. Therefore, the availability of phosphorus in the soil is greater after a fire. However, the increased availability of soil phosphorus after a fire may not be sufficient to sustain plants through years without fire. Obtaining phosphorus from insect prey, through carnivory, during years without fire is a potential solution to this problem.

Using a field experiment, we attempted to answer the question: why be a carnivorous plant in a fire-prone wet pine savanna in southeastern Mississippi? We focused on the carnivorous pale yellow pitcher plant, Sarracenia alata. We chose this species for two reasons. First, I had previously shown juveniles of the species had responded positively to fire. Second, pitcher plants are ideal for manipulating carnivory because one can reduce the amount of prey they naturally receive in the field by stuffing their pitchers with cotton. We measured growth and the amount of nitrogen and phosphorus within the leaf (pitcher) of adult plants in response to multiple combinations of treatments. These treatments included prey exclusion, neighbor reduction (by clipping neighboring plants), and addition of nitrogen fertilizer, phosphorus fertilizer, and ash to the soil. We tested two ideas: Nitrogen Limitation and Phosphorus Limitation. 

1) Nitrogen Limitation – Prey provide nitrogen, which is most beneficial under those conditions that occur after a fire (reduced neighbors, low soil nitrogen, addition of ash to the soil, sufficient soil phosphorus)

2) Phosphorus Limitation – Prey provide phosphorus, which is most beneficial under those conditions that occur before a fire (abundant neighbors, low soil phosphorus, no addition of ash to the soil, sufficient soil nitrogen). 

We evaluated the benefit of carnivory under each set of conditions. We did this by preventing the capture of prey and then measuring how much growth was reduced. We also measured which of the two nutrients in the leaf, nitrogen or phosphorus, was reduced more.

When we prevented the pitcher plants from capturing insects, we found that growth was reduced most when neighboring plants were not clipped. This was partially reversed by adding phosphorus fertilizer to the soil at the base of the pitcher plants. We also found that preventing prey capture caused a decrease in the amount of phosphorus in the leaf when nitrogen was added to the soil.

These results support the Phosphorus Limitation idea and suggest that carnivory in adult pitcher plants sustains a sufficient supply of phosphorus to the plants during years without fire in wet pine savannas, when soil phosphorus is lower. Interestingly, adding ash obtained from burned pine savanna vegetation did not reduce the benefit of carnivory. This suggests that fires in this system do not have a strong nutrient fertilization effect. In other words, adding ash to the soil does not add a lot of nutrients to the soil. What appears to be more important regarding the benefit of carnivory in fire-prone ecosystems is that soils in the wet pine savannas we studied lacked soil nutrients both before and after fires.

One limitation of the study is that we only looked at adult pitcher plants, which were generally not shaded by their neighbors. Previous work by our group with small, young plants showed that their growth was reduced more by shade from neighboring plants than by a lack of prey or soil nutrients in the absence of fire. Juvenile plants thus benefited from fire because it reduced shade. Prey increased growth of juvenile pitcher plants only after shade was reduced.

To the extent that our group’s research applies to other carnivorous plants in fire-prone ecosystems, we conclude that the association of carnivorous plants with fire is not puzzling. However, our experiments with the pale pitcher plant need to be repeated using other carnivorous plant species to evaluate this conclusion.

Written By: Dr. Stephen Brewer

Academic Editor: Neuroscientist

Non-Academic Editor: Fashion Designer

Original Paper

• Title: Growth and tissue nutrient responses of adults of Sarracenia alata to prey exclusion, nutrient addition, and neighbor reduction

• Journal: American Journal of Botany

• Date Published: 05 December 2022

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