The Fascinating Microcosm of the Pitcher Plant.
Who doesn’t love carnivorous plants? Even people who don’t really care about plants find carnivorous ones at least a little interesting. I think it’s because they violate our expectations of nature. We see plants as scenery and as something to be eaten. When they turn the tables and become the predators, we pay attention. From glittering, enticing sundews (Drosera) which entice their prey with a sweet smell and then trap them with sticky fly paper to Venus flytraps (Dionaea muscipula) which lie in wait and then snap shut at the touch of an unwary arthropod, plants evolved (multiple times) a wide array of ways to take advantage of animals for nutrition – typically in places like open marshes when nutrients are scarce in the soil.
But one of my favorite carnivorous plants is much less flashy than these characters Sarracenia purpurea, the purple pitcher plant relatively common throughout northeastern North America. At first glance, this species seems to have one of the least interesting methods of capturing prey. Specialized leaves collect rainwater into which insects (mostly) may fall and drown and be digested by the plant. No sticky leaves, no moving parts, not even dangerous looking spines. The plant is unassuming and it’s method of pray capture seems so laid back as to be, well, boring.
But S. purpurea is actually extremely interesting. In fact, there is a wealth of literature about this common marsh dweller. It has been intensively studied for more than 100 years and scientists are still learning things about and from it to this day. The purple pitcher plant is not a simple consumer but an entire ecosystem unto itself with a host of predators, prey, and detritivores living inside its pitcher. The pitcher plant could not function as it does without these organisms, as they make it possible for it to utilize the nutrients from its prey, while it provides them with a home and food as well.
We’ve known for a long time that various species of insect larva, particularly mosquito larva, can and do live inside the pitcher plant, but it wasn’t until the 1970’s that we began to pay attention to the structure and function of the community found there. In most areas, pitcher plants play host to three main species of larval insect – A sarcophagal fly (Blaesoxipha fletcherii), a mosquito (Wyeomyia smithii, actually known as the pitcher plant mosquito), and a midge (Metriocnemus knabi).
Aside: Doesn’t that sound like the beginning of a really nerdy entomology joke? “So, a fly, a mosquito, and a midge are hanging out in a pitcher plant…”
The presence of these larva is important to the plant, because while the pitchers do produce digestive enzymes, they actually don’t produce that much them, and that level goes down as the individual pitcher ages. Young, strong, newly opened pitchers produce the highest amount of digestive fluid and then produce less and less as they age. Likewise, the youngest pitchers capture the most prey, while older ones become less and less effective. But even the most vital pitcher bursting with digestive fluid would take awhile to reduce a whole insect into the essential minerals it needs to sustain itself.
That’s where the larva come in. They hang out in the pitcher because it attracts food they can eat too. Although it seems like they’re stealing the plant’s meal, they’re actually helping it. In the process of feeding on the dead insects trapped by the plant they tear the prey into progressively smaller pieces the plant can digest much faster than when the carcass was whole. In addition they, um…process…the prey by eating it and passing out some of the nutrients in their feces, which the plant can also use. This greatly speeds up the digest process for the plant, and provides a home and dinner for the larva.
You might think the larva species would be in competition in this scenario, but they actually have it all worked out in a way that works efficiently for them and the plant. In a textbook case of resource partitioning, each larva does something different. The aggressive fly larvae tend to float and they hang out at the top of the water level, feeding on the freshest and largest prey there. The midges stay at the very bottom of the plant and shred the pieces of food from the fly’s meal into even smaller pieces as they feed on them. The mosquito larvae hang out in the middle, filter feeding on suspended particles floating up from the midges’ shedding and down from the larvae feeding at the surface.
Not only do they partition the resources in space, but also in time. These species exhibit a succession within the plant as the pitchers age and the amount of prey coming in changes. The fly larva are found only in the youngest, pitchers that take in the highest amount of prey. As the amount of fresh prey dwindles, the fly larvae start to disappear and the population of mosquito larva increases. Near the end of the pitcher’s life, when very little fresh prey is coming in and there’s mostly old detritus at the bottom of the pitcher, the midge larvae are at their peak.
But even this is only part of the story. You see, it’s not just a few arthropods that call the pitcher home, but a host of other species including many different kinds of rotifers, mites, protozoans, and bacteria, each of which have a unique role in the pitcher plant inquiline food web. The mosquito larva doesn’t just filter feed on leftovers – it acts as a keystone predator for the community, feeding on protozoans and rotifers and bacteria, some of which also feed on each other, on the mosquitos leftovers, or on the pitcher plants prey. These relationships are still being studied today, and as scientists find out more about the community structure and food web within the pitcher plant, they are about to learn more about the basic principles of ecology itself.
The reason research on pitcher plant communities is so important goes far beyond the uniqueness of the system. This microcosm allows us to study a complete ecosystem in a way that is often impossible to do in a larger, more complex, and slower growing environment. The system in a pitcher plant is short lived (generally just a couple years) and allows scientists to study it from beginning to end in a workable time frame instead of the ten or hundreds of years other types of ecosystems might take to go through the same stages. It is realistically complex, but not impossibly so – all the organisms in a pitcher can be identified. And it is small and finite – scientists can manipulate the system and control for almost all variables or inputs.
This allows us to investigate large ecological ideas that would be impossible to test on a large scale. It’s much harder and more damaging to remove all the predators from a forest than it is to remove all the mosquito larvae from a pitcher plant. This is why, for many years, the pitcher plant and it’s community have been used as an experimental model in many ecological studies so that we can get an idea of what happens when you double the input of nutrients to a system or take away an entire trophic level – experiments that would be impossible in major ecosystem but that can help us understand more about how to manage and protect and even maybe restore our forests, lakes, rivers, and plains in a changing world.
Plant, P., Fish, D., Hall, D. W., American, S., Naturalist, M., & Jan, N. (1978). Succession and Stratification of Aquatic Insects Inhabiting the Leaves of the Insectivorous. Methods, 99(1), 172-183.
Kneitel, J. M., & Miller, T. E. (2002). Resource and Top-Predator Regulation in the Pitcher Plant (Sarracenia purpurea) Inquiline Community. Ecology, 83(3), 680.
Miller, T. E., Horth, L., & Reeves, R. H. (2002). Trophic interactions in the phytotelmata communities of the Pitcher Plant, Sarracenia purpurea. Community Ecology, 3(1), 109-116. doi: 10.1556/ComEc.3.2002.1.13.
Kneitel, J. M., & Miller, T. E. (2003). Disperal Rates Affect Species Composition in Metacommunities of Sarracenia purpurea Inquilines, 162(2).
Peterson, C. N., Day, S., Wolfe, B. E., Ellison, A. M., Kolter, R., & Pringle, A. (2008). A keystone predator controls bacterial diversity in the pitcher-plant (Sarracenia purpurea) microecosystem. Environmental microbiology, 10(9), 2257-66.