We all know pitcher plants trap bugs and digest them for nutrients. We are all wrong. Sort of.

It turns out that although pitcher plants do indeed trap insects (and other small animals!) to make up for the lack of nitrogen in their nutrient-poor soil, they often don't digest their prey all by themselves. In many pitcher plants, and particularly the purple pitcher plant (Sarracenia purpurea), there is a complex, thriving ecosystem responsible for breaking down the prey^ref.

Welcome to the pitcher plant! Come for the nitrogen, stay for the community! Source, by incidencematrix under CC2.

Dramatis personae

Pitcher plants

Let's start with the host for the whole shebang. The purple pitcher plant grows in boggy soil and needs nitrogen. To do this, it forms cup shaped leaves that fill with water (a kind of phytolema) and attracts insects with nectar. The insects, attracted to the sweet, sweet nectar, get trapped in the cup by means of (depending on the kind of plant) sticky walls, downward facing hairs, canopy lids, or even thin roof patches that let light in and act as 'decoy exits'.

The general view used to be that the pitcher plants secreted various enzymes to break down their prey, but we know now that some pitchers, such as our model purple pitcher, only secrete a small amount in the first few weeks^ref of a cup's useful lifetime - most cups last about a year and die-off during the winter.

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Bacteria

If an environment provides a way to live, you're probably going to find bacteria or archaea in it. The pitcher's phytolema is no exception - even in the rare cases where a pitcher species does not contain large residents (macrobiotic infauna), I wouldn't be surprised if there were microbes hanging about.

As best we can tell, there's no real 'core' bacterial community inside the pitcher.^ref It's just a random collection of generalist heterotrophs doing what generalist heterophs do: breaking down food into CO₂ and organic nitrogen (mineralization).

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Rotifers, ciliates, and other protozoa

Although physiologically interesting and often the stars of middle-school 'pond water microscopy', these are often the red-headed stepchild of microbial ecology. Lots of researchers generally just mention that they 'graze on bacteria' and move on. That's pretty much the case here, but there's one of group of protists which are getting some attention.

Colpodea colpodida is one of the protozoa found in pitchers and it can serve as a food source for mosquito larvae (more on those in a moment). However, other protozoa which move about more freely are more attractive targets to the mosquito larvae and when they are present, C. colpodida doesn't get eaten nearly as often. What's cool is that the genes expressed by C. colpodida are directly correlated to how much the mosquito larvae attack them - providing a nice little system for looking at ecosystem-influenced genetic expression in populations.

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Mosquito larvae

One of the most influential infauna members in a pitcher is a mosquito, specifically the larvae of Wyeomyia smithii. Not only is W. smithii considered to be a keystone species in the pitcher^ref, but the pitcher is supremely important to the mosquito, too. The larvae of W. smithii are only found in pitcher plants (obligate relationship); no pitchers means no W. smithii. In th pitcher plant world, fauna with this type of relationship are referred to as nepenthebionts, with the 'nepenthe' root coming from the genera of another popular pitcher.

So, what's so important about the pitcher to our mosquito friend? Simply, it's an amazing food source - the larvae are filter feeders and are capable of eating a lot of what's in the pitcher. They don't just limit themselves to the broken down parts of the pitcher's prey, they also eat the bacteria and protists I've already mentioned.

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Midge larvae

The other important nepenthebiont, Metriocnemus knabi is larva of a midge which has been described as "ravenously carnivorous". Like the mosquito larvae, they also eat the unfortunate prey of the pitcher. Interestingly, both the midge and mosquito larvae can exist in the same pitcher, despite (at first glance) competing for the same food.

They get along so well, in fact, that M. knabi larva won't even attack W. smithii larva, but they do attack the larva of other mosquitos. Clearly, there's some interesting ecological relationships going on in all this, but I want to highlight one more organism before we get into that.

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Crabs

Yes. That's right, crabs! I was completely unaware that crabs lived in some pitchers until I started reading more deeply to prepare this article. So, first off, I should mention they are more strongly associated with other pitchers than our purple protagonists, but it is too cool to not mention that there are crabs (Geosesarma malayanum) in Malaysia which regularly raid pitchers for food. Unlike the midges and mosquitos, they are not obligately dependent, so they are considered nepenthephiles, not nepenthebionts.

There is not a good copyright-free image of these little crustaceans, but Francesco Tomasinelli has taken some really cool snapshots of them in pitchers which you should check out. (The picture above is a related species in the same genus.)

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More relationships than a Jane Austen novel

Right, so an ecosystem consists the the environment, the organisms which inhabit it, and the relationships between those organisms. Not counting our cool little Malaysian crab but including the prey, there are six organisms involved, leading to 15 potential relationships for just nitrogen flow - and that's not even accounting for things like changes over time including the differences between 'partially chewed prey' and 'freshly caught prey'.

One possible schematic for the pitcher plant food web. Taken from Figure 2 of Mouqet et al, 2008. (Yay for open access!)

What kind of relationships are we looking at

You might remember terms like mutualism, symbiosis, and parasitism from science class. These are all examples of the forms these relationships may take. It turns out that there's a lot of these terms which can differ finely in shades of meaning (e.g. is it commensalism or just very lopsided mutualism?). Others can depend on how important it is to capture certain details - such as how resources are shared (e.g. syntrophy vs. processing based commensalism). To be clear here, let's stick to the basics relevant to our ecosystem and define our terms:

• mutualism: both organisms benefit
• commensalism: one organism benefits, the other is unaffected
• competition: both organisms require the same resource
• predation: one organism eats the other, for simplicity, I'm considering bacterial use of substrate as predation

In the purple pitcher plant, this is how it breaks down:

• The pitcher plant itself predates on the hapless prey, but it's not very good at breaking them down
• Bacteria decompose whatever is dead inside the pitcher, the smaller the surface area the better
• Midges predate on the prey, chewing up large chunks of the prey, making tiny particles
  • The bacteria are happy because this means that there's more surface area to eat away at and to grow upon (commensalism)
• The rotifers and other protozoa predate upon the bacteria
• The mosquito larvae predate upon the bacteria, rotifers, and chewed up bits of prey
  • Since the mosquitoes don't eat up the unchewed prey, they are not in actual competition with the midges. If fact, this is a form of processing chain commensalism
  • The bacteria both benefit from the mosquito larvae removing their predators and suffer from being eating by the larvae.
• All of the infauna are dying, and excreting waste, notably nitrogen, which the plant can absorb (commensalism). The larvae eventually become adults as fly away, taking the nitrogen in their body with them (competition)

The questions then, are, how do all these interactions work together, are they ultimately good or bad for the pitcher, and under what conditions does that change?

The short answer is that it all depends. The longer answer to this is that nobody's really quite sure and reasonable people have disagreed in the literature. An even longer answer lies in the idea that we can simulate all these interactions.

On models and experimentation

Some researchers have made a somewhat simplified computer model (in silico) and let it run with different population amounts to determine when and how each species benefits (or doesn't) from the others^ref.

According to their model, the pitcher actually does best when there are only bacteria present. That condition may be a mathematical maximum, but it is hardly realistic in a real bog. When other species are present, they show that things eating bacteria is bad (as we might expect) and that the mosquito levels have a sort of hump shape where whether or not they benefit the plant depends on their amount and the amount of detritus within the pitcher.

I also particularly like that in their conclusions they say that interactions are predominantly parasitic or mutualistic, emphasizing that these things are on a spectrum and not the clear cut relationship presented in so many texts.

Now, let me show you a picture then ask you a question.

Figure 3, illustrating the model used by Mouqet et al, 2008.(Again, yay for open access!)

The above is the schematic for the model used in the simulation. You definitely don't need to understand all of it, but I'd like you to look at the arrow between bacteria and protozoa. The part that says u_pBP is a parameter expressing how fast bacteria are consumed by protozoa, given their two population sizes. For the model to be run, that parameter and all the others have to be set to numbers. How do you pick the number? Experiments!

One great thing about purple pitchers is that they are tiny little ecosystems (microcosms) that're pretty easy to maintain, can be grown up in sterile conditions, and are (insofar as carnivorous plants go) easy to cultivate. It's actually possible to experimentally control the relative populations in these systems and figure out what those parameters are^ref. One of the great things about science is that you don't have to do it all, you can build on the work of others. And that is exactly what Mouquet did - they used existing experimental data from the literature.

Conversely, you can run the models with parameters and compare them to your experimental systems to validate your model. A note to all the budding ecologists out there - it's a BIG no-no to derive your parameters and validate your models from the same data. Do you want confirmation bias? That's how you get confirmation bias.

Why care

First of all, I'm going to assert that you should care simply because this is a cool system. It doesn't have to have any other value than itself. However, if you insist on a bigger picture, there's a whole slew of reasons to care, ranging form the the practical to philosophical.

At the most practical, pitcher plants are easily experimentally controlled complex ecosystems. Beyond exploring them for their own sake this makes them a great model system for exploring ecology. If, for example, you're interested in the effect of secondary predation upon the primary decomposers in a food web, this is perfect.

Apart from the direct food web questions, these systems are still incredibly useful practical models. For example, you can stress them out environmentally or population-wise to determine when and how different tipping points occur in the ecosystem. You can then extrapolate those results to other, less manageable ones.

I am particularly enamored with this set of quotes from Aaron Ellison, who studies environmental changes in lakes. "We would need to study a lake for 100 years to get the same information that we can get from a pitcher plant in less than a week" and "The same mathematical models can be used to describe either a pitcher plant ecosystem or a lake." (emphasis mine). This is the whole point of a model system. At the right level of abstraction, the math is the same; A model system lets you work with the same ideas more easily, economically, ethically, and/or rapidly.

Even without acting as models, these little plants can change the way we think about things. I hope by now you're convinced that the ecosystem they contain, while manageable, is complex and unpredictable. It may even seem a bit like hubris to assume you could easily predict what would happen if you altered the system.

Those of you who follow Hive account@steemstem may be aware of the really nice pair of articles in which Hive account@bitfairy laments the existence of mosquitos and Hive account@mountainwashere responds with some counterpoints.^†

The upshot is that removing mosquitoes has a lot of benefits, but also has a lot of (probably unpredictable) ecological side effects. The complexity of our little pitcher plant ecosystem is a humbling reminder of that. What do you think might happen to the pitcher population if you killed off all our W. smithii mosquitos? I am not sure, myself, but given that W. smithii is generally believed to benefit the pitchers, I suspect they would undergo a big die-off.

First, this would be lamentable on its own - the loss of biodiversity is always something saddening, but even more so when it involves such unique ecosystems and methods of living. Second, if you're not into the whole biodiversity-for-biodiversity's sake thing, let me tell you that pitcher plants may have some medicinal value and certainly have been used to make tasty cocktails.

Venturing even beyond those abstractions, this system can serve as an analogy for philosophical reasoning. I was inspired by Hive account@ced000's article using thermodynamics as analogy to understand STEEM. One of the big, perennial questions facing our community is how do various actions and types of users affect the health of the blockchain (And how do you define health anyway?).

I have to imagine that a better understanding of ecology can inform our understanding of community (and vice-versa!). The same tools we used to think about mosquitos and pitcher plants can help us ask and answer questions like 'are bidbots parasitic or commensal?' Or, how do you increase minnow retention in a way such that they benefit the blockchain? Does that make sense, or am I nuts?

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^†.Really cool things happen when articles are written as responses to each other. More of this please!