Life in a place of death? Pitcher plants as habitat

It shouldn’t be a surprise that there are things living in guts. Whenever I turn on ITV there’s an advert for yoghurt with Yummy Tummy Bacteria. Apart from microbes, there can be relatively complex invertebrates like worms. But you might not expect so much to be living the trap of a pitcher plant. Microbes maybe, but invertebrates? Didn’t these traps evolve to eat invertebrates? A forthcoming review for Annals of Botany, Traps of carnivorous pitcher plants as a habitat: composition of the fluid, biodiversity and mutualistic activities by Wolfram Adlassnig, Marianne Peroutka and Thomas Lendl shows it’s a lot more complex than that. The traps might digest prey, but they’re also home to small ecosystems. The creatures within include a microscopic zoo, but also larger animals like crabs, spiders and frogs.

How pitchers work

Diagram for a Pitcher Plant

Diagram for a Pitcher Plant by Adlassnig et al.

They start with an explanation of how pitcher plants work. They’re not as dynamic as some other carnivorous plants like Venus Fly Traps. At the top A there’s the hood with glands that produce nectar to entice prey in. However when they land on the rim B insects find the surface is slippy. Make a mistake and you’ll fall slightly down the trap. It then the insect discovers the rim is covered with inward pointing hairs, making it very difficult to climb up. Struggling here is likely make you slip into region C.

This zone is all about getting the prey down further. The hairs make it impossible to climb up and some pitchers also have loose wax crystals. The area labelled D is the bottom of the pitcher. This part of the plant exudes digestive enzymes that pool at the bottom of the pitcher E. Here insects drown.

You wouldn’t expect some of them to drown. Adlassnig et al. point out that some ants can run along the surface of pure water due to surface tension. They can’t in the pools of many pitchers. The reason is that the pools contain surfactants. Around the house you’d find them in things like soap and detergent. In the plant their role is to reduce the surface tension of the liquid allowing the prey to fall in and drown.

If pitchers are so bad, why so insects enter? Aside from the nectar glands in A and sometimes B, there’s another factor. It turns out these fluids might also have a narcotic effect, and the odour attracts insects to the plant where on the outside, F, they find a nice rough surface for any insect that wants to climb up and investigate. But it’s not just food that enters the pitcher.

Living in a pitcher

Among the crustaceans there’s Copepoda that live in the fluid, along with crabs that visit looking for food. Fly larvae also grow in the pitchers, with lunch supplied by the plant. More surprising to me was the frog Kalophrynus pleurostigma that spawns tadpoles in the pitcher of one species. The most stylish visitors though are the spiders.

Some spiders simply spin a web above the trap of the pitcher but Crab Spiders, Thomisidae, don’t. Instead they spin out a line and dive into the fluid to find something to eat. It’s tempting to call it bungee jumping, but the spiders that do this have a problem. The plant they inhabit is a pitcher with a honey-like fluid at the bottom that’s sticky and elastic. To get round this they have to move very slowly.

If all inhabitants were parasites then that would be bad news for the plants. In the case of some species though they clearly help as not all of the pitcher plants can produce the digestive enzymes they need. The obvious helpers are some bacteria, fungi and algae that can produce the enzymes. Adlassnig et al. also discuss the effect of animal excretion in pre-digesting prey for the plant. There are pointers to the discussion of the mass of Phosphorus and Nitrogen that rotifers and fly larvae produce by eating bacteria.

Where next?

If there is a hole in the paper, it’s at the microbiological level. I hesitate to call it a hole though as the authors make clear this is a major gap in our understanding of pitchers. Very few people are looking at microbiology of pitchers. For example the authors state: “The occurrence and importance of viruses in the pitcher fluid is completely unknown.” There’s little work done on the bacteria that could be fixing nitrogen or breaking down the prey for the plants. The exact nature of the environment of the fluid could have knock-on effects elsewhere. Adlassnig et al. point to research that records the use of pitcher fluid as eyewash by some peoples. Liquid used to digest creepy-crawlies doesn’t push my New Age buttons, so it could be hard to sell as a natural remedy. Still, a better understanding of the chemistry could medical implications.

There’s a lot of plants to cover too. The range of fluids is from the acidic to in some forms of Nepenthes to near rain water in Sarracenia. There’s also a huge range of pitcher volumes. There’s the big 1.5 litre traps that can catch small rodents down to 0.2 millilitre traps. Obviously the number and types of animals found in the pitchers will vary from species to species.

The trap of a pitcher plant

The trap of a pitcher plant by Adlassnig et al. 2011.

As papers go, this is a bit of a TARDIS. It might be just 14 pages, but the wealth of links makes it much bigger on the inside. Despite being comprehensive, it’s not intimidating. Flagging what isn’t known about these plants is as big an invitation to join in the research as I’ve seen in a scientific paper.

It also knocks down a few stereotypes. When seeing something new or unfamiliar I tend to interpret botany through simple analogies. For example respiration is a plant breathing – except the analogy breaks down when you look closely at what respiration means for a plant. Beyond a simplistic level, calling respiration breathing hides what is particularly interesting about a plant’s use of energy. Likewise I’ve tended to think of liquid filled pitchers as an open stomach for a plant, purely about digestion.

Some environments in Nepenthes and Cephalotus are clearly hostile to much life, but even they aren’t sterile. The complexity of interactions between pitchers and their inhabitants shows that simply focussing on the enzymes without mentioning the habitat loses most of what makes a pitcher interesting – including the sources of the enzymes in many cases. Thinking of pitchers as a place of death hides that a lot of the processes in the pitcher are symbiotic.

One of the perks of producing the AoB Blog is that I get to read papers like this and find there’s so much more that I simply don’t know about. However, it’s a perk you can share. If you’re a science blogger and you’d like to blog about an AoB paper get in touch and we’ll send you a copy, even if it’s behind the paywall. This review goes out in the Feb 2011 issue, and it’ll be free to access from Feb 2012.

About Alun Salt

When he's not the web developer for AoB Blog, Alun Salt researches something that could be mistaken for the archaeology of science. His current research is about whether there's such a thing as scientific heritage and if there is how would you recognise it?