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.

Alun Salt. ORCID 0000-0002-1261-4283

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?