New study on carnivorous plants makes headline writers batty

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Before we get started, let me say that I forgive those who write headlines like pollution makes carnivorous plants go vegetarian and carnivorous plants go vegetarian in response to pollution and new study finds that pollution turns carnivorous plants into vegetarians. They know not what they do. It's also tempting to go after the flashy, attention-grabbing headline. Just try to do a better job next time, ok? And while we're on the topic, let us thank those that presented reasonable titles, like the one by Liat Clark at Wired.co.uk: carnivorous plants capture less prey in polluted bogs. Thank you for getting it a bit better! It would also be a terrible oversight if I neglected to mention that the Southern Fried Scientist took this issue to task a few days ago. Bravo!

So, on to the science at hand. What's all the fuss about?

Drosera rotundifolia at Triangle Lake Bog, Ohio. Photo by kitkor.

Drosera rotundifolia, the round-leaved sundew. Or common sundew. Or "bloody hell that thing is everywhere." And it is: North America, Europe, Asia... Here in Ohio it is the most common species of Drosera that you'll bump into - the other being Drosera intermedia, but several sites it had been known from have now been developed. For those unfamiliar with carnivorous plants, you might be peripherally aware that it is thought that these species have evolved in nutrient-poor environments. Given this idea and our knowledge that most species possess a good deal of phenotypic plasticity in response to environmental cues, researchers decided to further test earlier experimental observations that Drosera rotundifolia reduced its investment in carnivory (as measured by stickiness in units of force used to remove a piece of filter paper from the leaf) when grown in the presence of more nitrogen (Thorén et al., 2003).

Here in the new study, the researchers, a team including J. Millett of Loughborough University, B. M. Svensson and H. Rydin of Uppsala University, and J. Newton of the Scottish University Environmental Research Centre, were more interested in the relative amount of nitrogen that came from prey captured by normal means and from the roots as a result of increased nitrogen available from atmospheric deposition due to increased air pollution.

Briefly, the authors identified three bogs in Sweden that represented a gradient of mostly pristine to somewhat polluted in terms of nitrogen deposition. Fifteen specimens were removed from the bogs, dried, and analyzed for stable isotopes of nitrogen. Once they had their isotope data, all they did was subtract surrounding Sphagnum isotope data from Drosera and divide that by (insect - Sphagnum), where insect represents the mean isotope number for prey captured on the plant at the time of collection. And there's an easy ratio!

So conclusions from this? Well, the authors state it very clearly in the abstract, which many of the headline writers must have missed: "Drosera rotundifolia plants in this study switched from reliance on prey N to reliance on root-derived N as a result of increasing N availability from atmospheric N deposition." (emphasis mine) No, headline writers, these plants were not "OMG BECOMING VEGETARIANS!" Wouldn't that be a plant eating plant matter? And, as strange and wonderful as nature is, we have two possible examples in Nepenthes ampullaria and Utricularia purpurea where the former seems well-adapted to catch leaf litter and the latter appears to primarily cultivate algae in its bladder-like aquatic traps. No, dear headline writers, increased pollution will not turn Drosera rotundifolia into a vegetarian. It may, however, given this work and that before it, be the cause of changing priorities in nitrogen uptake from primarily prey-derived to primarily root-derived. It should be noted, however, that the authors did not set out to assess prey capture rates in these areas, so any statement has to be carefully worded and specifically related to nitrogen assimilation from different sources. We don't know if the plants in areas with more nitrogen capture fewer arthropods. It's entirely possible that the plants that incorporate more nitrogen from their roots capture the same number of prey but preferentially assimilate the nitrogen from the roots.

More troubling, however, is that with increased nitrogen availability in these once off-limits landscapes, opportunistic species may find it easier to overcrowd the poor little perennial carnivorous herbs. (Of course, the increase in nitrogen in this study was not very large and probably would not be enough to allow non-bog-adapted species to thrive.) Most carnivorous plants are low to the ground and depend on high light conditions to thrive; if shaded too much, they may soon succumb to succession. Of course this is only a hypothesis and needs to be studied! I wonder what the headline writers will say then...

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References

Millett, J., Svensson, B., Newton, J., & Rydin, H. (2012). Reliance on prey-derived nitrogen by the carnivorous plant Drosera rotundifolia decreases with increasing nitrogen deposition New Phytologist, 195 (1), 182-188 DOI: 10.1111/j.1469-8137.2012.04139.x

Thoren, L., Tuomi, J., Kamarainen, T., & Laine, K. (2003). Resource availability affects investment in carnivory in Drosera rotundifolia New Phytologist, 159 (2), 507-511 DOI: 10.1046/j.1469-8137.2003.00816.x

A sundew makes a hasty retreat

I'm not yet ready to send out the heralds and call this a success on my first go at cultivating tuberous sundews, but I'm closer now than I was before. If you recall, I originally purchased a lovely specimen of Drosera peltata from California Carnivores in January 2012 and first posted about it in March. It started out as a cute little rosette of carnivorous leaves, then bolted to produce two lovely 6-inch tall stems bearing those irresistible peltate leaves. And then throughout the last few months it was happily going about the business of, well, what sundews do best: capturing prey to collect nutrients.

Most of the tuberous sundews are native to Australia where the winter is rainy and the summer is hot and dry as a bone. This lineage of sundews has evolved the handy adaptation of giving up trying to survive as a full-fledged leafy herb during that hot, dry, unforgiving summer. Instead, they retreat into the soil, packing up their nutrients into root structures called tubers, not unlike a potato in many ways though much smaller.

In just the last few weeks as we approach the hottest late May and early June weather in the Northern Hemisphere here in Ohio, this particular specimen I had was finally ready to make its scheduled retreat. The leaves and stem quickly browned from the tips in a matter of days, my cue to stop watering and let the soil go bone dry lest the tubers succumb to rot as they form. And then, a few weeks later, out of curiosity and because I knew the soil surface in the pot was much too hard for the new growth to break through next year, I dug through the soil to find the tiny tubers:

Those little cream-colored pearls are definitely not perlite! A closer look:

In the above photo, the two tubers toward the top were still attached to the root, the dark brown object leading from center to the bottom right. It was a bumper crop! After sifting through the remainder of the soil, I found ten tubers in all:

The next challenge will be keeping them in a nice, dark place until next fall when they begin to stir. I think the hardest part will be remembering that I have them stuffed away somewhere!

And also, thanks to my friends at Botanical Oddities, I now have tubers from Drosera auriculata. Thanks, guys! Here's hoping I have success with both as I imagine the difficulties of growing tuberous sundews arise when preparing the new soil mix - the sand can't be too sharp or the new growth will be torn up on its several inch ascent from below. It will, at least, be a fun challenge.