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Carnivorous plants respond to increased soil nitrogen, eco-news websites completely miss the point

Flowers of a venus flytrap. Photo by Andrew David Thaler.

Flowers of a venus flytrap. Photo by Andrew David Thaler.

Late last week, inspired by our newly flowering Venus Flytraps, I posted pictures of Amy and my carnivorous plant collection on twitter and on the Southern Fried Science Facebook page. After David’s recent post on a nurse shark that underwent major dietary changes following traumatic surgery and captivity, our wonderful readers must have been on high alert for trophic shifts following anthropogenic disturbance-type articles (or, more casually, “stuff that eats stuff now eats different stuff”), because this morning my inbox was filled with links to variations on the following article: Pollution makes carnivorous plants go vegetarian. Whenever human activity alters trophic interactions, there is potential for major ecological changes in an ecosystem. While ecosystems are dynamic, shape by continuous variation in community structure and resource and habitat variability, rapid changes can result in total collapse or permanent shifts to functional states.

Unfortunately, these “eating different stuff” articles rarely reflect the deep and nuance ecologic reality of trophic interactions and instead capitalize on the narrative of “even animals are going veggie to save the planet!” Allow me to revel in my cultural roots with a hearty “Oy vey!”

A few different websites have picked up on the carnivorous plants press release, including Planet Earth Online, Chem.Info, and our perennial favorite for greenwashed nonsense: Ecorazzi (in their defense, at least Ecorazzi had the decency not to just blindly copy the press release). The ‘razzi even gifts us with a lovely taxonomy fails–the picture associated with the article is a venus flytrap, the actual study was conducted on sundews (any botanist readers want to calculate the TFI on that?). Of course, their earth-shaking conclusion that “the harmful substances floating around in our air are doing more harm than good” is a marvel of circular reasoning (note to all journalists covering science news: link to the original source, link to the original source, link to the original source). Unfortunately, the study in motivated not by pollution response, but by evolutionary and ecologic questions regarding carnivorous plants. Anthropogenic N deposition provides an opportunity to study these phenomena.

So what’s really going on?

A sundew, though not the species from the study. Photo by Andrew David Thaler

A sundew, though not the species from the study. Photo by Andrew David Thaler

Carnivorous plants have evolved to exploit nitrogen and phosphorous depleted ecosystems by capturing and consuming prey species to supplement soil nutrients. This survival strategy is energetically expensive and provides only marginal advantage over pure autotrophy with soil uptake of nutrients. Carnivorous plants can thrive in regions with low soil nitrogen, where other plants cannot. Carnivorous plants are capable of root uptake of soil nutrients, and will, if nitrogen and phosphorous are available. In cases where soil nutrients are not limited, some carnivorous plants will invest more energy in root and leaf surface area, and produce fewer, and small, prey capturing devices. Ever so, carnivorous plants are not optimized for these conditions and are often outcompeted by strict autotrophic plants.

A hearty pitcher plant pitcher. Photo by Andrew David Thaler.

A hearty pitcher plant pitcher. Photo by Andrew David Thaler.

In Reliance on prey-derived nitrogen by the carnivorous plant Drosera rotundifolia decreases with increasing nitrogen deposition, the authors sampled sundews across a gradient of anthropogenic nitrogen input to measure the amount of nitrogen from soil uptake and from prey capture. What they found was that, when nitrogen was available in the soil, sundews incorporate more soil-derived nitrogen and less prey-derive nitrogen. Surprisingly, this shift happened at relatively low soil nitrogen concentrations. Plants that derived more nitrogen from prey capture were healthier than those that primarily relied on soil derived nitrogen.

While the discussion is well-written and contains some informative and interesting hypotheses for why these shifts happen, how plants respond physiologically, and what effects these shifts may have on the ecosystem, the study does not demonstrate that sundews become less sticky or that their leaves become less red, and it certainly doesn’t state that carnivorous plants will become extinct, as Ecorazzi reports. It also points to a latitudinal gradient and the variability of prey abundance as another possible reason for this shift in diet, especially considering that relatively low soil-derived nitrogen triggered the shift.

It seems silly to fabricate a touch of sensationalism when the actual science is interesting and compelling on its own.

Deep-sea biologist, population/conservation geneticist, backyard farm advocate. The deep sea is Earth's last great wilderness.

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