‘Double Whammy’ Could Wipe Out Carnivorous Plants

On a recent reporting trip out West, I found myself surrounded by hundreds of vicious carnivores, all soaking their prey in a corrosive mix of flesh-eating enzymes. In a California bog, I stood in a field of carnivorous plants. The sticky sundews held their sparkling, modified leaves just an inch or so off the ground, digesting insects ensnared in their numerous traps.

I was tagging along on a botany rescue mission to collect and preserve as many of the state’s rare and threatened plants as possible. One goal of that expedition: collecting seeds from the sundew. There are only a few known occurrences of the plant across all of California’s 160,000+ square miles. And in this case, the population is condensed to just two patches of floating muck, eking out an isolated existence.

Carnivorous plants showcase the splended oddities evolution is capable of. And although much about them remains mysterious, recent research has increased our understanding of why and how some plants flipped the script and started eating animals. Simultaneously, these meat-munching-marvels are increasingly threatened by habitat destruction, poaching, and climate change.

“Carnivorous plants face a double whammy,” Barry Rice, a botanist at UC Davis and an astrobiologist at Sierra College, told me in a phone call. “Not only are they facing all the stresses that all the other plants on the planet are facing, but they’re also, by their very nature, extra sensitive to it.”

To be considered carnivorous, a plant has to meet a few criteria. According to Rice, it must have clear adaptations intended to trap prey. Traps can include things like ensnaring hairs, deep pits, or clamshell maws that snap shut.

Then, a plant has to also have some way of breaking down (i.e. digesting) that trapped prey. Finally, it needs a means of absorbing the nutrients released by the digestion process.

“Imagine, for a minute, that you could turn your stomach inside out,” Aaron Ellison, a Harvard University carnivorous plant ecologist, told me. He explained that that’s basically what most of these plants do: invert their stomachs.

Be it via sundew glue, pitcher plant pit, or the quick-close response of a Venus fly trap, carnivorous plants snag their prey on what amounts to an external stomach. Then, they secrete some of the same enzymes that animals use to digest meals. Carnivorous plants have protein disassembling proteases, fat-melting lipases, plus other enzymes specialized for breaking down insects’ exoskeletons.

Together, these excreted compounds “go to work and turn the insect into soup,” said Ellison. That nutrient-rich slurry gets absorbed by specialized cells, similar to those that line your intestine.

Across nearly all known carnivorous plants, “it’s basically the same process,” he added. Some outliers work more like a cow’s stomach than our own, and a few pitcher plants excrete very little enzyme, instead relying on colonies of bacteria and protozoa to do the disassembly work for them.

You won’t find carnivorous plants in the dark depths of the forest. Instead, look for them in bright open space, with plentiful water.

Carnivorous plants are a prime example of an evolutionary trade-off. Making all those traps and digestive juices takes a lot of energy, so this survival strategy only that works in places where nutrients like nitrogen and phosphorous are at a premium.

Their identifying trait evolved as a way to cope with nutrient-poor environments, which is why they supplement with flesh. To make their tricky traps, carnivorous plants sacrificed some of their ability to photosynthesize, explained Ellison. “They’ve been modified in such a way that they just don’t do well with capturing light anymore,” he added. Meaning that they can only thrive in super-sunny habitats.

We know these botanical meat-munchers evolved to make the most of a low-nutrient niche. But how they got there is a different question. Most plants don’t produce digestive enzymes, so where did carnivorous plants get theirs?

There’s not a single, definitive answer to this. But at least some of the origins of plant carnivory are probably rooted in plant defense, said Rice. Those digestive enzymes may have begun as compounds meant to ward off insects.

In another example, Rice explained that the sticky, glue-trap hairs that are all over sundews could’ve once been something like the gummy, protective hairs that cover tomato stems.

Modern pitcher plants may have been preceded by an ancestor that simply had slightly cupped leaves, which helped it retain water or capture nutrients. Thus, what originated as an incidental trait variation became advantageous and selected for.

Recent research has only just begun to trace the origins of the genes that encode for plant carnivory. In one example, scientists determined that the prey absorption genes in the modified leaves of Venus flytraps and sundews are derived from DNA that would normally be expressed in a plants’ roots, according to a 2020 study published in Current Biology.

What’s clear is that it’s not that hard for plants to turn into meat eaters. Carnivory is thought to have emerged at least 11 times in different lineages across plant evolutionary history, said Rice.

The exact number isn’t settled, but there are more than 800 known species of carnivorous plants worldwide, living on every continent except Antarctica, said Ellison. And more are probably out there waiting to be added to the list.

All of those plants are united by their hunger for flesh, but they can look pretty different from one another and rely on varied strategies, in part because carnivory evolved independently in plants so many different times.

There’s the pitfall pitchers, “lobster pot traps” (which entice prey to crawl inside), sticky sundews, snap traps, and some even use suction—like tiny deadly vacuums.

Bladderworts, a rather unassuming type of carnivorous plant, ensnare small organisms like copepods suspended in water inside little sacks. To do so, the plant pumps liquid out of its flexible vessels, and then refills them with liquid when a potential prey item bumps a trigger hair, sucking the critter in. The whole process happens on the order of milliseconds and looks otherworldly.

It’s enough to make me feel extra grateful for my human size. Especially considering that not all carnivorous plants stick to invertebrates. At least one species of large pitcher plant has been documented digesting rats.

There are some basic unanswered questions about carnivorous plants out there. For one: how long have they been around? And it’s tough to get answers, because there’s not really a fossil record cataloging their evolution.

Which is something of a mystery in and of itself. Their wetland habitats are, in theory, “actually pretty conducive to fossils,” said Ellison. Yet, “very curiously,” few have been found. The oldest one documented thus far was found to be about 40 million years old, but carnivory in plants could be more than three times as ancient. All we know for sure is that the trait doesn’t precede the origins of flowering plants entirely, which emerged around 140 million years ago.

Another question that Ellison has: If carnivory has evolved so many times across different plant taxa, and it’s an advantageous trait—why aren’t way more bog plants meat-eaters? “If it’s so easy, why aren’t there so many carnivorous plants?” he asked. Sure, there are over 800—but that’s just a fraction of the more than 400,000 known plant species worldwide.

We’re currently in a bad era for life on Earth, what with the sixth mass extinction and all that. Climate change, habitat destruction, pollution, and other human activities are imperiling (most) biodiversity.

Unfortunately for carnivorous plants, things are even worse. “These plants are the canaries in the coalmine,” said Rice. They’re highly sensitive to pollution, because pollution usually means things like nitrogen and phosphorous. When a formerly nutrient-poor bog gets an influx of nutrients, “the ecosystem starts to malfunction,” and carnivorous plants are forced out.

Climate change is extra-bad news, too, because carnivorous plants are so fragmented across the landscape, said Ellison. They’re found in small pockets where the habitat conditions are just right. Unlike other plants that grow more continuously over large areas, carnivorous plants can’t easily spread north or up in elevation to stay in their ideal climate zone.

Plus, in some ways, the weirdness of pitcher plants, flytraps, and sundews is contributing to their vulnerability. There’s a large community of interested collectors, which can lead to poaching. “One person, in one afternoon, could wipe out a local population without any problem,” said Ellison.

The botanists I visited the sundews with in California were clear: Don’t reveal the plants’ exact location online. Plant poaching has become an increasingly big issue in the state.

Conserving these wild, wondrous plants—even in the face of multiple threats—is why the botanists were collecting sundew seeds to begin with. The seeds, which will be stored in a seed bank in California, are an insurance policy for the future of the state’s sundew population.

Habitat loss is a persistent threat closing in on carnivorous plants. Bogs are often drained to make way for development, knocking out everything that once lived there. It’s important to preserve as much as we can of what’s left, but it’s also important to have a back-up.