There are more than 800 known species of carnivorous plants. Despite their diverse designs, they all have one thing in common: they are built to trap and kill prey for survival.
Or so we’ve believed.
Now, a new study has thrown a botanical spanner in the works of more than a century of accepted knowledge of the predator-prey relationship between carnivorous angiosperms and the insects they lure. Researchers at the Okinawa Institute of Science and Technology (OIST) have found that this relationship is not so clean-cut, with one type of carnivorous plant letting 98% of its trapped insects leave unharmed.
And while every species – plant and animal – has individuals that, shall we say, aren’t so good at life, nature has a way of cutting them loose from the gene pool. Nonetheless, this carnivorous plant continues to thrive.
So the discovery has been, understandably, both exciting and puzzling for ecologists.
“Generally, we ecologists like to categorize relationships as just being one fixed, discrete type of interaction, such as predator-prey or competitive,” says the study’s senior author Professor David Armitage, from OIST’s Integrative Community Ecology Unit. “But what we’re becoming more aware of is that these ecological interactions are much more context-dependent and fluid.”
The carnivorous group in question is a type of pitcher plant, a group known for its specialized pit traps formed by modified leaves that lure insects with surface nectar. When the insect lands to feed, it slips into the plant’s bulbous pit to drown, providing the plant with nutrients.
This nectar is particularly alluring to vespid wasps – of which there are around 5,000 known species.
Asa Conover
During their fieldwork, the researchers noticed an oddity: Wasps would feed on the nectar from the carnivorous plant, but escape after getting their fill. This interaction had earlier been documented in 2005.
“If you hang out with pitcher plants enough, you’ll always see insects landing on them, feeding, or doing something, and then flying off,” says Armitage. “The capture rate is so low.”
In that 2005 study led by Philip Dixon, Californian Darlingtonia californica pitcher plants captured fewer than 2% of the wasps that landed on them for a feed, suggesting to scientists that the pit trap was rather ineffective if the aim was to lure a meal. Earlier studies had suggested there might be more at play than predator-prey dynamics here, but how this could be a mutualistic relationship has remained unclear.
Seeking a definitive answer for this anomaly of nature, Armitage and team traveled to California to collect samples – wasps, pitcher leaves that form the plant’s “trap door” and other flora. Using mass spectrometry to assess the biochemical levels (nitrogen in particular) in wasps found close to the pitcher plants compared with those further away, the researchers found that this nutrient is a driving force.
Essentially, the pitcher plants aren’t bad at their jobs, but could actually be controlling their own food source in a surprisingly creative way.
If you’ve studied botany or gotten into any kind of gardening, you’ll know the role nitrogen plays. Bacteria pull nitrogen from the atmosphere and convert it into forms that plants can access and absorb. Animals ingest that nitrogen when they eat plants, and then it’s passed on to predators at the next level of the food chain.
Along the way, the lighter form of nitrogen (nitrogen-14) is generally excreted as it’s easier to break down than the heavier kind (nitrogen-15), which will accumulate in an organism’s tissue. Because of this, heavy nitrogen levels can help scientists determine where on the food chain an animal is positioned.
When it comes to carnivorous plants, their nitrogen levels differ from those of non-carnivorous species due to their diet. And in D. californica, the traces of heavy nitrogen isotopes are present in the nectar the plant produces – which is, of course, then passed onto the wasps attracted to it.
But this is where it gets interesting, as the scientists discovered. The wasp samples found closest to the pitchers had elevated levels of nitrogen-15 compared to those found further from the carnivorous plants. This suggests that the wasps rely on this nitrogen-rich nectar and benefit from the reliable food source where the reward greatly outweighs the risk.
It’s worth noting that pitcher plants – often thought to be passive hunters compared to other carnivorous types – can adjust the turgidity of their leaf “trap doors”, most likely using this mechanism to pick off a wasp now and then, and otherwise offering a structurally sound platform for visitors to feed. In doing so, the low rate of trapping makes the plant more friend than foe.
“It is kind of cool to think about a plant cultivating an insect to eat,” says Armitage.
While the study shows that plant and animal interactions are not well understood, it also opens the door to investigating whether plants like these Californian pitchers play a much bigger part in shaping local ecosystems than we thought.
“The role of pitcher plants in some of these really dry, otherwise less-productive regions of the California mountains might be underestimated,” adds Armitage. “Rare and unique plants like Darlingtonia may even be considered foundation species, forming the basis for complex ecosystems akin to coral reefs or mangrove forests.”
The study was published in the journal Ecology.
