Much like the witch who tried to devour Hansel and Gretel, carnivorous pitcher plants employ a variety of enticements—such as bright colors, floral perfumes, and tasty nectar—to lure unsuspecting insects to their doom. But most visitors to pitcher traps evade this sticky fate and often exploit their would-be captors for food and shelter.
This phenomenon has led some scientists to wonder whether pitcher plants, rather than acting as strict predators, actually maintain a mutualistic relationship with local insect populations. To find out, a group of researchers set up honey traps of their own in several locations across northern California, where the carnivorous Darlingtonia californica grows in spring-fed fens, streams, and ponds. This plant uses chemical and visual lures to attract insects and spiders, which often lose their footing on its slippery walls. At each of these sites, the scientists hung up a single yellowjacket trap, which uses a similar strategy to capture and kill various species of social wasp.
The team tested the wasps for a rare stable isotope of nitrogen known as nitrogen-15, which naturally occurs at elevated levels in carnivorous pitcher plants. As the team reported last month in a paper posted to the preprint server bioRxiv, wasps living in pitcher fens had higher levels of this isotope compared with individuals of the same species occupying neighboring forest patches without carnivorous plants. These findings suggest that the wasps obtain enriched nitrogen originating from D. californica, either by directly consuming the plant’s nectar or by snacking on other insects that do.
“In either case,” the authors write, “the pitcher plants form the basis of this nutrient loop, channeling prey-derived nitrogen back into the insect populations they have evolved to attract.” Carnivorous plants, they add, may play “a more subtle ecological role” within their local food webs than traditionally thought.
Pitcher plants strike a sweet deal with insect prey
