Plants are famous for producing their own food, but sometimes the cupboard is just too bare. For several hundred plant species around the world, life in nutrient-poor habitats has expanded the menu with a very different food source: animals.
Carnivorous plants can still photosynthesize, but to make sure they get enough nutrients, they've also evolved a variety of tactics to capture small prey like insects and spiders. Some catch their victims in sticky mucilage or snap traps, for example, while others known as pitcher plants lure prey into bell-shaped leaves full of rainwater, where it eventually dies and decomposes into food for the plant.
Small prey is generally safer for carnivorous plants, which might suffer damage if they bite off more than they can chew. Most rely on a diet of invertebrates, but some of the biggest pitcher plants also trap frogs and lizards. A few species from the Old World tropics have even been known to catch small birds and mammals.
North America has a wealth of native carnivorous plants, including the world-famous Venus flytrap, but no vertebrate-eating monsters like those in other parts of the world. Or at least that's what the scientific record suggested, until researchers repeatedly found pitcher plants eating salamanders at a bog in Ontario.
Their discovery, published in the journal Ecology, sheds new light on North America's purple pitcher plant (Sarracenia purpurea), a widespread species that ranges across the Eastern U.S. and most of Canada. It also hints at how much we still don't know about the easily overlooked and quickly fading diversity of plant life all around us.
The new study began in summer 2017, when University of Guelph undergraduate student Teskey Baldwin visited Ontario's Algonquin Provincial Park for an ecology class. Baldwin found a salamander trapped in a purple pitcher plant, a relatively rare sight anywhere, especially outside the tropics. As one 2011 study put it, tropical pitcher plants may offer "the only example of vertebrate capture and digestion by a carnivorous plant that occurs frequently enough to be considered normal."
To investigate how normal this is in North America, a team of researchers conducted a survey at the park in August 2017, timed to coincide with the metamorphosis of local salamanders. They searched 144 pitcher plants, revealing mostly insects — especially flies, which accounted for 88% of the prey — but also eight juvenile spotted salamanders (Ambystoma maculatum).
They followed up with three more surveys in August and September 2018, this time covering more of the young amphibians' dispersal period after metamorphosis. The first survey looked at 58 pitcher plants in early August, finding mostly insects again but also three salamanders. The next two surveys took place in late August and mid-September, and revealed spotted salamanders in a surprising 20% of all surveyed plants. Several plants contained more than one salamander.
This coincided with "pulses" of young salamanders emerging from a nearby pond, where they had just changed from their larval state. There are no fish in this type of bog pond, leaving salamanders to fill key niches as both predators and prey in the local food web. These might have fallen into the pitchers while trying to eat insects trapped inside, the researchers note, or they might have been fleeing predators themselves and chosen a very bad hiding place. Some of the salamanders died within three days, while others survived in the pitcher for nearly three weeks.
'Unexpected and fascinating'
No one wants this to happen to salamanders, of course. They're as cute and charismatic as they are ecologically important, and many species are now in decline due to threats like habitat loss. Feeding native predators is part of their ecological role, though, and while this study does suggest pitcher plants could be "a non-trivial source of mortality for salamanders," the spotted salamander is still fairly common, with a Least Concern listing from the International Union for Conservation of Nature.
And despite scant evidence until now, spotted salamanders might also be a "substantial nutrient source" for some North American pitcher plants, the study's authors write, based on the numbers found in purple pitchers during these surveys.
It would be surprising enough if this was discovered in some remote, obscure wilderness. But it happened at one of Ontario's oldest and most popular parks, located near two major cities (Toronto and Ottawa) and accessible by a highway.
"Algonquin Park is so important to so many people in Canada. Yet within the Highway 60 corridor, we've just had a first," says study co-author Alex Smith, an integrative biologist at the University of Guelph, in a statement. He describes the discovery as an "unexpected and fascinating case of plants eating vertebrates in our backyard."
This is a rare moment in the spotlight for plants, which struggle for even a sliver of the attention we give our fellow animals. It's a helpful reminder that plants are full of surprises, both trivial and valuable, and that we'd be foolish to underestimate them. Still, if you're sad about the poor salamanders, try not to hold it against the plants for being good at what they do. Instead, you could channel the empathy to help your local salamanders, which might appreciate a new amphibian garden in your yard to offset habitat loss. (Maybe give it a little space from your bog garden, though.)