In parts of Florida and southern Georgia, two species of wolf spider eat the same insects as the pink sundew—a type of carnivorous plant.
Sundews catch bugs using a sticky mucilage on the tips of their leaves. The small plants then release digestive enzymes, which begin to process the trapped animals, leaving only their exoskeletons behind.
Sosippus floridanus spiders, meanwhile, build funnel webs slightly off the ground, at the same height as the sundews. And a wandering wolf spider species, Rabidosa rabida, actively hunts for the same insects the sundews tend to trap. (See spider web pictures.)
In the field the team saw that, when S. floridanus is in close quarters with the sundew, the spiders build larger webs farther away from the plants, presumably to snare more meals than the sundews' leaves.
This led the team to suspect that the spiders were hurting the plants via competition.
Laboratory experiments with the hunting spider R. rabida later confirmed that the presence of spiders can deprive the plants of bugs—and thus vital nutrients.
The plants become weaker overall, producing smaller leaves and fewer seeds, according to study co-author Jason Rohr, an ecologist at the University of South Florida.
There's no evidence so far that the plant responds in any way to the spiders' presence, though research is underway to investigate that, Rohr added. (Related: "Plants Can Recognize, Communicate With Relatives, Studies Find.")
Overall, the discovery contradicts a long-held assumption that competition for food mostly occurs among closely related taxa, or categories of organisms, he said.
"We have pretty convincing evidence that you get competition between very distantly related taxa."
Spider-Plant Study an "Interesting Twist"
Aaron Ellison, a Harvard University ecologist who studies carnivorous plants, said the study isn't necessarily surprising. For instance, it's well known that many types of plants compete with microbes for nutrients in the soil.
"In that sense, the results fall right in line with what we expect to happen when organisms share resources and resources are limited," Ellison said. (See pictures of killer plants.)
But the study does add "another interesting twist to how organisms interact," he said, adding that it's "a constant challenge to those of us who work with carnivorous plants to demonstrate that what we do has generality beyond these really weird plants."
As for study co-author Rohr and colleagues, they're moving on to bigger things: seeing if the presence of toads, which eat the same insects as the spiders and plants, affects the spider-sundew interaction.
It's "very possible," Rohr added, that similar plant-animal rivalry exists worldwide.
"It's an underappreciated set of interactions."
The spider-plant competition study was published online in May in the journal Proceedings of the Royal Society B.