Clash of the Kingdoms

On the African savanna, a cheetah fights with a lion over the carcass of a gazelle. In a North American boreal forest, a black spruce tree spreads its roots and leaves to capture more light and water from pines trying to do the same. Closely related species often compete aggressively for resources. But researchers have now found a remarkable exception: a plant competing for food with an animal.

The species in question are sundews and insect-eating wolf spiders. Sundews (Drosera capillaris) cover their leaves in a sticky mucous to trap insects and consume them with digestive enzymes, whereas the spiders (Sosippus floridanus) weave dense webs. Both species live close to the ground in the damp bogs of southern Florida, and both prey on a variety of bugs, including flies, ants, crickets, and springtails. This overlap led ecologist Jason Rohr of the University of South Florida in Tampa to wonder if the two species competed for resources.

Rohr and his colleagues surveyed field sites in Florida, counting the placement and number of sundews and spiders. They also trapped insects in the area in order to estimate the resources available to both. The team observed that the spiders built larger webs when sundews were around than when they were absent, ostensibly to catch more insects. “In web building, it seems the spiders make sensible decisions regarding the level of competition,” says Rohr.

To see how competition from the spiders affected the sundews, Rohr's team took the battle back to the lab. The researchers placed sundews and spiders in terrariums and introduced crickets for the organisms to feast on. As expected, in enclosures with no spiders, the sundews produced many leaves and seeds. But in the terrariums with both organisms, the spiders grabbed many of the insects before the plants could get their leaves on them. As a result, the sundews, suffering from limited resources, produced fewer leaves, seeds, and flowers, the researchers will report online tomorrow in the Proceedings of the Royal Society B.

Although plants and animals are not typically thought to limit each other’s food supply, Rohr says there must be other such competitions that scientists have yet to discover. He and his colleagues plan to study how the presence of oak toads, miniscule amphibians that also live in the bogs and eat insects, affects the sundews. “It’s very possible that it’s a widespread phenomenon, but we’d need to test it,” he says.

Ecologist Aaron Ellison of Harvard University says that the researchers produced “a really compelling paper.” Whereas other scientists have speculated about inter-kingdom competition, this study gives a nice confirmation, he adds. Ecologist Thomas Miller of Florida State University in Tallahassee is impressed to see an example of competition between an active, mobile spider and a plant that just sits and waits for prey to get stuck. “Beyond the plant and animal difference, these are really different types of predators, and yet they have significant effects on each other,” he says.