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Ecotrading in nature's marketplace
Ants and plants as model mutualists
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Ecotrading in nature's marketplace

Sipping nectar
isn’t just for bees and butterflies. Skidmore biologist Josh Ness does it too. Only he doesn’t drink it; he analyzes it.

As a way to illuminate ecological relationships, Ness studies plants that have extrafloral nectaries—nectar-releasing pores on stems or leaves—and the ants that frequent them. In a well-known symbiosis, the nectar nourishes the ants, and the ants attack leaf-eating bugs. It’s a useful evolutionary adaptation for the plant to sacrifice some carbohydrates in return for protection from herbivores; ditto for the ants to spend energy guarding their food source. Predation and competition may seem more dramatic, says Ness, but mutualisms underlie much of nature as well.

A painter and would-be architect in his school days, Ness likes the way form can convey meaning. Yet he warns that “too often we define, say, a cherry tree by basic traits like leaf shape, flower structure, fruit formation. But its interactions with other organisms are also crucial, defining traits”—for example, the chewing of caterpillars makes the leaves become more toxic, bees control which flowers get pollinated, and fruit-eating birds disperse the seeds.

This summer Ness and two students used grant funding to study Japanese knotweed, a non-native species that’s been aggressively spreading in the US, including the hillsides above Skidmore’s Northwoods Village. Some plants exude more or sweeter nectar when damaged—which presumably attracts more ant defenders—but that’s not been proved with Japanese knotweed. So Ness and crew snipped off parts of knotweed leaves, to simulate caterpillar damage, and later measured the sugar content of the nectar and the number of ants visiting the plants. On each vivisected plant, Ness located a tiny nectary on the underside of the stem and applied a droplet of purified water to it. A moment later he carefully sucked a bit of the water-nectar solution into a tiny glass pipette. His student partners measured the sample and placed it on the screen of a hand-held refractometer: when aimed toward the sun, it can identify various chemicals, including fructose, by the way they bend light.

While the damaged knotweeds had more ants on them, they didn’t always produce higher-fructose nectar. In the future Ness wants to check the seasonality of peak nectar production and insect arrivals. He’s also curious whether Japanese beetles (fellow ex-pats, and common garden pests in the US) might have a taste for their native cuisine, which could help check the spread of knotweed. He says, “I love fieldwork. Depending on my observations each day, I can adjust my research design to study other factors. I’m usually doing multiple experiments at a time.”

In fact, serendipitous mission creep actually helped Ness with his PhD. His research began with parasitic wasps’ effects on the behavior of their caterpillar hosts on catalpa trees. “But ants kept harassing the caterpillars. So I told them, OK, I’ll study you then.” He found native ants were using the plant for nectar, while invasive fire ants, common in disturbed or urban-edge habitats, “had no interest in nectar and were just hunting caterpillars. They would have done the same on a fencepost or a car.”

Fire ants also figure in Ness’s research in Arizona’s Sonoran Desert, where he shares a study plot. There, one cactus species with extrafloral nectaries has a chemical in its petals that repels some ants, keeping them away from the bees that pollinate it. But certain fire-ant relatives still patrol the flowers and attack bees, who soon learn to avoid those cacti. Freer of pests, the cacti produce lots of fruits, but since they weren’t as well pollinated the fruits contain fewer seeds. Now armed with six years’ worth of observations in droughts, monsoons, and other weather, Ness is eager to try extrapolating the data to predict the effects of global climate change on these plant-and-animal interactions. —SR