[Another version of this article first appeared on Mongabay.com]
Imagine a ‘plant’. Now remove the soil, for it’s not essential. In fact, about one-tenth of plant species have liberated themselves from soil and evolved into epiphytes—plants that grow on other plants.
Epiphytes, except for the mistletoe, do not steal from their host plants. Adapted to the dangers of lofty living, epiphytes grip their host plants with specialized roots that cannot absorb nutrients. Often mistaken as parasites, epiphytes merely ride their host plants into the canopy for more sunlight. Most often, epiphytes are hitchhikers that do not harm their hosts.
A recent publication in the journal Biotropica suggests however, that epiphytes may even protect their host trees from pest damage.
Scientists led by Edd Hammill, then a postdoctoral with University of British Columbia, surveyed bromeliads on orange trees in Costa Rica to study the relationship between bromeliads and their host trees. Bromeliads (family Bromeliaceae) are herbaceous plants native to Florida, Central and South America; according to conservative estimates by the Marie Selby Botanical Garden, Florida, half of the 3352 bromeliad species are epiphytic.In Costa Rican forests, trees are crowded with bromeliads and other epiphytes; Hammill wondered if host trees would benefit from shouldering the extra load. An earlier study in natural forests showed that bromeliads can attract ants that then deter herbivorous beetles from the host trees. Forest ecosystems however, complicate investigation. Hammill turned to a much simpler ecosystem— pesticide-free orange farms—next to the forests for clues. Many of the orange trees hosted bromeliads, especially that of the genus Vriesa. Some of the Vriesa house predators, like the trap-jaw ants Odontomachus, spiders, scorpions (and snakes!). Could these predators living in Vriesa protect the orange trees from herbivores?
Hammill addressed that question with a survey. He located ten orange trees without bromeliads and twenty each with a large Vriesa bromeliad (>30cm diameter); half of the Vriesa were colonized by trap-jaw ants whereas the rest were ant-free. Hammill then measured damage on orange tree leaves near the Vriesa (within 50cm radius) and those farther away (>150cm).
On trees with bromeliads that housed trap-jaw ants, Hammill found that herbivores spared nearby leaves (2.5% damage) but attacked leaves farther suffered away (22.5% damage). Expectedly, herbivores would not linger near bromeliads colonized by ferocious trap-jaw ants that earned their namesake with jaws that snap a thousand times faster than an eye blink.
Farther away from their bromeliad colonies, the trap-jaw ants’ influence waned and herbivores damaged leaves. Hammill was surprised, as the ants often search for food far from their nests. He noted however, that the ants do not forage equally across the tree, instead spending more time around their bromeliad nests. “Increased level of ant presence near the bromeliads”, Hammill proposed, might explain the lower leaf damage observed there.
As early as the 4th century, citrus farmers in China have recruited aggressive weaver ants to fight caterpillar pests. That this ancient practice might be bolstered with bromeliads is a new and—Hammill agrees—untested idea. Current agronomic practices like pruning and pesticides can discourage bromeliads from citrus trees, added Professor J. Howard Frank, entomologist at University of Florida. Frank further warned that this approach might even backfire if “the pests were mainly scale insects and mealybugs”—referring to insects that suck plant sap and produce honeydew to feed the ants that guard them.
“Bromeliads serve as shelter for many predators” –Hammill
Yet bromeliads might offer more than just ants. On orange trees without ants, leaves close to bromeliads suffered minor damage (6%) whereas those farther away suffered herbivory (16%) as high as leaves on trees without bromeliads. Bromeliads seem to protect tree leaves even without trap-jaw ants.
“Bromeliads serve as a shelter for many predators,” said Hammill who has found spiders and scorpions in bromeliads. These predators do not seem to share bromeliads with ants, and might deter herbivores like ants do.
Hammill however, is not advocating bromeliads to farmers yet, though Costa Rican farmers welcome new techniques and ideas. He recognizes that his survey cannot confirm mutualism between bromeliads and orange trees. For that, he hopes to secure funding to manipulate removal/addition of bromeliads on trees and examine consequent effects on host trees. “I believe our study to be the start,” said Hammill “rather than the end of this story.”
Beyond farms and in the tropical forests, mutualism among ants, epiphytes and host trees is “probably a widely occurring phenomenon,” according to Emeritus Professor David Benzing of Oberlin College. Benzing, author of several books on bromeliads, added that organisms interact in all manners from parasitism to mutualism, and that Hammill has revealed “but a small part of a very complex system.”
Such interactions might be best observed among organisms that share a long co-evolutionary history, prompting Bruce Holst, director of botany at Marie Selby Botanical Garden, to suggest that Hammill examine bromeliad relationships with native host trees instead of the Asia-originated orange tree.
Stereotyped as parasites by most people and treated as impartial hitchhikers in ecology textbooks, epiphytes turn out to be all these and mutualistic too. Adaptations in nature “clearly wander in and out of the cut and dried target roles,” says Prof. Daniel Janzen, an influential tropical ecologist, “that humans would like to think can be used to categorize things.”
Perhaps we should take a leaf out of epiphyte’s book, and free ourselves from paradigms—like epiphytes liberated from soil—and explore the world with fresh minds. Surprising discoveries await.