One of the clearest examples of commensalism in the tropical rainforest is the relationship between epiphytes and the trees they grow on. Orchids, bromeliads, and ferns anchor themselves to tree trunks and branches to reach sunlight in the canopy, gaining everything they need from the air and rain around them, while the host tree is neither helped nor harmed. But epiphytes are far from the only example. Tropical rainforests are full of commensal relationships, from birds that trail army ant swarms to moths that spend their adult lives nestled in sloth fur.
How Commensalism Works
Commensalism is a relationship between two species where one benefits and the other is unaffected. It sits between mutualism (both benefit) and parasitism (one benefits, one is harmed). In practice, the line between these categories can blur. A relationship that looks commensal at first glance sometimes turns out to carry hidden costs or benefits. But several rainforest interactions fit the definition well enough that ecologists use them as textbook cases.
Epiphytes on Trees
Epiphytes are the classic example. These are plants that germinate and grow entirely on other plants, never making contact with the soil and never parasitizing their hosts. Unlike mistletoe, which taps into a tree’s vascular system, epiphytes simply use the tree as a platform. They pull water and nutrients from the air, rain, and organic debris that collects around their roots.
The scale of this relationship is enormous. Epiphytes account for 25 to 50 percent of all plant species in tropical forests and represent roughly 8 to 10 percent of all known vascular plant species on Earth. A single large canopy tree can host dozens of orchid, fern, and bromeliad species on its branches. The success of these plants depends on host tree traits like bark roughness, branch angle, and canopy structure, but the trees themselves are rarely affected by the arrangement. The epiphyte gets access to light it could never reach on the forest floor. The tree carries on as before.
Army Ant Followers
When a massive swarm of army ants moves across the rainforest floor, it flushes out everything in its path. Grasshoppers, katydids, crickets, and other insects take flight to escape the advancing colony. Certain bird species, including antbirds, woodcreepers, and ovenbirds, have evolved to exploit this chaos. They follow the swarm and pick off the fleeing insects, essentially letting the ants do the hunting for them.
The ants gain nothing from the birds’ presence, but they lose nothing either. The birds aren’t eating the ants or stealing their prey in any meaningful way. They’re catching insects the ants would never reach, the ones that escape by flying. Some of these bird species are so specialized that they rarely forage any other way, spending their days tracking swarm movements through the forest. This is commensalism driven by opportunity: the ants create a disturbance, and the birds capitalize on it.
Moths Living in Sloth Fur
Pyralid moths in the genus Cryptoses spend their adult lives living in the coarse fur of sloths. The relationship has long been classified as commensal: the moths need the sloth to complete their life cycle, but because they don’t feed on the sloth, they impose no cost on their host.
The life cycle is unusual. When a sloth descends from the canopy to defecate (something it does roughly once a week), pregnant female moths leave the fur and lay their eggs in the fresh dung. The larvae develop entirely inside the dung pile, feeding on it as they grow. Once they emerge as adults, they fly up into the canopy to find a sloth and settle into its fur, where they mate and begin the cycle again.
More recent research has complicated the picture slightly. As moths die in sloth fur, they decompose and release nitrogen, which fuels the growth of green algae on the sloth’s coat. That algae may provide camouflage or even a supplemental food source for the sloth. If true, this would push the relationship closer to mutualism. But the moth-sloth pairing on its own, where the moth benefits and the sloth is unaffected, remains a widely cited example of commensalism.
Frogs in Bromeliads
Many species of poison dart frogs use the small pools of water that collect in bromeliad leaves as nurseries for their tadpoles. A female frog deposits her eggs in the cup-shaped center of the plant, where the water is sheltered from predators and buffered from temperature swings. Some species return repeatedly to feed their developing tadpoles with unfertilized eggs. The bromeliad provides a safe microhabitat, and the frog depends on it.
Most research has treated this as commensalism, with the frog benefiting and the plant remaining unaffected. A 2017 study, however, found that nitrogen in the frogs’ waste acts as a fertilizer, potentially boosting the bromeliad’s growth. If confirmed broadly, that would make the relationship mutualistic. For now, the frog-bromeliad pairing is still commonly used as a commensalism example in ecology courses, with the caveat that the bromeliad’s side of the equation may be more complex than it appears.
Mosses and Liverworts on Bark
Mosses and liverworts blanket tree trunks and branches throughout tropical rainforests. Like epiphytes, they use bark surfaces as growing platforms without penetrating the tree’s tissue. Their distribution depends on factors like bark texture, light availability, and humidity, but they draw no resources from the tree itself.
Ecologists have generally classified tree-moss associations as commensal: mosses benefit from the microhabitat conditions the tree provides, while the tree experiences no obvious harm or benefit. There is some debate about whether trees hosting very large moss biomass might face costs from the added weight or reduced light to bark surfaces, but this remains an open question rather than an established finding.
When Commensalism Gets Complicated
Strangler figs illustrate how a relationship can shift between categories over time. A strangler fig begins life as an epiphyte, its seed deposited in the canopy by a bird or bat. In this early stage, the relationship looks commensal: the fig uses the tree for support while causing no harm. As the fig grows, however, it sends roots down the trunk to the ground and gradually encircles the host tree, competing for light and sometimes killing it. What started as commensalism becomes competition or even parasitism.
Interestingly, research has found that the relationship isn’t always lethal. Trees with large attached strangler figs were more than four times as likely to survive tropical storms compared to their nearest neighbors without figs. The figs’ aerial roots may act like guy-lines on a tent, stabilizing the host tree and limiting movement in high winds. Their leaves can fill gaps in the canopy, reducing wind exposure, and their root networks may physically reinforce the trunk. So even in this seemingly hostile interaction, the host tree can benefit under certain conditions.
These blurred boundaries are part of what makes rainforest ecology so rich. Pure commensalism, where one species benefits and the other is completely unaffected, is harder to prove than it sounds. But epiphytes on trees remain the gold standard example, and army ant followers, sloth moths, and bromeliad frogs each demonstrate how one species can build its survival strategy around another without taking anything away.