Jun. 12, 2012

Epiphytes and the Hidden Biodiversity of Rainforests

by Kara Rogers

Click to enlarge images
High up in the rainforest canopy, at heights that elude casual observation, exists an amazing above-ground tapestry of vegetation that consists primarily of plants known as epiphytes. Defined simply as plants that grow upon plants, these atypical species, which often produce flowers of remarkable color and have leaves of unusual shape or size, make up a significant portion of rainforest biodiversity, accounting for as much as 25 percent of all vascular plant species in the tropics alone.

While epiphytes are defined by their growth on other plants, most commonly trees, they do not steal nutrients away from their hosts. Rather, they rely on specialized aerial root systems that absorb water and nutrients directly from the air. There are more than 30,000 known species of these “air plants,” about half of which are found in the Neotropics (the biogeographical region extending from Florida to South America). Some of the richest epiphyte diversity in this region occurs in montane forests at elevations between 1,000 and 1,500 meters, where humidity levels tend to be relatively high. The four major groups of plants that contain epiphytic species that grow abundantly at these elevations include orchids (Orchidaceae), ferns (Pteridophyta), plants of the arum family (Araceae), and bromeliads (Bromeliaceae).

The diversity of epiphytes is reflected in their different growth and survival strategies, several of which challenge the notion that epiphytes are nonparasitic on their host trees. Strangler figs (genus Ficus), for example, begin their lives as epiphytes but eventually grow roots down to the ground, enabling the plants to draw nutrients from the soil and thereby out-compete their host trees for nutrients and sunlight. Because strangler figs ultimately become trees themselves, they are considered to be hemiepiphytes, as opposed to true epiphytes. Other epiphytes that may harm their hosts include species of nonphotosynthetic (achlorophyllic) orchids that are mycorrhizal, partnering with a species of fungus that grows on the orchid's roots and siphons off nutrients from the host tree for the orchid. In most cases, the fungal partner in these relationships is parasitic on the host tree and may even cause disease.

Epiphytes further influence rainforest biodiversity by supporting the survival of other tropical species. For instance, bromeliads, which have long overlapping leaves that form bowls, collect water that provides habitat for animals such as snails, frogs, and worms. In fact, more than 250 different species of animals have been discovered in the water “tanks” of bromeliads. When animals trapped in the tanks die, they decompose, providing the plant with an additional source of nutrients.

Epiphytes also support a variety of birds and insects, which serve as the plants' primary pollinators. As a result, the rainforest canopy is literally buzzing with life, with animals such as hummingbirds, beetles, bees, and moths flitting among their preferred epiphytes, sipping nectar and transferring pollen from plant to plant.

Although much is known about epiphytes, scientists are working to find new ways to explore life in the rainforest canopy, suggesting that more will be revealed. The sheer number of different types of plants that live epiphytic lifestyles further suggests that new species of not only plants but the animals that visit them are awaiting discovery.
About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.
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