In the rainforests of the Amazon basin soils are poor in nutrients. In fact, tropical forest soils are among the poorest on Earth, and around 95% of all nutrients in the Amazon are locked up in plant matter.
Among the flora of the Amazon’s rainforests, over countless generations competition for light and nutrients has resulted in specialized adaptations and massive biological diversity. In the plant world, some species have evolved to such a degree that they do not need to grow in the thin soils of the forest floor. Instead, they live on other plants.
These plants are known as epiphytes (from the Greek “epi”, meaning “on”, and “phyton”, meaning “plant”), and epiphytic species are believed to constitute some 25% of all lowland tropical forest plant species. Epiphytes are a particularly diverse family of plants. Almost 90% of epiphytic species are flowering plants, or angiosperms, and they include certain large species such as ferns, orchids, bromeliads and even cacti, as well as smaller plants, such as algae, mosses and lichens. Neotropical habitats are home to at least 15,000 species of epiphytes.
Also known as “air plants”, because of their ability to thrive virtually in mid-air, epiphytes are not parasites. Whereas a parasitic plant will attach itself to a host and live off or cause harm to that host, an epiphytic species attaches itself to a host merely for support.
And so, while epiphytes have no attachment to the ground or to other obvious sources of nourishment, nor do they engage in a parasitic relationship with the plants that support them. In fact, because they are so efficient in taking up water, some epiphytes positively benefit their host plants by producing a cooling effect that can reduce water loss through transpiration. However, in tropical forest ecosystems, epiphytes are often so successful that their sheer weight can bring down the trees that have supported them.
The majority of the world’s epiphytes live in moist tropical forests. In such habitats, their ability to grow above the ground gives them access to sunlight in what at the level of the forest understory is a predominantly densely populated and shaded environment.
Epiphytes obtain the water they need from rain and water vapor in the air. They can absorb water via their roots, or in the cases of some species through the specialized hairs on the surfaces of leaves that have adapted to take in moisture. In addition to the nutrients they are able to gather from rainwater, epiphytes absorb nutrients from the small amounts of organic debris they trap from the plants that support them, or from soil carried by the wind.
In addition to forming part of the nutrient cycle and biomass of tropical forests, some epiphytes are important to certain species of rainforest fauna. Creatures including frogs and some arthropods live in the water reservoirs captured by epiphytic species.
In South America, the epiphytic tank bromeliad is able to hold up to two gallons (eight liters) of water, constituting a valuable source of drinking water for some species of fauna, as well as an entire miniature ecosystem for insect species and the tadpoles of rainforest frogs. In other parts of the world, epiphytes from the genus Myrmecodia contain within them networks of chambers that house ant colonies, which in their turn provide the plant with nutrients.