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Ecology

Tropical Rainforests The Cradle of Global Biodiversity

Rainforests purify the planet’s air and help regulate climate, global temperature, and the water cycle (one fifth of the Earth’s fresh water is found in its rivers and lakes). They also act as humanity’s natural pharmacy: many of the medicines we use were discovered in this fascinating ecosystem.

Text and Photos: Javier A. Pinzón

Tropical forests are the heart of global biodiversity and, although they occupy just 2% of the Earth’s surface (in the Tropical Zone), they are home to more than half the planet’s living beings.

This unique ecosystem is home to millions of micro-ecosystems. It is estimated that just 2.5 acres of rainforest can contain 42,000 species of insects, 807 species of trees, and 1,500 species of other plants, including two thirds of the world’s angiosperms (flowering plants).

Nature has met the challenge of accommodating so many plants and animals in such a reduced area by creating a three-tiered world: the “forest floor” is home to species living underground or on the surface; the “understory” is the area between the canopy and the forest floor; and high above, many species live in the “canopy” or treetops. In this way, many different species can share the same tree and live related and interdependent lives without ever meeting, just like in modern apartment buildings.

In this world of intricate relationships, life and death battles are fought over an invaluable asset: light. The winner is the first to reach the sunlight above the canopy. The canopy in these forests may rise up to 165 feet above ground, which means a long journey for a little tree just beginning to germinate. In a healthy forest, the canopy is so dense that plants and animals below receive just 2% of the sunlight and, therefore, only those requiring low light can live there.

The Forest Floor

Although jungle soil doesn’t contain many nutrients, simply lifting a stone or a fallen log reveals hundreds of insects and litter-decomposing fungi, which are responsible for producing fertilizer for the trees that keep the forest on its feet. Leaves and dead branches fall to the forest floor, as well as dead animals, but the cycle of life doesn’t end with death: insects and fungi recycle all the nutrients to make them available to the trees. Termites are the most dominant decomposers, with more than a thousand individuals per square foot. They, along with ants, process a third of the organic material that falls to the forest floor. Fungi, worms, and slugs finish off this process. The transformation is quick: the recycling of nutrients in the soil takes only about six weeks, while in other forests it can take a year. But this speed has disadvantages: the nutrients don’t have enough time to permeate below the first two inches of soil.

The Understory

The understory, between the ground and the canopy, is home to many birds, mammals, insects, and reptiles. The plants that inhabit this story of the large “building” receive only 5% of the light. Their neighbors are shrubs, ferns, vines, herbs, and grasses. To take advantage of the limited light, some of these plants, like the Monstera deliciosa, have developed large leaves that can grow up to three feet long and 2.5 feet across. The reason: the larger a leaf’s surface, the more light the plant receives.

Crawling mammals in this neighborhood climb up and down the tree trunks in search of frogs, snakes, and hundreds of insects among the leaves. Many are colored in such a way that allows them to blend in with the environment and avoid being a larger predator’s lunch. Flowers, however, use vivid colors and strange shapes to attract pollinators and help them do their job as efficiently as possible. Butterflies and hummingbirds have evolved along with the plants they pollinate, which is why the hummingbird’s bill fits perfectly inside its favorite flowers.

The Canopy 

The largest trees, rising up to 165 feet above the ground, stretch their tall trunks to connect inhabitants on the ground, in the middle, and higher up. Their leaves form the forest ceiling and are responsible for limiting the light that reaches those in the middle and below. Large forest trees such as ceibas and cuipos support plants seeking sunlight that are unable to reach it on their own. This explains the presence of epiphytes, such as orchids and bromeliads, on the tree trunks. Larger birds like toucans and orioles, as well as monkeys and sloths, frequent the canopy. And the largest birds, like eagles, live on the canopy and fly from treetop to treetop looking for food.

When one of the larger trees falls to the ground, the separation between those at the top, in the middle, and on the bottom no longer exists. The sun reaches the ground and the dormant seeds of large trees, waiting patiently for an opportunity, quickly emerge and begin a race for the canopy, where they will fill in the natural forest ceiling. A good example of this extreme fight for light is the strangler tree, the seeds of which can be transported to the canopy by a bird. Once there, using energy provided by the sun, they germinate very quickly on the host tree and gradually send down roots in search of the ground. Over time, these roots become big and strong and wrap around the host tree to strangle it.

Separate but Equal

There are many mouths to feed in the forest, so the battle for food is intense. It’s important to go unnoticed by predators. To achieve this, certain butterfly species have developed chemicals unpleasant to their predators; others have evolved creatively to mimic the colors of the toxic butterflies, avoiding the effort of developing the toxin, but still “disguised” to deceive their enemies.

In the forest nothing is wasted. Every resource is used so that there is enough for everyone. Monkeys, for example, eat the fruits off thick branches, birds eat those growing on thinner branches, and parrots make use of the most inaccessible because they can hang upside down to harvest them.

The fruit of the huge almond tree, which rises above the canopy, is highly sought after and is eaten by many. Monkeys eat the sweetest parts only and discard the rest, which drops to those below. There, coati eat the soft parts and leave the hardest parts, a delicacy for ñeques. Bats take advantage of the peaceful nighttime hours to look for food in almond trees, but to avoid becoming easy prey for predators they carry the fruit to a safe place, and in the process drop a good portion, which falls and becomes available to those below. At dawn, the ñeques find more fruit on the ground than they can eat and bury the surplus for times of scarcity. But they never go back for it and, unbeknownst to them, are responsible for keeping this food cycle going by planting the fruit of the almond tree, which will feed future generations.

Space in the forest, like food, is scarce and fought over. Therefore, both plants and animals have developed strategies to make the best use of every inch. For example, woodpeckers make holes in the branches of trees to use as nests; if these nests are abandoned, they are fought over by countless species. Meanwhile, bromeliads in the upper canopy serve as tiny water basins where frogs live and raise their tadpoles.

Rainforests and Man

Although tropical forests lie so far away and seem so removed from city life, the truth is that we’re all linked to them in some way, no matter where we are. These majestic ecosystems purify the planet’s air and help regulate global climate and temperatures, as well as the water cycle, since a fifth of freshwater on Earth is found in rivers and lakes. In addition, tropical forests are humanity’s natural pharmacy: more than a quarter of natural medicines were discovered there, including 70% of cancer drugs.

And yet this ecosystem is also one of the planet’s most threatened. Fragmentation and destruction of forestland disrupts interactions between species and is one of the major causes of extinction, along with hunting, logging, mining, and water pollution. It is the task of all the world’s inhabitants to ensure the safety of tropical forests; although few in number, they contain so much of what we need.