Climbing the Food Chain
By this point, you might be wondering why all the emphasis has been on plants. (As an animal yourself, you probably think first of the animals in any biome.) Part of the answer lies in that second law of thermodynamics that you probably skimmed through several pages ago. In any ecosystem, ~90% of the energy that is captured by any trophic level (groups of organisms that get energy in the same way) stays there. Most is either used or wasted as heat. Only about 10% of the energy that is obtained ultimately becomes available to the next trophic level.
Almost all biomes on Earth's surface* are named for the climax plants that grow there, not the animals. (The only exception is the coral reef, where the coral animal with its calcium skeleton forms the backbone of a complex ecosystem.) That doesn’t mean the animals aren’t crucial for the existence and the balance of the system! If we were somehow able to calculate the total mass of all of the organisms in an ecosystem, we’d find that the mass of first order consumers (plant eaters) would be only a fraction of the total mass of plants, and the mass of second order consumers (animals that eat the plant eaters) would be only a fraction of that. “Top carnivores” would be far fewer. It's all about energy. Consider the sloth at the left; minimizing energy use is its specialty!
When students learn about food webs, they often draw quite simple representations, with only a few plants or animals. That’s never really accurate, but it’s most deceptive when we think about a rainforest--an area that can have hundreds of trees and thousands of animal species in a single hectare. Each of those species occupies a specific role (called a niche) in the forest; each might be indispensable in maintaining balance.
The very diversity of rainforest makes the ecosystem more stable. Unlike a desert or tundra biome (where only a few animals may control the plant population) the high biodiversity in the rainforest contributes to its ability to survive and thrive. (In the third week, you'll read about a controversial theory--that much of the diversity in the forest was actually created by humans!) There are even food webs within food webs. In the fur of the tree sloth (above) lives an entire community of algae, fungi, and insects. It’s a pleasant place to make a home, since the sloth only moves once every two or three days—when it’s raining so hard that it can safely descend to the ground to do its bathroom duties. Then it climbs back up for another few days of well-deserved rest. You’ll learn more about the sloth in Week 2.
*The ecosystems in deep ocean trenches are totally different, getting energy from hydrothermal vents and radiation.
Meet a Fragile Resident
Like the plants, rainforest animals are diverse and highly specialized. One example is the hoatzin, a reclusive bird with adaptations that make it especially well-suited for the dark, wet environment.
Hoatzins are first order consumers, eating the leaves of Arum as well as fruits and seeds. They have a uniquely primitive third stomach (crop) where the "water lettuce" is digested; its chemicals make the hoatzins taste terrible. Like the plants in their world, the hoatzins also use a unique chemical defense—a powerful odor that can be emitted from a specialized gland—to resist predators. They build their nests on limbs overhanging the water. The young swim well, so they can drop from their nest into the water for protection whenever a predator appears. A unique claw on each of their wings that helps them climb back up to the nest once the predator has departed the area. On your trip to the Amazon, you may find it in a secluded oxbow lake near the lodges.
Hoatzins have evolved over tens of thousands of years to occupy a unique niche in the rainforest. They can survive in the canopy, but only if it overhangs calm, open water with plenty of water lettuce. Clearing the water’s edge (for roads or human residences) deprive it of its escape route and make survival far more difficult. Like many rainforest denizens, it is so highly specialized that it can be easily threatened to extinction. Now that the peoples of the Amazon (who traditionally moved every few years) now are being given permanent title to strip farms along the river, changes are coming more quickly.
This is just one example of an organism for which a small change in the environment could create tremendous population pressures. There are many others. Animals such as jaguars and pumas require large areas to hunt. Fragmentation of a forest (by cutting roads) can limit their ability to hunt or find diverse mates. (Inbreeding in small forest areas weakens a species over time.) Road building also increases human access to the forests, attracting hunters who kill jaguars, monkeys, deer, and agoutis (giant rodents).
While the food web of the rainforest is diverse and complex, it is still fragile. The adaptations of each of the species are often so specific, and the competition from other species with different adaptations so intense, that there’s little room for change. You’ll learn more about adaptations, changing environments, and evolutionary mechanisms in Week 2.