Perhaps the best symbol of the Amazon rain forest is the three-toed sloth, a gentle herbivore that dwells exclusively in the canopy of the forest. It moves literally at a snail’s pace and this, together with its cryptic coloration, protects it from its only major predator, the harpy eagle. Viewed up close, the sloth appears as an hallucination, an ecosystem unto itself that softly vibrates with hundreds of exoparasites. The sloth’s mottled appearance is due in part to blue-green algae that lives symbiotically within its hollow hairs. A dozen varieties of arthropods burrow beneath its fur; a single sloth weighing a mere ten pounds may be home to over a thousand beetles.

The life cycles of these insects are completely tied to the daily round of the sloth. With its excruciatingly slow metabolism, the sloth only defecates once a week. The animal climbs down from the canopy, excavates a small depression at the foot of the tree, voids its feces and then returns back up. Mites, beetles, and even a species of moth leap off the sloth, deposit an egg in the dung, and climb back on to their host for a ride back up the tree. The eggs germinate and, in one way or another, the young insects find another sloth to call home.

Why would this animal go down to the base of the tree, exposing itself to all forms of terrestrial predation, when it could just as easily defecate from the treetops? The answer provides an important clue to the immense complexity and subtlety of this ecosystem. Biologists have suggested that in depositing the faeces at the base, the sloth enhances the nutrient regime of the host tree. That such a small amount of nitrogenous material might actually make a difference suggests that this cornucopia of life is far more fragile than it appears. In fact, many ecologists have called tropical forest a counterfeit paradise. The problem is soil. In many areas, there essentially is none.

Forests have two major strategies for preserving the nutrient load of the ecosystem. In the temperate zone, with the periodicity of the seasons and the resultant accumulation of rich organic debris, the biological wealth is in the soil itself. The tropical ecosystem is completely different. With constant high humidity and annual temperatures hovering around 80F (27C), bacteria and micro-organisms break down plant matter virtually as soon as the leaves hit the forest floor. Ninety per cent of the root tips in a tropical forest may be found in the top four inches (10cm) of earth. Vital nutrients are immediately recycled into the vegetation. The biological wealth of this ecosystem is the living forest itself, an exceedingly complex mosaic of thousands of interacting and interdependent living organisms. It is a castle of immense biological sophistication built quite literally in a foundation of sand.