Structure of the tropical rainforest

SYMBIOTIC RELATIONSHIPS IN THE RAINFOREST

INTERDEPENDENCE AND COMPLEX SYMBIOTIC RELATIONSHIPS

These interdependent relationships have been developing for millions of years and form the basis for the ecosystem. Each species that disappears from the ecosystem may weaken the survival chances of another, while the loss of a keystone species—an organism that links many other species together, much like the keystone of an arch—could cause a significant disruption in the functioning of the entire system.

For example, Brazil nut trees (Bertholletia excelsa) are dependent on several animal species for their survival. These large canopy trees found in the Amazon rainforest rely on the agouti, a ground-dwelling rodent, for a key part of their life cycle. The agouti is the only animal with teeth strong enough to open their grapefruit-sized seed pods. While the agouti eats some of the Brazil nut's seeds, it also scatters the seeds across the forest by burying caches far away from the parent tree. These seeds then germinate and form the next generation of trees. For pollination, Brazil nut trees are dependent on Euglossine orchid bees. Without these large-bodied bees, Brazil nut reproduction is not possible. For this reason, there has been little success growing Brazil nut trees in plantations—they only appear to grow in primary rainforest.

Life in the rainforests is competitive and countless species have developed complex symbiotic relationships with other species in order to survive. A symbiotic relationship is a relationship where both participant species benefit mutually. Symbiotic relationships appear to be the rule and not the exception in the rainforest. For example, ants have symbiotic relationships with countless rainforest species including plants, fungi, and other insects. One symbiotic relationship exists between ants and caterpillars. Certain caterpillar species produce sweet chemicals from "dew patches" on their backs, upon which a certain ant species will feed. In return, the ants vigorously protect the caterpillar and have even been observed carrying the caterpillar to the nest at night for safety. This relationship appears to be species specific in that only one caterpillar species will cater to a particular ant species.

DIVERSITY

All tropical rainforests are characterized by tremendous biological diversity. Section 3 concentrates on the diversity of the tropical rainforest.

Review questions:

• What is a symbiotic relationship?
• What is a keystone species?
• Why are agoutis important in the rainforest ecosystem?

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Continued / Next: Rainforest biodiversity