The Understory

AMPHIBIANS OF THE RAINFOREST FLOOR

Amphibians are common on the forest floor, although not as abundant as those of the trees. Among the best known of rainforest amphibians are the tiny but brilliantly colored poison dart (arrow) frogs (members of the Dendrobatidae family). These striking frogs secrete powerful toxins from glands on their backs and use their color to advertise their toxic composition to potential predators. The potency of the toxin varies according to the species, and rainforest dwellers have been using these skin secretions for centuries to poison the tips of their blow-darts. The most toxic frog known is the yellow-gold Phyllobates terribilis, of Western Colombia, which is said to be potentially fatal if held in the hand. Indians need only rub and arrow tip across the frog's back and the arrow is good for a year. Other poison arrow frogs must be roasted to extract their poison. The skin secretions of poison arrow frogs have human healthcare applications as evidenced by the story of Epipedobates tricolor and ABT-594/epidatidine.

Dart frogs derive their toxicity from the ants and mites they consume. Frogs that are kept in captivity are generally not poisonous.

Not all rainforest frogs are so brilliantly colored. In fact more amphibians take the opposite approach to defense: camouflage. Several species throughout the world, including the horned toad and two unrelated frogs in Brazil, look like dead leaves and when disturbed stretch out their back legs and become totally still for 30 minutes.

Amphibians are on the decline worldwide—more than 150 are known to have gone extinct since the early 1980s. Several notable species including Costa Rica's Golden toad (Bufo periglenes) and the Gastric Brooding Frog (Rheobatrachus silus) of Queensland, Australia, have disappeared in recent decades. Habitat loss, introduced diseases like the chytrid fungus, over-harvesting, the effects of climate change, pollution, and invasive species are driving amphibian decline. More than two out of every five amphibians assessed by the IUCN are considered threatened.

Because amphibians have highly permeable skin and spend a portion of their lives in water and on land, they are sensitive to environmental change and can act as the proverbial canary in a coal mine, indicating the relative health of an ecosystem.

To keep up-to-date on the latest amphibian developments, be sure to check the Amphibian news feed.

Review questions:

• Why are poison dart frogs colorful?
• Why are frogs endangered?

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Continued / Next: Ground Invertebrates