Tortoise meets tourist on the Galapagos

This aerial view of Isla Sombrero Chino reveals the Galapagos' geological origins. Up to 5 million years ago, magma bubbling up from a hot spot in the Earth's crust cooled and hardened into these remote islands. They lie nearly 1,000 kilometers (621 miles) off the Ecuadorian mainland in South America.

Secluded by their distance from other lands, plants and animals on the islands evolved independently - meaning the Galapagos are rich in endemic species, or those that can be found nowhere else on earth. This lava cactus is one, perfectly adapted to thrive on inhospitable lava fields where very few other organisms survive.

One of the most unusual of the archipelago's endemic speces is the marine iguana - the only lizard in the world that makes a life foraging in seawater. They graze on algae, and can dive up to 9 meters deep. But the marine iguana is at risk from human-introduced pathogens - as well as pigs, dogs and cats, which were brought to the islands by people and, given the chance, will feast on iguana eggs.

Red rock crabs are not found exclusively on the Galapagos Islands. But the local population has a unique adaptation: The crabs have been observed feeding on ticks from the islands' marine iguanas, in a symbiotic relationship thought to benefit both lizard and crustacean.

Frigatebirds are found across the tropics, favoring remote islands where they breed in colonies of up to 5,000 birds. A common sight on the Galapagos Islands, they can fly thousands of kilometers at a time. Yet one of the archipelago's two frigatebird species - the magnificent frigatebird - is now recognized as genetically distinct from relatives elsewhere.

Giant tortoises survive only on two remote archipelagos, the Galapagos and Aldabra in the Indian Ocean. The lumbering reptiles can live for more than 100 years. Hunting reduced Galapagos tortoise numbers from around 250,000 in the 16th century to just 3,000 in the 1970s. Conservation efforts have seen populations rebound, and 20,000 now inhabit in the islands.

Hundreds of thousands of tourists visit the Galapagos each year. But careful steps are taken to minimize their impact on biodiversity. Cruise ships anchor offshore, and visitors can only get on to the islands by dinghy. They're not allowed to bring food, or touch local wildlife. Most Galapagos animals have no fear of humans, because they evolved without exposure to predators.

Galapagos sea lions breed exclusively on the remote archipelago and on Isla de la Plata, just off the Ecuadorian mainland. Their loud bark, playful nature and agile grace in the water make these gregarious animals a favorite with tourists. They have little fear of humans - but once in the water must take care not to end up as lunch for sharks or orcas.