The Galapagos Islands have a low biodiversity because the islands are 600 miles from the nearest land and this huge expanse of inhospitable ocean in-between makes it very difficult for new kinds of plants and animals to reach the islands. Marine organisms, such as green sea turtles and corals, probably came on their own, swimming, or as floating larvae. Sea birds are all strong flyers that frequently make long journeys across the open sea. But most of the Galapagos life forms reached the islands by accident, and all had a long sea voyage. During that trip, both plants and animals were exposed to saltwater, drying winds, and intense sunlight. They had no fresh water or food. Galapagos reptiles are also more likely than land birds or mammals to be able to survive under these conditions. As a result, animals of the Galapagos Islands are species whose ancestors were already well suited for its harsh environments. Compared to elsewhere in the tropics there are few birds or Galapagos mammals, and many important groups are missing.

• Mammals: 32 species recorded
• Reptiles: 28 species recorded
• Darwin Finches: 13 sub-species recorded
• Sea Birds: 42 species recorded
• Shore Birds: 34 species recorded
• Water Birds: 21 species recorded
• Land Birds: 49 species recorded

The Galapagos Islands are celebrated throughout the world for their unique natural wonders and the rare and intriguing wildlife that played such an important role in Darwin's theories of Evolution & Natural Selection. The wildlife that amazed Charles Darwin still invites travelers to this treasured region today. Isolated from the mainland for thousands of years, some of the most unusual species in the world have evolved here, including giant tortoises, marine iguanas, blue-footed boobies, and even the world's only equatorial penguin. The islands comprise the Galapagos National Park and were recently named a "World Heritage Site".

The Galapagos bird life is world-renowned for its finches - the tiny little birds that helped inspire Darwin during his exploration of the Islands. But bird life on the islands goes far beyond its famous finches. This unique habitat is home to a wide variety of species, from colorful flamingos and comical blue-footed boobies, to showy frigate birds and bizarre cormorants. The distinct features of the birds make for easy identification even for the novice birder, while the savvy will be pleased to spot the rare and treasured species found only in the Galapagos Islands. 

While most of the world finds that mammals are the predominant land animals, the Galapagos Islands' land animals are dominated by reptiles. Unlike mammals, reptiles are able to survive long periods without water, which equipped them with the ability to make the epic 620-mile journey to the volcanic islands from the mainland millions of years ago. Visitors to the Islands will have plenty of opportunities to discover unique creatures like giant tortoises, land and marine iguanas, snakes, and lizards. In fact, the Islands draw such inspiration from these gentle reptiles that the archipelago was even named after one of its most famous species - the giant tortoise. 

Fish and rays and sharks, oh my! The Galapagos Islands are considered to be one of the natural wonders of the underwater world. A serendipitous combination of conditions have made the archipelago's impressive marine biodiversity possible – right on the equatorial line, the Islands are affected by three different currents: the cold Humboldt and Cromwell currents and warm Panama currents. Roughly 17% of the marine life found in the islands are endemic to the Galapagos, meaning that they cannot be found anywhere else in the world. Snorkelers and divers are treated to a spectacular gathering of aquatic wildlife, from rare whale and hammerhead sharks, to the colorful Sally light-foot crab. Although treasured by wildlife enthusiasts, Galapagos marine animals are also sought after by the fishing industry. Sharks and sea cucumbers have been particularly vulnerable; while sharks are protected by the Galapagos Marine Reserve, they are still hunted illegally for their highly-prized fins to make shark fin soup. Other fascinating underwater species you might come across include all manner of rays, from manta to golden to stingrays, a veritable rainbow of tropical fish, sea turtles, corals, seahorses, and their hungry avian predators that plunge into the seas to snag their next meal from high above. Close to the shore, you'll likely see marine iguanas diving to graze on seaweed and algae - their perfectly evolved tails, claws, and digestive system allow them to be the only sea-going lizard in the world.

Many visitors recall swimming with sea lion pups or watching dolphins race along the bow of their ship as some of their favorite Galapagos memories. But ironically, there are a very limited number of mammals on the islands - instead the Galapagos' land animals are predominately reptiles. Because of its isolation, there are very few native mammals that currently reside in the Galapagos Islands, and there have never been more than 14 native mammals on the islands. 

Because the Galapagos Islands were never part of mainland South or Central America, they are inhabited by a very limited number of invertebrate and insect species. Visitors will have the best luck spotting the different species of insects after a heavy rain or at night - ships' lights particularly catch the attention of the Islands' green hawkmoth and fringed notcuid. Butterflies are usually a popular insect and are the easiest to identify by their brightly colored wings. The small creatures that inhabit the islands range from the colorful large painted grasshopper to the peculiar giant long-horn beetle. 

Based on http://www.adventure-life.com/galapagos/articles/galapagos-wildlife