An Ecuadorean oil tanker that ran aground about 550 yards off one of the Galapagos Islands began spilling oil on Friday, posing a major threat to the rare bird and marine life in the area. About 150,000 gallons of diesel fuel have already escaped from the 240,000-gallon tanker, creating an oil slick of at least 117 square miles, and experts fear the ship could break up and send more oil into the water at any moment. Ecuadorean Environment Minister Rodolfo Rendon described the environmental damage from the spill as “extremely grave.” The islands, famous for their tortoises and Charles Darwin’s work on evolution, are the country’s main tourist attraction. The World Wildlife Fund yesterday called for the limits to shipping traffic around the Galapagos in the future.