What is the biggest challenge that the Galapagos Conservancy faces in its marine conservation initiatives?

"Since the discovery of Galapagos in 1535, humans have brought many alien species to the islands-some intentionally, including goats, pigs, cats, and both ornamental and food plants (vegetables and fruits), to name a few-while others, including rodents, insects, and weedy plants, have been carried to the islands accidentally."