For many Galapagos visitors, plants are of secondary interest. It's nice to eat fruit or enjoy the salads served up on tourist boats. Yet from a biological standpoint the Galapagos flora (comprised about 600 species), are as unique and special as the fauna. Remember that without the plants, most of the native land animals would not survive. The islands are as fragile a habitat for the plants as much as the animals. A dozen or so species are in danger of extinction (according to Michael Jackson in Galapagos: A Natural History). Since 1985, over 100 alien plant species have been introduced to the Galapagos, bringing the total of introduced species to around 200. In the absence of natural herbivores, many of these invaders are taking over, especially on the four inhabited islands. The flora of Galapagos Islands are recognized to be of global importance, declared by the IUCN to be a Center of Diversity. Around 170 species are endemic — found nowhere else in the world. Of these, 82 are threatened with extinction and two have already gone extinct. This is partly due to introduced plants and partly due to introduced animals, especially goats which are a major pest on some islands (see Animals > mammals). As you drive up a bumpy road on one of the larger islands, you progress through a series of distinct vegetation zones, each characterized by its own community of plants. These zones are one of the reasons for the variety of plant life found on the islands, a selection of which you can read about in the following pages. (Learn more about vegetation zones.)