Since the first sailors visited the Galapagos Island visitors have introduced foreign organisms. These now pose the greatest threat to the islands flora and fauna. Scientists estimate that since 1950 over 400 alien plant species have been accidentally or (worse) deliberately introduced to the islands. The number of introduced species now rivals the number of native species. Introduced plants threatend native plants by outcompeting them and reduce their range. Quinine, introduced to Santa Cruz as an anti-malarial (even though malaria is unknown on the islands), is drastically altering habitat in the Miconia zone (see Vegetation zones). When foreign plants become established, the animals that depend on native plants are deprived of food or negatively affected in other ways. For example, lantana (Lantana camara) has colonized large areas of Floreana Island, formerly nesting habitat for the endangered dark-rumped petrel (Pterodroma phaeopygia). Since the birds cannot get through the vegetation to their burrows, their breeding numbers have dramatically dropped.