The clearance of the palms to make the settlements led to their extinction almost 350 years ago. The toromiro tree "(Sophora toromiro)" was prehistorically present on Easter Island, but is now extinct in the wild. However, the Royal Botanic Gardens, Kew and the Göteborg Botanical Garden are jointly leading a scientific program to reintroduce the toromiro to Easter Island. With the palm and the toromiro virtually gone, there was considerably less rainfall as a result of less condensation. After the island was used to feed thousands of sheep for almost a century, by the mid-1900s the island was mostly covered in grassland with "nga'atu" or bulrush ("Schoenoplectus californicus tatora") in the crater lakes of Rano Raraku and Rano Kau. The presence of these reeds, which are called "totora" in the Andes, was used to support the argument of a South American origin of the statue builders, but pollen analysis of lake sediments shows these reeds have grown on the island for over 30,000 years. Before the arrival of humans, Easter Island had vast seabird colonies containing probably over 30 resident species, perhaps the world's richest. Such colonies are no longer found on the main island. Fossil evidence indicates six species of land birds (two rails, two parrots, one owl, and one heron), all of which have become extinct. Five introduced species of land bird are known to have breeding populations (see List of birds of Easter Island).