By that time, 21 species of trees and all species of land birds became extinct through some combination of over-harvesting, over-hunting, rat predation, and climate change. The island was largely deforested, and it did not have any trees taller than . Loss of large trees meant that residents were no longer able to build seaworthy vessels, significantly diminishing their fishing abilities. One theory is that the trees were used as rollers to move the statues to their place of erection from the quarry at Rano Raraku. Deforestation also caused erosion which caused a sharp decline in agricultural production. This was exacerbated by the loss of land birds and the collapse in seabird populations as a source of food. By the 18th century, islanders were largely sustained by farming, with domestic chickens as the primary source of protein.