Jared Diamond suggested that cannibalism took place on Easter Island after the construction of the moai contributed to environmental degradation when extreme deforestation destabilized an already precarious ecosystem. Archeological record shows that at the time of the initial settlement the island was home to many species of trees, including at least three species which grew up to or more: "Paschalococos" (possibly the largest palm trees in the world at the time), "Alphitonia zizyphoides", and "Elaeocarpus rarotongensis." At least six species of land birds were known to live on the island. A major factor that contributed to the extinction of multiple plant species was the introduction of the Polynesian rat. Studies by paleobotanists have shown rats can dramatically affect the reproduction of vegetation in an ecosystem. In the case of Rapa Nui, recovered plant seed shells showed markings of being gnawed on by rats. Barbara A. West wrote, "Sometime before the arrival of Europeans on Easter Island, the Rapanui experienced a tremendous upheaval in their social system brought about by a change in their island's ecology... By the time of European arrival in 1722, the island's population had dropped to 2,000–3,000 from a high of approximately 15,000 just a century earlier."