Water and land pollution are the most significant problems in São Tomé and Príncipe. The purity of the nation's water supply is questionable due to the lack of adequate water treatment systems. The nation's forests are also threatened due to overuse and there is currently no regulatory policy to regulate their preservation. The nation's cities are threatened by inadequate sewage treatment. Soil erosion and soil exhaustion are other major environmental problems. Rare or threatened species include the São Tomé short-tail and São Tomé sunbird. São Tomé and Príncipe is a party to the Law of the Sea agreement and has signed but not ratified international agreements on biodiversity and desertification.