The Okomu River forms the western boundary. Rainfall is between 1,524 and 2,540 mm per year. Soils are acidic, nutrient-poor sandy loam. Vegetation is Guinea–Congo lowland rain forest, including areas of swamp-forest, high forest, secondary forest, and open scrub. Among the common trees are Kapok, Celtis zenkeri, Triplochiton scleroxylon, Antiaris africana, Pycnanthus angolensis and Alstonia congoensis. The park is probably the best example of mature secondary forest in southwest Nigeria. The park is accessible to tourists, and has well marked trails. There are two tree houses, one 140 feet high in a silk-cotton tree, from which visitors can view the park from above and observe bird life. visitors can stay at chalets built on stilts, just outside the park entrance, surrounded by fig trees that are often occupied by Mona monkeys. Guides are available for forest walks, and will point out such things as termite nests and the many medicinal plants. Fauna The park has diverse fauna, with 33 species of mammals including the African buffalo and the endangered African forest elephant. Elephant sightings are rare, although in 2007 a one-year-old elephant carcass was found, unlikely to have died from natural causes. Park officials claim that elephant poaching no longer occurs, despite the high prices commanded for ivory in Lagos. There is a population of the vulnerable white-throated guenon, a primate. Although no thorough study of the primate population has been done since 1982, chimpanzees were reported to be present in the region in 2009. The number of chimpanzees estimated to live in the Okomu Forest reserve was guessed to be 25–50 in 2003, and some may use the national park at times. Other animals found in the park include dwarf crocodiles, red river hog, sitatunga, warthog, civet cat, Maxwell's duiker, grass cutter, mona monkey, Thomas's galago and tree pangolin. About 150 species of birds have been identified. These include Angolan pitta, grey parrot, wrinkled hornbill, fish eagle, hawks, woodpeckers, great owl, grey hornbill, cattle egret, black-casqued hornbill, yellow-casqued hornbill, Sabine's spinetail, Cassin's spinetail, black spinetail, white-breasted negrofinch, chestnut-breasted negrofinch, pale-fronted negrofinch and yellow-throated cuckoo. Terrestrial molluscs seem exceptionally vulnerable to extinction, and low diversity may indicate subtle environmental problems. A survey of land molluscs in a small area of the forest found 46 species in 11 molluscan families, of which Streptaxidae snails accounted for over a third. This is much lower diversity than has been found in Cameroon and