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Approximately 40% are free-living, the remaining 60% are either endoparasite, ectoparasite, or a combination of both. The diversity of parasitic animal nematodes surpasses that of parasitic plant nematodes with a ratio of 3 to 1. Free-living species are distributed in extremely diverse landscapes (from freshwater to marine to terrestrial environment; from mountainous terrains to ocean depths; from polar regions to tropical zones), though many are found in soils and sediments. Many known parasitic species are highly host-specific, some are even beneficial to us as they infect and kill pests. Although 26,000+ species have been described, total diversity is estimated to be at 100,000 to 10 mil species.