The term "myrmecophile" is used mainly for animals that associate with ants. An estimated 10,000 species of ants (Formicidae) are known, with a higher diversity in the tropics. In most terrestrial ecosystems, ants are ecologically and numerically dominant, being the main invertebrate predators. As a result, ants play a key role in controlling arthropod richness, abundance, and community structure. Some evidence shows that the evolution of myrmecophilous interactions has contributed to the abundance and ecological success of ants, by ensuring a dependable and energy-rich food supply, thus providing a competitive advantage for ants over other invertebrate predators. Most myrmecophilous associations are opportunistic, unspecialized, and facultative (meaning both species are capable of surviving without the interaction), though obligate mutualisms (those in which one or both species are dependent on the interaction for survival) have also been observed for many species.