Tigers are generally at the top of the food chain and classified as apex predators, so they aren't afraid of any other animal. One exception to this is the case of tigers living in the range of particular bear species, as they sometimes are killed by bears when competing for food or disputing a kill.
Tigers are fierce predators who sometimes compete with brown bears and may clash in competition for available food sources. Tigers may kill both bear cubs and adults. In some ranges, tigers also routinely prey upon Asiatic black bears. The sloth bear is a particularly aggressive species that are fierce enough to sometimes scare tigers away from the food that they kill.
A big threat to tigers all over the globe is destruction of their habitats. At the start of the 20th century, there were an estimated 100,000 tigers worldwide. In 2015, the tiger population is estimated to be from between 1,500 to 3,500 individuals. An additional threat is the use of parts of the tiger in traditional Chinese medicine.
Although geographically limited, instances of tigers being killed by mugger crocodiles, pythons and leopards also threaten the tiger population. For the most part, tigers coexist with other top predators through strategies such as hunting at different times in order to avoid competition.