Jaguars have been seen eating anacondas, the deadly water snakes that live in South America. The main threats to the jaguar is deforestation and poaching. Since jaguars prefer dense forests, removing the trees will cause them to be seen more out in the open, and when seen they are shot on sight. Ranchers also kill jaguars, fearing that they will kill their livestock. Ranchers allow their livestock to free-range, and become almost feral, so the jaguar takes livestock as prey often.
Humans also compete with the jaguar for available prey.
Fortunately, sport hunting jaguars for their skins has declined since the 1970's, so this is not as big of a threat to the jaguar. Wild Cats of the World. United Kingdom, 1998. Stillwater, Minnesota, 1993.