Being the only big cat in the Western Hemisphere, they have little competition where prowling the ground for food, stalking prey such as peccary, deer, monkeys, turtles, and anything else which comes their way. Jaguars also evidently catch fish from streams. The jaguar and leopard are very similar in appearance, but there are many things that can help you distinguish between them.
Location is the first and most obvious distinction.
The Jaguar is native to the New World and the leopard is native to the Old World. In other words, if you see a big spotted cat in Brazil, it's a jaguar, whereas if you see something very similar in Zaire, it's a leopard. Beyond this, another way to tell them apart is subtle differences in their appearance. A jaguar has orange spots with the black rings fairly spread out, while a leopard's spots are much more condensed in the center with a more solid black ring. Lastly, if you are facing a big cat and still are not sure which species it is, you might want to consider its behavior.