Life tends to be richest at its borders. Here, on the international border between U.S. and Mexico, a host of nature's borders converge climate, community, land form and geography. Only 5% of the native landscape remains on the lower river and its nearby reaches, yet the diversity within these fragments adds up to an astonishing 1,200 types of plants, 700 species of vertebrates (including nearly 500 bird species) and 300 kinds of butterflies. You'll find 11 different biological communities, from the Chihuahuan thorn forest to tidal wetlands. Numbers alone cannot depict the true value. A rare ocelot merges with the shadowy brush. A pair of crested caracaras glides above the river. A Mexican bluewing butterfly flutters into view, while great kiskadees cry an insistent "kis-ka-dee, kis-ka-dee".