Moisture from a morning rain hangs heavy in the air, clinging to spider webs, flower petals and the Spanish moss draped over the shady canopy of trees at Santa Ana National Wildlife Refuge. A tiny tree frog rests upon the wood railing along a trail, while plain chachalacas gobble and complain and green jays and malachite butterflies appear in bursts of tropical color through the deep green of the forest. The landscape masquerades as Costa Rica, but in reality this refuge lies in Texas' Lower Rio Grande Valley, just north of the border with Mexico. Here, since the 1940s, the national wildlife refuge system has sheltered a rare treasure trove of life. "The four most southern counties in Texas constitute one of the most biologically diverse regions in North America," says Nancy Brown, public outreach specialist at the South Texas Refuge Complex. "We have a documented 1,200 species of plants, 513 species of birds and nearly 300 butterfly species." Many of the creatures that dwell here exist nowhere else in the United States.