More than 5,500 species of birds, mammals, amphibians and reptiles are bought and sold on the worldwide animal market, a volume that is around 50 percent higher than earlier estimates, a study published in Science said Thursday. The legal and illegal trade of wildlife as pets or for animal products is a multi-billion-dollar industry, and recognized as one of the most severe threats to biodiversity. But the extent of the trade has remained poorly understood. The research by scientists at the University of Florida and University of Sheffield found that threatened and endangered species were disproportionately represented.