Can you imagine a world without butterflies and pandas?  Soon, we might not have to. The number of monarch butterflies has dropped 97 percent since the mid-1990s. Only 1,600 pandas are left in the wild. Several other species are on the brink of extinction because of habitat loss, climate change, and other events caused by human activities.  But there’s hope. Thanks to the work of conservationists, species such as the lion-tailed macaque and the California condor have recovered. Sept. 4 is National Wildlife Day, so we celebrate five of the most innovative efforts to rescue the world’s imperiled animals.