New York Times: Two lavish, cautionary nature documentaries go head to head on Sunday in a fight for the worried viewer. Their messages of ecological doom are similar, but their methods couldn’t be more different. "Crash: A Tale of Two Species" on PBS, an installment of "Nature," is quiet, personal and specific, a filmmaker’s attempt to show how one tiny thread of the environment is unraveling. "Six Degrees Could Change the World," on the National Geographic Channel, is bombastic, superficial and …