Extinction May Have Trickle-Down Effect WASHINGTON (AP) -- Conservationists concerned about the extinction of plants and animals may be overlooking the danger to thousands of other species that depend on the threatened ones.    A team of researchers led by Lian Pin Koh of the National University of Singapore studied some 12,200 plants and animals considered threatened or endangered, and calculated that an additional 6,300 dependent insects, mites, fungi and other species could be considered endangered...