The poem imagines what became of the last mountain lion in Pennsylvania, though the title's "lion" suggests the big cat once hunted there. Instead of a dramatic death, the speaker gives us a disappearance that turns into a kind of afterlife. She is never found because the forest absorbs her: beetles carry her underground, trees take her up, her ears become leaves, and her roar lives on in the wind-struck tops of pines.
Then the mood shifts. With her predator gone, the deer have lost their wariness and multiplied, and they eat without restraint. The lion passes into them too, and through their eyes she sees "a growing emptiness, / a desert with trees." The beauty of the opening gives way to quiet unease. The speaker seems to feel both wonder at how nature recycles even a loss and sorrow that the forest, though still green, is being hollowed out. The ending trails off in ellipsis, leaving the damage unresolved.