I can't say if its "authoratative", as such questions confuse me (especially when I can't believe what's being said).
Here it is again: "Man is the only creature on earth that God has willed for its own sake."
However thinking over this quote, it seems to me that the question of the meaning of the earth somehow underlies it. Just as the earth quite literally underlies and encompasses the life of man. Significantly, although man can be called a "creature on the earth", the earth itself cannot be . . . when man begins to view other earthly creatures as valuable relative only to himself, does not the word creature cease to have its divine connection, and begin to mean those creatures that man has distinguished as such by his means/end dominion? -- thus first becoming separate and distinct "creatures on earth" rather than shadowy messengers springing up into the lightening glade from the earth's dark secrecy where man himself is molded, and his destiny waits.
All of this seems to be bound up in an over hasty reading of Genesis and the mission of mastery, which in Genesis is related to gardening and naming, now one could read gardening as piling up necessary stock for human use, whether nutritive, instructive, or aesthetic. But these very "goods" seem to lose their meaning in such an alienated context. Nutrition, building, pleasure -- their nature is bound to fostering the secret of the earth in all its appearing generations. Naming could be assigning utilitarian roles such as pollenator for the bee, or protein source for the beetle, but these are not names. We do not, cannot in fact, give names to things that do not in some way speak for themselves and thus grow within and into the name that is their own. That these "creatures on earth" have names -- "whatever he called each one, that was its name" -- by which they can be called indicates to me that they do have a sake of their own which calls them forth into being. I would argue that to have a name of your own that truly calls you, is what it means to have a sake of your own, to live and move for your name's sake, a name that calls you, gathers you toward the caller of all names, and holds you in His heart.
Ratzinger often quotes Holderlin's saying in "Introduction to Christianity" that the greatness of God is his inability to limited by the greatest things and yet his allowing himself to be encompassed within the smallest. Propping up the destiny of man by making contradistinction between him and other creatures seems like a total inversion of this truth and moreover does little more than defining the greatness of man by the very things that he supposed to be greater than, thus erecting a faulty and precarious structure that will come tumbling back down to the good earth that still waits quietly underlying it all, and giving it the lie.