They mention the naturalist Joseph Wood Krutch and how he believed that the fact that birds sing not just to warn of danger or attract a mate, but out of joy as well, is a sign of "an excess." That "joy itself is not necessary, useful, or productive to the workings of the natural world" (where the law of economy reigns) BUT -- and this is the kicker: "In a universe limited by the economy of the essential, joy is proof of a surplus."
The song of the bird, like the joy of a human, is not a passive acquiescence to what is, an acceptance of the conditions of life. It is an energy-infused celebration of that life, a recognition of its giftedness (Kindle Version Loc 601).I think it's telling to then refer to 2 Nephi 2:27 "...men are that they might have joy." The law of economy operates but isn't the goal. Ultimately we are meant for "surplus."
Terryl and Fiona Givens go on to explain that our sense of taste is more "refined" that survival necessitates and the sensitively of our ability to differentiate different smells also seems to exceed what a law of economy would dictate. Therefor, "If we are made in God's image, we can see His joyful nature reflected in the arsenal of access He gave us, to a variegated world of color and sound and texture and taste and smell" (Kindle Version Loc 609). The beauty of the modern world does not exist solely for itself, but to accomplish its ends as well as provide joy for us. "Nature's purposes and God's purposes are not in competition but work in tandem." The law of economy still holds, but there rides alongside it a divine "generosity" (Kindle Version Loc 618).
God's purpose "is to enlarge the sphere of human joy, and we discover the marvelous truth that our joy is His joy. What greater motivation could there be for us to seek out and secure our own, our friends', our families' happiness, than to know it adds to His." (Kindle Version Loc 635).