Over the long term, maintaining soil fertility may require nurturing, creating and sparing plant and microbial diversity. After all, biodiversity itself appears to control the elemental cycles—carbon, nitrogen, water—that allow the planet to support life. Only by acting in conjunction with one another, for example, can a set of grassland plant species maintain healthy levels of nitrogen in both soil and leaf. "As soil fertility increases, this directly boosts biomass production," just as in agriculture, Reich notes. "When we reduce diversity in the landscape—think of a cornfield or a pine plantation or a suburban lawn—we are failing to capitalize on the valuable natural services that biodiversity provides."
At least one of those services is largely unaffected, however, according to Hooper's study—decomposition. Which means the bacteria and fungi will still happily break down whatever plants are left after this sixth extinction. But thousands of unique species have already been lost, most unknown even to science—a rate that could halve the total number of species on the planet by 2100, according to entomologist E. O. Wilson of Harvard University. Ghosts of species past haunt ecosystems worldwide, which have already lost not just one or another type of grass or roundworm but also some of their strength at sustaining life as a whole.