In 1994 biologists seeded patches of grassland in Cedar Creek, Minn. Some plots got as many as 16 species of grasses and other plants—and some as few as one. In the first few years plots with eight or more species fared about as well as those with fewer species, suggesting that a complex mix of species—what is known as biodiversity—didn't affect the amount of a plot's leaf, blade, stem and root (or biomass, as scientists call it). But when measured over a longer span—more than a decade—those plots with the most species produced the greatest abundance of plant life.

"Different species differ in how, when and where they acquire water, nutrients and carbon, and maintain them in the ecosystem. Thus, when many species grow together, they have a wider set of traits that allow them to gain the resources needed," explains ecologist Peter Reich of the University of Minnesota, who led this research to be published in Science on May 4. This result suggests "no level of diversity loss can occur without adverse effects on ecosystem functioning." That is the reverse of what numerous studies had previously found, largely because those studies only looked at short-term outcomes.

The planet as a whole is on the cusp of what some researchers have termed the sixth mass extinction event in the planet's history: the wiping out of plants, animals and all other forms of life due to human activity. The global impact of such biodiversity loss is detailed in a meta-analysis led by biologist David Hooper of Western Washington University. His team examined 192 studies that looked at species richness and its effect on ecosystems. "The primary drivers of biodiversity loss are, in rough order of impact to date: habitat loss, overharvesting, invasive species, pollution and climate change," Hooper explains. Perhaps unsurprisingly, "biodiversity loss in the 21st century could rank among the major drivers of ecosystem change," Hooper and his colleagues wrote in Nature on May 3. (Scientific American is part of Nature Publishing Group.)

Losing just 21 percent of the species in a given ecosystem can reduce the total amount of biomass in that ecosystem by as much as 10 percent—and that's likely to be a conservative estimate. And when more than 40 percent of an ecosystem's species disappear—whether plant, animal, insect, fungi or microbe—the effects can be as significant as those caused by a major drought. Nor does this analysis take into account how species extinction can both be driven by and act in concert with other changes—whether warmer average temperatures or nitrogen pollution. In the real world environmental and biological changes "are likely to be happening at the same time," Hooper admits. "This is a critical need for future research."

The major driver of human impacts on the rest of life on this planet—whether through clearing forests or dumping excess fertilizer on fields—is our need for food. Maintaining high biomass from farming ecosystems, which often emphasize monocultures (single species) while also preserving biodiversity—some species now appear only on farmland—has become a "key issue for sustainability," Hooper notes, "if we're going to grow food for nine billion people on the planet in the next 40 to 50 years."

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.