I once saw a cartoon, perhaps a Larson, with an older man in a white lab coat at a chalkboard explaining a long equation with many terms, pointing at a section of the (nonsense) equation. The caption read "And then a miracle occurs". It often seems that there are many white coated folks who expect miracles.

Ecosystems containing many different plant species are not only more productive, they are also better able to withstand and recover from climate extremes, pests and disease over long periods of time. . .

"This is exciting because it shows that biodiversity can be used to produce a sustainable supply of biomass for biofuels," Tilman says.

Gee, that's all there is to it? Fertility, moisture and all that don't matter?

Biodiversity of global ecosystems has decreased as global population has increased because diverse ecosystems such as forests and prairies have been cleared to make way for agricultural fields planted with monocultures, buildings and roads.

Tilman's research has shown that ecosystems containing many different plant species are more productive than those containing only one of those species. A return to biodiversity may prove to be the key to meeting energy needs for the growing number of people on the planet and for restoring global ecosystems.

"Diverse prairie grasslands are 240 percent more productive than grasslands with a single prairie species," Tilman says. "That's a huge advantage. Biomass from diverse prairies can be used to make biofuels without the need for annual tilling, fertilizers and pesticides, which require energy and pollute the environment. High diversity allows us to produce biofuels with low inputs, and this means that we can get more energy from an acre of land, year after year, with high certainty. Because they are perennials, you can plant prairie grass once and mow it for biomass every fall essentially forever."

That would be miraculous! On earth it doesn't work that way. When you harvest stuff and cart it away you in effect strip mine the soil. Whatever was removed must be replaced to be "sustainable". You can haul in manure, compost, junk hay or some other sort of organic matter from another field to replace the lost fertility, or you can spread fertilizers. The amounts you need depend on what was removed and what can be replaced naturally. For example rain has nitrates in it synthesized by lightning energy during storms. Legumes harbor symbiotic bacteria that can in effect suck gaseous nitrogen from the air and mineralize it for plants. But there's no help for phosphorous or potassium, and there's no help for calcium or other minerals. Deep roots can mine the subsoil for these minerals, but that's not sustainable, it just depletes the soil to a deeper level.

Do you believe in magic?

Besides. This stuff has been known for eons and eons. Total dry matter production for diverse swards is always higher than for monocultures. And if the various species have different growth habits - some deep rooted and others less so - they can make better use of available biotope space, each drawing from different soil levels to achieve production comparable to a larger area.

But that doesn't relieve the need for replenishment. If nothing is removed, or is circularly replaced, then things can continue for a very long time. Maybe grazers - everything from ruminants to insects - remove material from one place but they deposit it in another nearby. These gifts are reciprocal. Even passing birds make deposits, and eventually they will die and be eaten. Dust to dust.

Humans haul things off to remote locations and seldom return the materials after use. If they did return what was taken - even the carcasses of their defunct friends and relatives - that would be different, more natural and sustainable. Failing that they must pay their debts to the land in another currency. The more they take the more they owe. There's no free lunch.