Q.Is it possible that productivity and diversity of a natural community remain constant over a time period of, say one hundred years?

Yes — it is possible, but only in a community that stays stable over that period, i.e. one that is not seriously disturbed and whose species diversity itself is not eroded.

Ecologists have long debated whether species-rich communities are more stable than species-poor ones. Long-term outdoor-plot experiments by the ecologist David Tilman provide strong evidence: plots with more species showed less year-to-year variation in total biomass, and increased diversity also contributed to higher productivity. In other words, a community with high species diversity tends to be more resistant or resilient to occasional natural or man-made disturbances (drought, disease outbreaks, minor fires), so its productivity does not swing wildly from year to year. …

Whether the number of species in a community really matters to how the ecosystem functions is a question ecologists have not been able to answer definitively. For many decades, ecologists believed that communities with more species generally tend to be more stable than those with fewer species. A stable community should not show too much variation in productivity from year to year; it must be either resistant or resilient to occasional disturbances, and it must also be resistant to invasion by alien species.

The clearest evidence for this comes from the American ecologist David Tilman's long-term ecosystem experiments using outdoor plots. Tilman found that plots with more species showed less year-to-year variation in total biomass — that is, their productivity stayed more constant over time. He also showed that, in his experiments, increased diversity contributed to higher productivity. This is directly relevant to the question: a natural community with a rich, largely intact species diversity is exactly the kind of community whose productivity would be expected to stay roughly steady across decades, including over a period as long as a hundred years.

Anchor the answer to the chapter's own evidence on the diversity-stability question: Tilman's long-term plot experiments, which directly measured tha …