Diversity and ecosystem stability

Whether the number of species matters to how an ecosystem works is a question ecologists have not answered definitively. For decades they held that species-rich communities tend to be more stable.

A stable community does not vary too much in productivity from year to year, is resistant or resilient to occasional natural or man-made disturbances, and resists invasion by alien species.

David Tilman's long-term experiments on outdoor plots gave tentative answers: the richer a plot was in species, the less its total biomass varied from one year to the next.

In the same experiments, greater diversity also raised productivity.

Rich biodiversity is essential for ecosystem health and imperative for the survival of humans, even though exactly how richness helps is not fully understood.

Paul Ehrlich's rivet popper hypothesis: an airplane (the ecosystem) is held together by thousands of rivets (species). Passengers popping rivets one by one (extinctions) may not affect flight safety at first, but the plane grows dangerously weak as more go.

Which rivet goes also matters. Rivets on the wings stand for key species that drive major ecosystem functions; losing them is far more serious than losing a few from the seats or windows.