What are the effects of biodiversity on ecosystem functioning?

(D) Ecologists are not entirely clear about the relationship between species richness and ecosystem functioning.
Historically,it was believed that communities with more species generally tend to be more stable than those with less species.
$(i)$ $A$ stable community should not show too much variation in productivity from year to year. (ii) It must be either resistant or resilient to occasional disturbances (natural or man-made). (iii) It must be resistant to invasions by alien species.
Experiments conducted by David Tilman showed that plots with more species showed less year-to-year variation in total biomass. Increased diversity contributed to higher productivity.
While we do not fully understand how species richness contributes to ecosystem health,it is certain that rich biodiversity is essential for the survival of the human race. The 'Rivet Popper Hypothesis' explains this: An ecosystem is like an airplane where thousands of rivets hold all parts together. If every passenger starts popping a rivet (species extinction),initially,it may not affect the flight safety (ecosystem functioning). However,as more rivets are removed,the plane becomes dangerously weak (ecosystem becomes unstable). Furthermore,which rivet is removed is also critical; if a 'key' rivet (keystone species) is removed,the ecosystem may face serious consequences.