What do you understand from the observations of David Tilman to species richness ?

Ecologists believe that communities with more species tend to be more stable than those
with less species. This was confirmed by David Tilman experimentally who showed that:
(i) Productivity should not vary too much from year to year.
(ii) It should be resistant to occasional natural and man-made disturbances.
(iii) It should be resistant to invasions by alien species.