State the contributions of the following scientists:
$(i)$ Alexander von Humboldt
$(ii)$ David Tilman
$(iii)$ Paul Ehrlich

(N/A) $(i)$ Alexander von Humboldt: He observed that within a region, species richness increased with increasing explored area, but only up to a limit. He gave the species-area relationship equation: $\log S = \log C + Z \log A$, where $S = \text{Species richness}$, $A = \text{Area}$, $Z = \text{Slope of the line (regression coefficient)}$, and $C = Y\text{-intercept}$.
$(ii)$ David Tilman: He performed long-term ecosystem experiments using outdoor plots and showed that plots with more species showed less year-to-year variation in total biomass. He also showed that increased diversity contributed to higher productivity.
$(iii)$ Paul Ehrlich: He proposed the 'Rivet Popper Hypothesis' using the analogy of an airplane to explain the importance of species diversity. He compared the ecosystem to an airplane and species to rivets. If every passenger pops a rivet (extinction of a species), the flight safety (ecosystem stability) is initially unaffected, but eventually, the plane becomes weak and crashes.