Statement I: Increased diversity in ecosystems leads to significant variations in productivity from year to year.

Statement II:  David Tilman discovered that species diversity contributes to higher productivity in ecosystems.

Only statement II is correct.

The correct answer is Option (2) -Only statement II is correct.

For many decades, ecologists believed that communities with more species, generally, tend to be more stable than those with less 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 (natural or man-made), and it must also be resistant to invasions by alien species.  David Tilman found that plots with more species showed less year-to-year variation in total biomass. He also showed that in his experiments, increased diversity contributed to higher productivity.

Communities with greater species diversity tend to be more stable for several reasons. In ecosystems with a wide variety of species, there is a greater chance that some species can thrive when faced with environmental changes, disturbances (such as natural disasters or human impacts), or invasions by alien species. This resilience is due to the fact that different species play different roles in an ecosystem, and some may be better suited to adapt to new conditions, thus maintaining the overall balance and functioning of the community.

Additionally, in species-rich communities, if one species declines or is lost, other species may fill its ecological role, preventing large disruptions to the ecosystem's functioning. For example, if a specific pollinator species is lost, other pollinators may still be present to maintain plant reproduction. This functional redundancy helps prevent significant losses in productivity or ecosystem services, ensuring stability over time.

In contrast, ecosystems with fewer species lack this buffer, making them more vulnerable to disturbances or changes. If a key species is lost in a less diverse community, the ecosystem is more likely to experience a breakdown in its structure and function, leading to a less stable environment.