**Explanation:**

The highest level of species richness is observed in tropical rainforests. There are several factors that contribute to this phenomenon:

**1. Climate:** Tropical rainforests are characterized by high annual rainfall and constant warm temperatures. These conditions provide a stable and favorable environment for a wide variety of species to thrive. The consistent rainfall ensures a constant supply of water, while the warm temperatures create a suitable habitat for many organisms.

**2. Biodiversity Hotspot:** Tropical rainforests are considered biodiversity hotspots, which means they have an exceptionally high number of species within a relatively small area. These hotspots are typically found in regions near the equator, where the combination of favorable climate and geographical factors promotes high levels of biodiversity.

**3. Habitat Complexity:** Tropical rainforests have a complex structure consisting of multiple layers, including the forest floor, understory, canopy, and emergent layer. Each layer provides different microhabitats and niches for a wide range of species to occupy. This diversity of habitats allows for the coexistence of numerous plant and animal species.

**4. Nutrient Cycling:** The high levels of rainfall and temperature in tropical rainforests create ideal conditions for nutrient cycling. The rapid decomposition of organic matter, coupled with efficient nutrient uptake by plants, results in a continuous supply of nutrients. This nutrient-rich environment supports the growth and survival of a diverse array of species.

**5. Evolutionary History:** Tropical rainforests have been relatively stable environments for millions of years, providing ample time for the evolution and diversification of species. The long-term stability of these ecosystems has allowed for the development of intricate ecological relationships and the coevolution of many species.

Overall, the combination of favorable climate, habitat complexity, nutrient cycling, and evolutionary history makes tropical rainforests the most species-rich ecosystems on Earth. The high species richness in these forests highlights the importance of conserving and protecting these unique and biodiverse habitats.