The cause / causes for species richness of tropics is / are

Options

A. Tropics had more evolutionary time
B. They receive more solr energy, hence more productivity
C. Less seasonal variation
D. All

Answer

### Step-by-Step Solution:

1. **Understanding Species Richness**:
Species richness refers to the number of different species present in a particular region. The tropics are known for having a high level of biodiversity.

2. **Evaluating Factors Contributing to Species Richness**:
Several factors contribute to the high species richness in tropical regions. These include climate, evolutionary history, and ecological productivity.

3. **Climate Stability**:
One of the primary reasons for high species richness in the tropics is the relatively stable climate. The tropics experience less seasonal variation compared to temperate regions. This stability allows species to thrive without the stress of extreme seasonal changes.

4. **High Solar Energy Availability**:
The tropics receive a significant amount of solar energy throughout the year, which supports high levels of photosynthesis and, consequently, plant productivity. However, while solar energy is abundant, it is not the sole reason for species richness.

5. **Optimal Rainfall**:
Tropical regions typically have optimal rainfall, which supports diverse ecosystems. The availability of water is crucial for the survival of various species.

6. **Evolutionary Time**:
While evolutionary time can play a role in species richness, it is not the primary factor in the tropics. The long-term stability of the environment allows for the evolution of diverse species, but the immediate factors like climate and productivity are more influential.

7. **Conclusion**:
The main cause for the species richness of the tropics is the less seasonal variation in climate, which creates a stable environment conducive to a wide variety of species.

### Final Answer:
The primary cause for species richness in the tropics is the less seasonal variation in climate, leading to a stable environment that supports a diverse range of species.

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