How do ecologists estimate the total number of species present in the world?

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks how ecologists estimate the total number of species present in the world. This involves understanding the methods used by ecologists for species estimation.

2. **Statistical Comparison**: Ecologists do not physically count every species in every part of the world. Instead, they use statistical methods to estimate species richness. This involves comparing data from well-studied groups of organisms.

3. **Example of Species Richness**: For instance, ecologists might study a specific group of insects in both temperate and tropical regions. They analyze the species richness (the number of different species) in these areas.

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