The estimated total number of species present on Earth as calculated by Robert May is 7 million. Let's break down the explanation into headings and key points:

1. Robert May:
- Robert May is a prominent British scientist who has made significant contributions to the field of ecology and biodiversity.
- He is known for his work in quantifying and understanding the complexity of ecosystems.

2. Estimation of Total Species:
- Robert May conducted extensive research to estimate the total number of species present on Earth.
- He used various mathematical models and statistical techniques to arrive at his estimates.

3. Factors Affecting Estimation:
- Estimating the total number of species is a challenging task due to several factors:
- Many species have not yet been discovered or documented by scientists.
- Some species are very small, cryptic, or live in remote and inaccessible areas, making them difficult to study.
- The diversity of microorganisms, including bacteria and fungi, is still not well understood.

4. May's Estimate:
- After considering these factors, Robert May arrived at an estimated total of 7 million species on Earth.
- This estimation takes into account both known and unknown species.
- It is important to note that May's estimate is just an approximation and may vary depending on future discoveries and advancements in scientific techniques.

5. Significance of Species Diversity:
- The high number of species on Earth highlights the incredible biodiversity present on our planet.
- Each species plays a vital role in maintaining the balance of ecosystems and contributes to the overall health of the planet.
- Understanding and conserving this biodiversity is crucial for the well-being of humans and other organisms.

In conclusion, Robert May's estimation of the total number of species on Earth is 7 million. This approximation takes into account various factors and highlights the immense biodiversity that exists on our planet.

7 million !!!