To solve this problem, we need to determine the global species diversity according to Robert May.$\newline$Robert May, a renowned ecologist, estimated the global species diversity to be around 7 million.$\newline$Let's analyze the options provided:$\newline \bullet$Option 1: 20 million$\newline \bullet$Option 2: 50 million$\newline \bullet$Option 3: 7 million$\newline \bullet$Option 4: 1.5 million$\newline$According to Robert May's estimation, the correct answer is 7 million.$\newline$Therefore, the correct option is Option 3.