A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the study. It gives a broad overview of the issue of animal populations. It does not use any pronouns or definite articles that need prior context. Why other lines are not chosen? Paragraph 2 starts with "Often this is due..." which uses a pronoun that refers back to something not yet mentioned. Paragraph 3 starts with "A healthy environment..." which assumes prior knowledge of the topic. Paragraph 4 starts with "This recent study..." which also uses a pronoun that refers back to the study, making it dependent on previous context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion. It summarizes the problems caused by the issues mentioned in the previous paragraphs.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it explains why animal populations are becoming sparse. This shows a cause and effect relationship.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 because it discusses the importance of a healthy environment. It gives a broader view after explaining the problems.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it shows the problems related to the balance of animal populations. It continues the topic of the study.