The main focus of the investigation described in the portfolio could be determined based on the context provided. However, you would need to read through the portfolio to identify which of the options aligns most closely with its contents. Among the given options, it seems like a typical investigation might focus on one of those areas, such as predator-prey relationships, pollution effects, decomposer roles, or resource availability impacts.
If you have specific details from the portfolio, I could help you analyze them to determine the most accurate answer. Otherwise, I recommend selecting the option that you believe corresponds best to the main theme of the portfolio.