Maximum price tag is for ___________(ecosystem service).

Explanation

**Step-by-Step Solution:**

1. **Understanding Ecosystem Services**: Ecosystem services are the benefits that humans receive from nature. These include services such as pollination, water purification, climate regulation, and nutrient cycling.

2. **Research on Valuing Ecosystem Services**: Robert Costanza and his colleagues conducted research to assign monetary values to these ecosystem services. They aimed to quantify the economic value of nature's life support systems.

3. **Estimation of Total Value**: According to their findings, the average price tag for all fundamental ecosystem services is approximately **US dollar 33 trillion per year**. This figure represents the total economic value of these services provided by ecosystems.

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