2. A biologist studied a nonnative plant species growing on a riverbed. She studied the plant over three years, measuring average growth, monitoring water quality, and observing the effect of this plant on other river organisms. The biologist concluded that the plant was invasive and that it was lowering the amount of oxygen in the river. She wants river authorities to remove the plant to keep it from spreading to other rivers and lakes. River authorities want proof that the explanation is valid. Which of these is the BEST way for the biologist to get validity for this study?