Final answer:
Protected areas can become less effective over time due to factors such as their small size, encroachment, and the movement of biodiversity and threats across their boundaries.
Step-by-step explanation:
Protected areas can become less effective over time due to several factors. Firstly, if the protected areas are small in size, they may not be able to adequately protect the biodiversity present within them. Moreover, protected areas may face encroachment and illegal activities like deforestation and poaching that can degrade their effectiveness. Additionally, as biodiversity and threats to biodiversity move freely across protected area boundaries, expanding protected areas or targeting them with greater attention to biodiversity protection may be necessary to ensure the long-term persistence of species and ecosystems.