Final answer:
Human activity, such as clearing land for agriculture, is the most likely cause of an ecosystem changing from grasslands to farmland.
Step-by-step explanation:
The most likely cause of an ecosystem changing from grasslands to farmland is humans. Human activity, such as clearing land for agriculture, can lead to the conversion of natural grasslands into farmland. This process often involves removing the native vegetation, leveling the terrain, and introducing crops or livestock.
Learn more about Human impact on ecosystems