Final answer:
A disease in the grassland ecosystem would likely decrease vegetation, lower net primary productivity, and increase soil erosion, corresponding to option A.
Step-by-step explanation:
If a disease were to impact the grassland ecosystem, it is likely that the vegetation in the grasslands will decrease, leading to a decline in the herbivore population that relies on it for food. With a decrease in vegetation, the ecosystem's net primary productivity would also decrease as there would be fewer plants to convert sunlight into usable energy for other organisms.
Additionally, with less plant cover to protect the soil and bind it together, soil erosion would likely increase due to wind and water. This is inferred from the reference that describes a scenario where vegetative cover is lost, leading to increased erosion and negative effects on ecosystems, such as coral reefs being buried by sediment discharge.
Therefore, the most accurate answer to how a disease would likely impact the grassland ecosystem is that the vegetation in the grasslands will decrease, and soil erosion will increase, which corresponds to option A.