How does a decrease in biodiversity impact an ecosystem?

Biodiversity is essential to an ecosystem's resistance against viruses and famine. In history, there has been many cases in which a lack of biodiversity has allowed a virus or disease to endemically wipe out an entire species of plant. This will subsequently affect all consumers of each trophic level. Ex: A disease that attacks the DNA of only one certain species of plant would cause decreases in the population of the organisms that rely on eating that plant, the organisms that rely on eating those organisms, and so on. This is caused by a decrease in food source, thus not being able to sustain the ecosystem's demand, thus naturally killing off any organisms that would not be able to find a sufficient food source.

Its either a) or b),wind and water erosion gradually widen the canyon