High biodiversity improves an ecosystem by making it less susceptible to disaster and increasing plant reproduction rates. All the species in an ecosystem rely on one another in some way, and with less biodiversity, the ecosystem is more vulnerable to collapse.
As an example of the importance of high biodiversity, a disease once destroyed the majority of eastern North America's chestnut trees, but the ecosystems pulled through because other trees weren't affected by the disease. In an area with less biodiversity, such a disease could have resulted in massive habitat loss, leading to potential species displacement and extinction.
High genetic diversity, which is another type of biodiversity, affects ecosystems by improving the health of offspring. Species that are not genetically diverse are more prone to extinction as a result of susceptibility to disease and birth defects. A species is healthiest when several populations are thriving.
Extinction as a result of a lack of biodiversity can have a significant negative impact on an ecosystem, especially if that ecosystem already has low biodiversity. If there are no other species in that ecosystem that can fill the role of the species that went extinct, the entire ecosystem is in danger of collapse. Because the species in an ecosystem rely on one another, one extinction can easily lead to another.