Which natural disaster actually increases biodiversity over the long term?

All natural disasters affect the ecosystem.  They affect lands, forests, and coasts and cause death to people, plants and animal species, the spread of invasive species, and loss of habitat. In the short term, they cause climate change but over time, there are some types of natural disasters that increase biodiversity in the long run. Examples of these are earthquakes, landslides, volcanic eruptions, and wildfires. They play an important role in rejuvenating the ecosystem that they once destroyed.

In this question, let us cite one specific example:

1.      Volcanic eruption.  The eruption has an immediate negative effect on surrounding, but through primary succession, the forest begins re-colonization almost immediately. Many plants, insects, and animal species arrive from adjacent places to take up residence. These life forms are adapted to survive in the severe conditions following volcanic eruption causing a new and more diverse forest ecosystem that will last a 150 year period.

the right option is; decreased genetic variation  

decreased genetic variation describes the effect of this environmental change.

genetic variation is the naturally occurring genetic diversities or differences between organisms of the same species in a population. genetic variation allows the survival and adaptability of a species when they encounter changing environmental conditions. when an endangered species reduces, it loses genetic variation which may not be restored even if the species rebound and will only be recovered through the gradual collection of mutations over many years.