How would you define loss of biodiversity?
A reduction in the number of species present in an ecosystem.
Biodiversity is basically how much variety there is in an ecosystem. The variety generally refers to the number of species present. Ecosystems like tropical rainforests tend to be extremely biodiverse because of the sheer number of species present at each trophic level.
Let's take this rainforest example and say that a virus swept through a particular area and wiped out all populations of a bird species to extinction. The whole ecosystem is down one species. Even if this doesn't have a massive impact on the overall ecosystem, it's fair to say that it's a reduction in biodiversity.
In sum: every time an ecosystem experiences a reduction in the number of species that live in it, it's considered a loss of biodiversity.