Biodiversity is specifically the number of different species. An area with large populations of few species is not biodiverse.
If the numbers of one species are affected, there are almost always knock-on consequences. A simple food chain is:
algae → zooplankton → sand eel → puffin → arctic skua
If the numbers of zooplankton are reduced by pollution, such as plastic waste, then more algae will grow and the population of other consumers will fall.
Ecosystems with higher biodiversity have fewer species that depend on just one other for food, shelter and maintaining their environment. With the example above, puffins could also eat molluscs and worms. Ecosystems with higher biodiversity are more stable as they can easily adjust to changes.
We are slowly realising that the future of our species on Earth depends on maintaining high biodiversity. There are a number of reasions for this: