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 if the numbers of sand eels decreased. Many food chains will be linked together to form a food web.
Ecosystems with higher biodiversity are more stable as they can easily adjust to changes.
A keystone species is one that plays a critical role within the ecosystem. Social insects are often thought of as keystone species because of the many contributions they make to the functioning of the ecosystem.
Examples of this include termites that decompose organic waste material, which releases many nutrients into the ecosystem and bees that play a vital role in pollinating many plant species. If these keystone species were removed the ecosystem could collapse.
The future of our species on Earth depends on maintaining high biodiversity. Activities that create air and water pollution, are reducing biodiversity in many ecosystems. Conservation of species and habitats by charities, governments and individuals helps to maintain the range of biodiversity.