Monocultures and biodiversity

One of the many issues with invasive species is that they decrease the biodiversity. Biodiversity is the diversity and amount of different species in an ecosystem. It is very important to the ecology of an ecosystem because of all the interactions between species.⁸ 
Food webs are a good example of how species interact together, when one species is knocked out it affects both species who eat them and who are eaten by them. Other interactions act much in the same way with plants providing housing, insects pollinating, and roots holding nutrients in the soil. 

An example of biodiversity in a Washington forest

When invasive species invade many times they get room in the ecosystem by out competing and replacing native species.³ When one species gets eliminated from an area all of the species that have interactions with it suffer, damaging its chances for survival. This effect grows exponentially until the only thing that can survive is the invasive species, creating a mono-culture.¹¹

Apparent forest floor monoculture of English ivy

Monocultures are dangerous because with only one species the area is vulnerable to any sort of disturbance, disease, or other threats.⁹ Most species cannot survive without any other species supporting it and so building the area back up to the maximum diversity is very difficult.
Biodiversity has many benefits, including intrinsic values, monetary values, and undiscovered potential. One of the keys to preserving an ecosystem is keeping biodiversity.