There's another argument: For aesthetics. It's nice to have variety, we like diverse environments more than the alternative, and we want to preserve the beauty of the natural world as much as possible.
Variety has a non-aesthetic value as well -- more complex ecosystems have greater resilience to shock (e.g. climate change or invasive species introduction).
A broader variety of species allows for the loss / decrease in a particular function to be more quickly replaced by a similar species. As opposed to a simpler, more limited ecological system, where there may only be a single species fulfilling a given role. A single species that might be highly vulnerable to a new toxin / predator / fungus / virus / different average temperature.
A broader variety of species allows for the loss / decrease in a particular function to be more quickly replaced by a similar species. As opposed to a simpler, more limited ecological system, where there may only be a single species fulfilling a given role. A single species that might be highly vulnerable to a new toxin / predator / fungus / virus / different average temperature.