(a) The broadly utilitarian argument says that biodiversity plays a major role in many ecosystem services that nature provides.
For instance,
(1) Oxygen: About 20 percent of the total oxygen is produced by Amazon Forest which is essential for our survival.
(2) Pollination: Without which plants cannot give us fruits or seeds.
(b) Ethical reason of conserving biodiversity: We should realise that every species has an intrinsic value and we have moral duty to care for their well-being.