What is the biggest reason for the decline in biodiversity?

The biggest reason for the decline of biodiversity is due to human activities such as land use changes, pollution and climate change. 

Habitat destruction due to the expansion of human population is further contributing to decline in biodiversity.

For more relevant articles refer to the links given below:

• UPSC Environment and Ecology Questions
• Paris Agreement (COP21)
• UPSC Environment and Ecology