The most important cause for the loss of biodiversity is

The most important cause of the loss of biodiversity is Climate change.
Climate change is one of the main drivers of biodiversity loss, but destruction of ecosystems undermines nature's ability to regulate greenhouse gas (GHG) emissions and protect against extreme weather, thus accelerating climate change and increasing vulnerability to it.
Loss of biodiversity can lead to land degradation, effects on water supply and changes in farming productivity.