Complex sentence: This has seen a rapid depletion in the number of land crabs, which are vital to Christmas Island's biodiversity-land crabs are a keystone species in the forest ecology: they dig burrows, turn over the soil, and fertilise it with their droppings.
Split into simpler sentences: This has seen a rapid depletion of land crab numbers which are vital to Christmas Island's biodiversity. They are a keystone species in the forest ecology by digging burrows, turning over the soil, and fertilising it with their droppings.