Complex sentence: The long-term decline of the water vole resulted from a loss of habitat and changes in farming practices, but the acceleration in the rate of this decline in the 1980s was down to increasing predation by feral American mink.
Split into simpler sentences: The long-term decline of the water vole resulted from a loss of habitat and changes in farming practices. However, the acceleration in the rate of this decline in the 1980s was down to increasing predation by feral American mink.