Complex sentence: Research conducted in the 1990s suggests that by the end of that decade the area of limestone pavement had halved since pre-Famine times, c.1840, while from the 1970s to the 1990s alone, about 4% of pavements were converted into pasture or covered by roads.
Split into simpler sentences: Research conducted in the 1990s suggests that by the end of that decade the area of limestone pavement had halved since pre-Famine times, c.1840. From the 1970s to the 1990s about 4% of pavements were converted into pasture or covered by roads.