Complex sentence: Cyrene's chief local export through much of its early history, the medicinal herb silphium, was pictured on most Cyrenian coins, until it was harvested to extinction, and commercial competition from Carthage and Alexandria reduced the city's trade.
Split into simpler sentences: Cyrene's chief local export through much of its early history was the medicinal herb silphium, which was pictured on most Cyrenian coins. Silphium was harvested to extinction, which, in conjunction with commercial competition from Carthage and Alexandria, resulted in a reduction in the city's trade.