Complex sentence: An explanation for the scarcity in Europe of this otherwise common species is the barrier effect of the Sahara which generates unfavourable winds, such as the Sirocco, whose dryness makes dragonfly passage almost impossible.
Split into simpler sentences: An explanation for the scarcity in Europe of this otherwise common species is the barrier effect of the Sahara. This makes unfavourable winds, such as the Sirocco, whose dryness makes dragonfly passage almost impossible.