Complex sentence: The species has declined there owing to the accidental introduction of rats (a.k.a. longtail); the rats have, however, recently been removed from the Calf of Man allowing Shearwater numbers to increase.
Split into simpler sentences: The species has declined on the latter, owing to the introduction of rats (a.k.a. longtail). Rats have, however, been recently removed from the Calf of Man allowing Shearwater numbers to increase.