Rats Gone, Seabirds Thrive on Anacapa Island

Anacapa Island, part of Channel Islands National Park, has seen a dramatic resurgence in its native seabird populations following the successful eradication of invasive black rats. These rats, likely introduced before 1940, had severely impacted local wildlife, particularly preying on the eggs and chicks of Scripps's murrelets. Around 40% of murrelet nests on the island showed signs of predation by these rodents.

The eradication efforts were conducted in stages, with East Anacapa treated in 2001 and the Middle and West islets in 2002. Subsequent studies published in the Journal of Wildlife Management revealed striking improvements. The number of occupied Scripps's murrelet nests surged from 11 in 2001 to 60 by 2014, representing a 4.45-fold increase. Additionally, nighttime counts of murrelets at sea and hatching success rates saw substantial gains.

Hatching success for Scripps's murrelets nearly tripled, rising from approximately 30% during the 2001-2002 baseline to about 85% between 2003 and 2010. This improvement was largely attributed to the sharp reduction in egg predation by rats, with native deer mice becoming the primary cause of remaining losses. The recovery also included a consistent annual increase in nesting activity, with previously vacant sites being occupied post-eradication.

The positive impacts extend to other seabird species as well. Cassin's auklets, which were absent as breeding birds for years, began returning to Anacapa shortly after the rats were removed. Confirmed nests were found as early as 2003, and by 2012, auklet nesting was detected in multiple shoreline areas, further demonstrating the island's recovering biodiversity.

This ecological restoration underscores the profound effect of removing introduced predators on island ecosystems. The National Park Service, in collaboration with Island Conservation and other partners, continues to monitor the island, observing ongoing increases in Scripps's murrelet nests and high hatching success. The project serves as a clear example of how targeted conservation efforts can successfully restore habitats and protect vulnerable species.