May 4, 2010
Bald eagles have made a remarkable comeback. They were nearly wiped out in the mid-twentieth century due to DDT (the chemical causes female birds to lay eggs with thin shells), but following the 1970s ban on the chemical, the birds have recovered so well they were taken off the Endangered Species List in 2007. But that doesn’t mean that they have managed to return to their previous population numbers or to everywhere they once lived. Quite the contrary.
On California’s Channel Islands, researchers have attempted to re-introduce the eagles to the islands since 1980, with varied success. High levels of DDT remain in the marine ecosystem and though chicks have hatched, there have been abnormally high rates of egg failure. And it gets more complicated: a new study, published this week in PNAS, suggests that if the eagles manage to establish themselves in sufficient numbers, the consequences for other threatened and endangered species could be dire.
The favorite food for bald eagles is fish, either marine or fresh, but they are “opportunistic generalists” that will eat a wide range of creatures, either hunting and killing it themselves, scavenging carcasses or stealing from others. In the new study, the biologists analyzed chemical isotopes in bone collagen and feather keratin from a historic nest on San Miguel Island and also material from paleontological sites and other historical sites throughout the Channel Islands to figure out what the birds ate. The scientists determined that seabirds were important prey for the eagles for thousands of years, and after humans introduced sheep to the islands in the 1840s and 1850s, the eagles fed sheep meat to their chicks (I’m assuming they were scavenging sheep carcasses, but I could be wrong).
These findings have important implications for the reintroduction of the eagles to the Channel Islands. The sheep and feral pigs have been removed from the islands, and many seabirds have declined in numbers. The biologists suggest that without these species, the eagles may turn to pinnipeds, which are abundant in the area, or endangered island foxes.
The foxes are the bigger concern. Golden eagles were removed from the islands after they began to prey on the foxes, to prevent their extirpation, but what happens if bald eagles start to kill foxes? Would people support removing bald eagles? And that brings up a larger question: When looking at the natural world as a whole, how do we decide what to save?
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