A study of marine mammals and other protected species finds that several once endangered species, including the iconic humpback whale, the northern elephant seal and green sea turtles, have recovered and are repopulating their former ranges. The research, published in the June edition of Trends in Ecology and Evolution, suggests that some species, including humpback whales, have reached population levels that may warrant removal from endangered species lists.

But returning species, which defy global patterns of biodiversity loss, create an urgent new challenge for policymakers and communities, the study suggests. While many people embrace the environmental and economic benefits of returning species - many of them large predators - others interpret the animals' recovery as a hostile invasion, encroaching on key fishing and recreation areas, researchers say.

"Most people support the idea of saving endangered species," says lead author Joe Roman of the University of Vermont's Gund Institute for Ecological Economics. "But when native species return, it can be a struggle for communities. After generations away, these forgotten species can suddenly be seen as newcomers - or even pests."

The return of North Atlantic gray seals has been blamed in Massachusetts for declining fishery yields and attracting sharks to Cape Cod. Some fishermen in Alaska and Washington State blame returning whales for reducing black cod and salmon stocks. In California, harbor seal pupping has resulted in temporary closures of public beaches.

"The takeaways here are that conservation clearly can work, which is important to celebrate given the trend of declining global biodiversity," says Roman, who is also a professor at UVM's Rubenstein School of Environment and Natural Resources. "But wildlife managers need to do a better job of planning for the return of these species to avoid future conflicts."