However, non-natives shouldn’t all be lumped together as representing a major extinction threat to native species, the ecologists wrote.

“Indeed, recent analyses suggest that invaders do not represent a major extinction threat to most species in most environments—predators and pathogens on islands and in lakes being the main exception. In fact, the introduction of non-native species has almost always increased the number of species in a region.”

A point Carroll emphasized in the essay is that while ”the effects of non-native species may vary with time, and species that are not causing harm now might do so in the future...the same is true of natives, particularly in rapidly changing environments.”

They also ask if the costs are worth it, such as the efforts to eradicate tamarisk shrubs (Tamarix spp), introduced from Eurasia and Africa into the United States in the 19th century. The U.S. Congress authorized $80 million for tamarisk control and eradication between 2005 and 2009 alone. Studies show that tamarisks are not the water thieves initially thought to be (they use water “at a rate comparable to that of their native counterparts”). Also,  it’s the preferred nesting habitat of the endangered southwestern willow flycatcher(Empidonax traillii extimus).

“Policy and management decisions must take into account the positive effects of many invaders,” they declared, advocating that these decisions be based on “sound empirical evidence and not on unfounded claims of harm caused by non-natives. Another valuable step would be for scientists and professionals in conservation to convey to the public that many alien species are useful.”

“We are not suggesting that conservationists abandon their efforts to mitigate serious problems caused by some introduced species, or that governments should stop trying to prevent potentially harmful species from entering their countries,” they clarified. “But we urge conservationists and land managers to organize priorities around whether species are producing benefits or harm to biodiversity, human health, ecological services and economies.”

“Nearly two centuries on from the introduction of the concept of nativeness, it is time for conservationists to focus much more on the functions of species, and much less on where they originated.”

Earlier this year Carroll published “Conciliation Biology: the Eco-Evolutionary Management of Permanently Invaded Biotic Systems,” in Evolutionary Applications. Eradication, Carroll wrote, may be "ill-advised when non-natives introduce beneficial functions.“

Carroll called for a “conciliatory approach to managing systems where novel organisms cannot or should not be eradicated. Conciliatory strategies incorporate benefits of non-natives to address many practical needs including slowing rates of resistance evolution, promoting evolution of indigenous biological control, cultivating replacement services and novel functions, and managing native–non-native coevolution.”

“Rather than signaling defeat, conciliation biology thus utilizes the predictive power of evolutionary theory to offer diverse and flexible pathways to more sustainable outcomes,” Carroll concluded.

Other California ecologists who co-authored the Nature essay: Geerat J. Vermeij, UC Davis Department of Geology; Katharine N. Suding, Department of Environmental Science, Policy, & Management, UC Berkeley; and Joseph Mascaro, Department of Global Ecology, Stanford University.