Why do we classify lionfish populations near the eastern United States as an invasive species? What suggests that these new populations are not an expansion of the lionfish’s natural range? View Available Hint(s) Why do we classify lionfish populations near the eastern United States as an invasive species? What suggests that these new populations are not an expansion of the lionfish’s natural range? Lionfish are native to the tropical Pacific. There are no natural water routes of warm water between the Pacific and Atlantic oceans.