Earlier this year, the northern snakehead — that flesh-eating, land-crawling invasive fish — colonized a Maryland pond and an entire country’s imagination. But the snakehead is small fry compared to the larger problem of nonnative species in the U.S.: According to a report released yesterday by the National Wildlife Refuge Association, 8 million of the 94 million acres in the wildlife refuge system are infested with invasive plant and animal species. Exotic plants and animals can have a significant and difficult-to-reverse impact on native species, as well as other undesirable effects. For instance, the introduction of the zebra mussel into the U.S. led to five native species being listed as endangered and caused billions of dollars to be spent on unclogging and repairing of water pipes and filters in the Great Lakes region. The report urges Congress to mobilize 5,000 volunteers into 50 rapid-response strike teams to combat the unwelcome species.