How invasive species spread. Invasives move from garden to garden in many ways — when birds eat the fruit and drop the seeds, by lawnmowers and snowplows.

How invasive species spread. Invasives move from garden to garden in many ways — when birds eat the fruit and drop the seeds, by lawnmowers and snowplows.

From river-clogging plants to disease-carrying insects, the direct economic cost of invasive species worldwide has averaged about $35 billion a year for decades, researchers said Monday.