When I was just old enough to help in the garden, my mom explained to me that the difference between flowers and weeds is this: a weed is a flower that grows where it’s not wanted. I thought about this distinction almost daily when I visited Australia for the first time in 2011. The influence of introduced plants and animals that thrive at the expense of the continent’s native species could be seen everywhere. The fact that I was an invader myself was always on my mind.