It’s been called the world’s most successful weed species because it has grown so quickly in numbers and spread so far geographically. Everywhere this species has gone, it has taken over local ecosyst...It’s been called the world’s most successful weed species because it has grown so quickly in numbers and spread so far geographically. Everywhere this species has gone, it has taken over local ecosystems. Is the species in question a weedy plant like the dandelions pictured here? No; that dubious honor has been given to our own species, Homo sapiens.