Historically, the diversity and complexity of life on an island was determined largely by its size and distance from other landmasses; if animals couldn’t easily travel to and from an island, few new species would arrive and establish themselves.

But with the shipping trade now allowing species to stow away from one island to the next, commerce has overtaken geography as the key factor in cross-island speciation.

“Humans have switched the system to one that in the past was dominated by speciation but now is dominated by colonization,” said Matthew R. Helmus, an ecologist with the Amsterdam Global Change Institute and the lead author of the study, published in Nature. “That’s really a fundamental shift to the system.”