Yaël: We often hear about how human activity and expansion can endanger wildlife      habitats. But can you think of any animals that might be threatened by the      disappearance of humans?

Don: It must be something that depends on humans for food or shelter. What about      rats?

Y: You're on the right track. I was thinking of house mice. Have you heard the story of      the extinct St. Kilda House Mouse?

D: No.

Y: St. Kilda is a small group of islands about one-hundred miles off the west coast of      Scotland. The isolated archipelago was inhabited by humans for more than two      thousand years, from the Bronze Age until 1930. In 1930 the few remaining residents      of St. Kilda were permanently evacuated because of sickness, crop failure and      casualties of World War I.

     Scientists believe that as early as 500 BC, Norse settlers arrived in St. Kilda and      brought along a few unwanted stowaways--European house mice. Isolated from      their mainland relatives, these house mice evolved over time into a distinct species,      larger and shaped differently from their ancestors.

     St. Kilda is also home to a unique subspecies of field mouse, that probably also      arrived as stowaways and evolved into a new species.

     But the St. Kilda house mouse needed the warm houses, farms and dropped food      crumbs of its human neighbors to survive. Within three years of the humans      evacuating, all the St. Kilda house mice had died off. In contrast, the field mice      survived and are still living on St. Kilda today.

D: Is that because they didn't depend on humans for food or shelter?

Y: Exactly.

D: Wow. What a fascinating example of evolution in action!  

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