When disease strikes an island species

Jennifer Viegas writes an interesting story about a new study that shows the extinction of Christmas Island rats was driven by black rat diseases:

Co-author Ross MacPhee, a curator of vertebrate zoology at the American Museum of Natural History in New York, N.Y., explained that Charles Andrews of the British Museum documented at the time that black rats were first brought to the island via the S.S. Hindustan in 1899. The ship-jumping black rats then carried a protozoan known as Trypanosoma lewisi. A related organism causes sleeping sickness in humans.
"Fleas are the intermediate host for one of the developmental stages of Trypanosoma, and the only likely method (of disease spread) is infected fleas crossing from black rats to endemic rats," MacPhee told Discovery News.
After the Hindustan's arrival, the native island rats were observed staggering around deathly ill on footpaths. Shortly thereafter, they were never seen again.

Credit must go to whoever wrote down their observation of “staggering” rats!

The study found trypanosomes in post-contact rats but not pre-contact ones, and so concluded that they triggered the extinction. This is fairly weak; not all the postcontact rats had the trypanosomes in their analysis, and they fall back on an argument about critical numbers for species survival, etc. Personally, I would fall back on competition with the black rats.

In that sense, there’s little difference between the Christmas Island rats and, say, English red squirrels – which are largely being annihilated by a disease brought by the American grays, but also face strong competition with the larger grays when their numbers decline.

The point is, it’s never good to compete with a species that brings their own pathogens along with them. Which humans learn repeatedly, both in history and in science fiction!