Via language log, I came across an interesting interview with linguist David Harrison.
It turns out there are 7000 languages on the planet; one one dies off every fortnight.
Apparently it's down to the kids. Children are little barometers of what is cool/valued, and if they sense their community thinks more highly of one language than another, they opt for the 'better' one. (Total tangent: it reminded me of that UK study where children were shown a series of photos of kids and asked who would they like to be friends with. Everyone - irrespective of ethnicity - wanted to be friends with the pasty white kid. Little barometers indeed...)
One point that Harrison made was that most knowledge about the local environments and ecosystems of the planet are held within these very local languages. As the words die so does all of that accumulated wisdom.
Western science often misses out by not talking to the neighbourhood folk, going off on its own to 'discover' a species of plant or animal. I was talking with the Codebreakers recently: they lived in Madagascar in the 1960s, and the Life on Earth crew came to film there. Apparently David Attenborough made a big deal about catching a glimpse of one super rare something-or-other. The locals found it hysterical, as there were thousands of the creatures about a mile away.
No-one disabused him of his notions.