A good attempt

The first thing I noticed was the amount of people mentioned in the credits at the beginning of the film, this appears to be a Spanish movie made with EU funding. It is a lot better than many US straight to DVD releases, with a good build up of tension in places. However, I didn't understand the alien sequence at the beginning of the film - it appears if the alien who has crashed after the collision with the fighter plane is killed by the evolved spider, so was this brought by the alien or was it already on the island? If the former, what is the relevance of the alien sequence?