A Day for Damnation

In the near future, humanity is decimated by a series of plagues, after which large areas of earth are taken over by a rapidly expanding alien ecology populated by organisms so voracious that terrestrial organisms stand no chance, humans included. A graduate student is enlisted by the US military to help in the struggle to understand the workings of the alien flora and fauna and destroy them, but begins to think that perhaps those two tasks are not irrevocably linked.