Planet of Dread by Murray Leinster

Humans have expanded to myriads of worlds throughout the galaxies but they have found that the only way for colonies to be self sustaining, was to reproduce the total ecology of their home world; the original Earth. This meant bringing the entire ecosystem, the good, the bad and the ugly. Viruses as well as grass, goats as well as stink bugs and allowing the whole mixture to ultimately produce an inhabitable world for humans. But what happens when this system is not properly supervised? Moran and the others in the space yacht Nadine find a world where strange things have been brewing for over a hundred years and may or may not survive an environment gone mad.