I read a book many, many years ago and cant recall the name of the book or author. One of the major points I remember was the planet ecosystem wasstrange. Small organisms would overrun the area then be eaten by larger ones, than those organisms eaten by still larger. Until finally the penultimate predator appears and for some reason the humans were trying to (unsuccessfully) communicate with these large predators. Some of the people ultimately being eaten. I cant recall they wanted to gain communication. For some reason the term "Ptorr" sticks in my mind. Any help would be appreciated.