What made Buck's first day at Dyea so terrifying in Call of the Wild?

Buck has been thrust in a world which is totally alien to the one he is used to, which is what makes his first day at Dyea so terrifying.  Having grown up on Judge Miller's place in California's Santa Clara Valley, he has lived an idyllic life, eminently civilized, where all he has had to do each day if he so desired was lie around in the sun.  When he arrives at Dyea after his kidnapping,

"every hour (is) filled with shock and surprise.  He (has) been suddenly jerked from the heart of civilization and flung into the heart of things primordial...here (is) neither peace, nor rest, nor a moment's safety.  All (is) confusion and action, and every moment life and limb (are) in peril".

Buck, whose only previous canine companions have essentially been house dogs, finds that there is now "imperative need to be constantly alert", for the dogs among whom he finds himself here in Dyea are "savages...who (know) no law but the law of club and fang".  Buck realizes this reality immediately and in a grizzly manner, when he witnesses the team attack Curly, one of their own, and rip her to pieces within minutes.  In this new world, there is "no fair play...once down, that (is) the end of you".

A second adjustment Buck must quickly make is to his treatment by the humans in his life.  Upon arrival at Dyea beach, he is unceremoniously fastened into "an arrangement of straps and buckles" and set to work like a draught animal.  His master is "stern, demanding instant obedience, and by virtue of his whip receiving instant obedience", and Buck instinctively understands that to rebel would be futile (Chapter 2).