There is justification for concern over human impact on the environment.
To begin, frogs and crocodiles are getting hit, hard. As I recall, we may have driven one species of frog over the edge lready. Why is this significant? Because frogs and crocodiles are not supposed to go exticnt, ever.
At the end of the Cretaceous period, the dinosaurs, the pterosaurs, the mosasaurs the plesiosaurs, the pliosaurs, the ammonites, several lineages of toothed birds and a few other more obscure lineages went extinct in what in geological terms is the blink of an eye. The frogs and crocodiles, however, were next to unaffected (one of the reasons the asteroid hypothesis doesn't make much sense).
There are K strategists, like rhinos and elephants and whales, which take care of one youngster and focus a lot of energy into it. There's the foil stratergy, R strategists. These animals put energy into churning out lots of young so that at least one has a good chance of reaching adulthood. R strategists, like rats roaches and such are very much harder to kill off.
The dinosaurs, from what we know of their nests (quite a bit actually) and bone texture were R strategists par excellance. Even the largest sauropods laid huge clutches, as evidenced by well preserved nests of titanosaurs from Patagonia.
So, if what killed off the dinosaurs, which were adaptable and very fast reproducing beasts, was not able to kill of pond turtles, frogs and crocodiles, why are we?
The recent wave of pleistocene extinctions is a compicated event. In North America, there is evidence that the climatic changes that changed the flora had a drastic effect on the megafauna of the time. Also, several lineages went extinct several hundred thousand years before humans got to the americas, so they died on their own. In several other cases though, butchery sites, folsam spear points in mammoths and the like, it is appearant that people were hunting the large mammals very effectively indeed. Large mammals are almost always K stategists, and since the fossil record is only a small percentage of what actually went on, one can readily imagine that paleo indians driving a thousand Bison antiquis off of a cliff on a regular basis drove their numbers down beyond their reproductive capacity.
Elsewhere, however, the picture is far clearer. In Australia, New Zealand, New Calidonia, Madagasgar, Hawaii and several other islands in the carribean and mediterranean, the arrival of humans coincides with the extermination of all the large ground animals, especially flightless birds, and in many cases severe enivironmental changes. In Hawaii, the local avifauna is about 80% extinct and the local flora isn't doing a whole lot better. All of this is directly attributable to human action.
Humans, and the pets and parasites they take along with them unitentionally (Rats in Hawaii that eat the eggs of native birds) or intentionally (Mongeese in Hawaii that were supposed to eat the rats, but don't and eat the local birds instead) are incredibly destructive, especially to island habitats. We wiped the steller's sea cow out within twenty-seven years of it's descovery.
Extinction is a fact of life, but right now the amount going on is far beyond background levels, and it's happening in animals that generally are not affected by mass extinction events. While there are species whose demise we certainly had nothing to do with (e.g. phorusrhacids), there are others we clearly killed off, or other we pushed close to the brink and are now slowly on the way out.
The actions of paleo indians are doubtless blameless, they had to eat something, and I sincerely doubt they knew the recklessness of their actions. We, however, do know what impact we have, and are now at a crossroads. If we value biological diversity, which has furnished us with countless medicines, mechanical inspirations, and just plain beauty, then it is now that we can try to preserve what's left.
When the very first people came to Australia, the dogs they brought with them (unintentionally, the evidence seems to show IIRC) brought the thylacines to their knees, and eventually out of existence in Australia. Oops. In Tasmania however, where the humans were docile and remarkable in technological (they couldn't start fires), the thylacines lived on. If the settlers had been able to interpret the fossil record, the rock paintings of thylacines in Australia, and the fact that thylacines really didn't have that big of an impact on sheep, they could have easily kept the magnificent beast from being reduced into a dictionary entry.