The United States went through a roughly 100 year period or so where conservation efforts were... er, not terribly great. It took about 100 years for humans to shoot an incredibly large passenger pigeon population into extinction, and bison which once numbered millions on the plains was reduced to mere hundreds. Fortunately, in the mid 19th century to early 20th century in particular, people began to notice and started protecting areas. Thank goodness for that, but a large portion of ecosystems did get wiped out (to give one example, roughly 95% of the California redwood forests).
Megafauna do get most of the attention, but a lot of the hidden economic / scientific benefits of maintaining biodiversity I think comes from smaller parts of the ecosystem -- for instance, drug discovery. Or, the widely used polymerase chain reaction (PCR) technique for copying DNA... which for all we know is a technique that might not even exist today had preservationists not had the foresight to preserve the Lower Geyser Basin of Yellowstone National Park, where the crucial enzymes for this technique were found.