Sure the story of the passenger pigeon has its appeal: There were billions of them, humans came along, then there were thousands, and then there were none, all in the space of just a few hundred years. The stories of mega-flocks blackening the skies and trampling the landscape are amazing. But wouldn't you rather see a mega-bird? How about one that is 6 feet tall and carnivorous? Or a bird that is 12-feet tall? What about the biggest raptor that ever lived?

for me, the interesting part of the passenger pigeon story isn't the bird... it's the the story behind it, which is relatively unexplored.... Why has particularly North America been host to so much imbalance in our ecosystems?  Passenger Pigeon is one example. So were the massive unsustainable Bison herds, and the first near-extinction of coyotes, and of pre-columbian horses, as well as the great insect swarms which infected the continent repeatedly during the 18th and 19th centuries.  Why does North America have such limited bio-diversity compared to other regions?  THAT's what's interesting to me... :-)

Australia and Europe have limited biodiversity, too.

The former because of biomes and the latter because of human population density.

Perhaps North America has some of each?

Causes, Oversimplified (must research!): 

Studies over the last two decades have demonstrated that more biologically diverse ecosystems are more productive. As a result, there has been growing concern that the very high rates of modern extinctions -- due to habitat loss, overharvesting and other human-caused environmental changes -- could reduce nature's ability to provide goods and services like food, clean water and a stable climate.

EFFECTS!! OMG!!

But until now, it's been unclear how biodiversity losses stack up against other human-caused environmental changes that affect ecosystem health and productivity.

"Some people have assumed that biodiversity effects are relatively minor compared to other environmental stressors," said biologist David Hooper of Western Washington University, the lead author of the Nature paper. "Our new results show that future loss of species has the potential to reduce plant production just as much as global warming and pollution."

In their study, Hooper and his colleagues used combined data from a large number of published studies to compare how various global environmental stressors affect two processes important in all ecosystems: plant growth and the decomposition of dead plants by bacteria and fungi. The new study involved the construction of a data base drawn from 192 peer-reviewed publications about experiments that manipulated species richness and examined the impact on ecosystem processes.

The global synthesis by Hooper and his colleagues found that in areas where local species loss this century falls within the lower range of projections (loss of 1 to 20 percent of plant species), negligible impacts on ecosystem plant growth will result, and changes in species richness will rank low relative to the impacts projected for other environmental changes.

In ecosystems where species losses fall within intermediate projections (21 to 40 percent of species), however, species loss is expected to reduce plant growth by 5 to 10 percent, an effect that is comparable in magnitude to the expected impacts of climate warming and increased ultraviolet radiation due to stratospheric ozone loss.

At higher levels of extinction (41 to 60 percent of species), the impacts of species loss ranked with those of many other major drivers of environmental change, such as ozone pollution, acid deposition on forests, and nutrient pollution.

"Within the range of expected species losses, we saw average declines in plant growth that were as large as changes seen in experiments simulating several other major environmental changes caused by humans," Hooper said. "I think several of us working on this study were surprised by the comparative strength of those effects."

http://www.sciencedaily.com/releases/2012/05/120502133106.htm

Plenty of great links through which to meander .....