Didn't know this thread was still going;
blames the decline on changes in farming practices that mean there's less food for birds in winter. Wow! A far cry from your implications
Which is why they are no longer seen in inner city parks? Even at locations, like St. James Park in London, where they were so used to being fed year-round, with a little stillness and a fistful of grain you could have a handful of sparrows?
As for examples of species loss, there are plenty as I believe you must know.
Tropical deforestation is leading to the loss of thousands of species, many of which were undiscovered. Well documented historical examples include the nearly complete clearing of Mauritius and Rodriguez islands resulting in the extinction of at least 12 bird species. The complete clearing of Cebu island in the Philippines resulted in the extinction of all 10 of its endemic birds. The clearing of St. Helena led to the extinction of 80 of the islands' more than 100 endemic plant species, most of its land snails and 3 of its 4 native land bird species. Many of the great apes are in serious trouble because of deforestation; the orangutan is expected to be extinct in the wild within the next 20 years, largely because of loss of habitat.
Deforestation may have been responsible for the mysterious collapse of Mayan civilization in Guatemala about 500 years before the arrival of Christopher Columbus in the West Indies. The Peten region had one of the densest human populations of any time in human history, with almost 2,600 people per square mile in the cities. Then over a period of about 100 years these people vanished. It is being suggested that this was an example of what happens when a human population exceeds the carrying capacity of its environment. The government of Guatemala is now seriously concerned that history is about to repeat itself.
The incredible rate of destruction of tropical forests was first brought to the attention of the public by a book called "The Sinking Ark" by Norman Myers published in 1979. At that time Myers estimated that of the original extent of about 6.2 million square miles of tropical moist forests only about 3.6 million square miles was left: about 44% of the original tropical moist forest on the earth had already been lost. Enormous amounts of tropical forest have been lost in Central America, Brazil, Africa, and Southeast Asia. Recommended book:
Trees of Life : Saving Tropical Forests and Their Biological Wealth (A World Resources Institute Guide to the Environment) by Kenton Miller, Laura Tangley, Gus Speth. Paperback - 218 pages (April 1991)
Obviously proving unknown species are lost due to deforestation is a bit like proving god doesn't exist.
As for your queries as to the extent of deforestation, I draw your attention to "about 44% of the original tropical moist forest on the earth had already been lost" in the above quote.
It would take many more years than 20,000 to "evolve"
Speciation can occur in less than 20,000 years, even in higher animals. I'm pretty sure a lot less variety of such life would exist if it couldn't.