Quote: Alfred Wallace On Human Driven Extinction

He writes in The Malay Archipelago (1869):

It seems sad that on one hand such exquisite creatures should live out their lives and exhibit their charms only in these wild, inhospitable regions, doomed for ages yet to come to hopeless barbarism; while on the other hand, should civilized man ever reach these distant lands,  and bring moral, intellectual, and physical light into the recesses of these virgin forests, we may be sure that he will so disturb the nicely-balanced relations of organic and inorganic nature as to cause the disappearance, and finally extinction, of these very beings whose wonderful structure and beauty he alone is fitted to appreciate and enjoy. This consideration must surely tell us that all living things were not made for man. 

Via - The Song of the Dodo: Island Biogeography In An Age Of Extinction.

This was apparently a refutation of the argument made by the Duke of Argyll, that beauty in nature is evidence of God's handiwork. Wallace though was also clearly worried that European expansion and demand for natural resources would put these ecosystems at grave risk.

You can read The Malay Archipelago at Wallace Online.

Volcanism And The Demise Of Neanderthals

In addition to the many proposed reasons, something more to think about:

From Geology (early edition)-

Campanian Ignimbrite volcanism, climate, and the final decline of the Neanderthals - Benjamin A. Black, Ryan R. Neely, and Michael Manga

The eruption of the Campanian Ignimbrite at ca. 40 ka coincided with the final decline of Neanderthals in Europe. Environmental stress associated with the eruption of the Campanian Ignimbrite has been invoked as a potential driver for this extinction as well as broader upheaval in Paleolithic societies. To test the climatic importance of the Campanian eruption, we used a three-dimensional sectional aerosol model to simulate the global aerosol cloud after release of 50 Tg and 200 Tg SO2. We coupled aerosol properties to a comprehensive earth system model under last glacial conditions. We find that peak cooling and acid deposition lasted one to two years and that the most intense cooling sidestepped hominin population centers in Western Europe. We conclude that the environmental effects of the Campanian Ignimbrite eruption alone were insufficient to explain the ultimate demise of Neanderthals in Europe. Nonetheless, significant volcanic cooling during the years immediately following the eruption could have impacted the viability of already precarious populations and influenced many aspects of daily life for Neanderthals and anatomically modern humans.

Widely varying climatic conditions and resource availability may have hit Neanderthals more than "modern" humans. A number of reasons are given including the ability of "modern" humans to set up long distance networks facilitating exchange of technology and ideas.... Off course some would argue that the Neanderthals  never really became extinct. Their genetic legacy lives on in us. There is no doubt that interbreeding between the two human populations means that Neanderthal genes are with us today, but certainly a way of life, a particular morphology, social mores and perhaps a unique language (s) did disappear.

A Survivor From The Namib Desert

This beautiful passage from Robert Krulwich's essay on the plant Welwitschia mirabilis, a survivor, the last representative of its genus, isolated and endemic to the Namib desert in South Africa:

Welwitschia, when you finally get to see one, sits apart. It's very alone. All its relatives, its cousins, nieces, nephews have died away. It is the last remaining plant in its genus, the last in its family, the last in its order. "No other organism on earth can lay such a claim to being 'one of its kind,' " writes biologist Richard Fortey. It comes from a community of plants that thrived more than 200 million years ago. All of them slowly vanished, except for Welwitschia. It has survived by doing very little, very, very slowly — sipping little wafts of dew in the early mornings, otherwise minding its own business, as the big, busy world goes by.

With much recent attention given to the question of de-extinction i.e. bringing back extinct species, a more urgent focus needs to be on species and populations which are alive today and hanging by a thread. Genetics will play a role here too along with old fashioned conservation of the ecology in which these creatures persist despite all odds.