Why does the Sixth Extinction continue?

'The invention of agriculture accelerated the pace of the Sixth Extinction. Phase two of the Sixth Extinction began around 10,000 years ago with the invention of agriculture-perhaps first in the Natufian culture of the Middle East. Agriculture appears to have been invented several different times in various different places, and has, in the intervening years, spread around the entire globe.
Agriculture represents the single most profound ecological change in the entire 3.5 billion-year history of life. With its invention:

• humans did not have to interact with other species for survival, and so could manipulate other species for their own use
• humans did not have to adhere to the ecosystem’s carrying capacity, and so could overpopulate

Humans do not live with nature but outside it.

Homo sapiens became the first species to stop living inside local ecosystems. All other species, including our ancestral hominid ancestors, all pre-agricultural humans, and remnant hunter-gatherer societies still extant exist as semi-isolated populations playing specific roles (i.e., have “niches”) in local ecosystems. This is not so with post-agricultural revolution humans, who in effect have stepped outside local ecosystems. Indeed, to develop agriculture is essentially to declare war on ecosystems - converting land to produce one or two food crops, with all other native plant species all now classified as unwanted “weeds” — and all but a few domesticated species of animals now considered as pests.
The total number of organisms within a species is limited by many factors-most crucial of which is the “carrying capacity” of the local ecosystem: given the energetic needs and energy-procuring adaptations of a given species, there are only so many squirrels, oak trees and hawks that can inhabit a given stretch of habitat. Agriculture had the effect of removing the natural local-ecosystem upper limit of the size of human populations. Though crops still fail regularly, and famine and disease still stalk the land, there is no doubt that agriculture in the main has had an enormous impact on human population size:

Earth can’t sustain the trend in human population growth. It is reaching its limit in carrying capacity.

• Estimates vary, but range between 1 and 10 million people on earth 10,000 years ago.
• There are now over 6 billion people.
• The numbers continue to increase logarithmically — so that there will be 8 billion by 2020.
• There is presumably an upper limit to the carrying capacity of humans on earth — of the numbers that agriculture can support — and that number is usually estimated at between 13-15 billion, though some people think the ultimate numbers might be much higher.

This explosion of human population, especially in the post-Industrial Revolution years of the past two centuries, coupled with the unequal distribution and consumption of wealth on the planet, is the underlying cause of the Sixth Extinction. There is a vicious cycle:

Overpopulation, invasive species, and overexploitation are fueling the extinction.

• More lands are cleared and more efficient production techniques (most recently engendered largely through genetic engineering) to feed the growing number of humans — and in response, the human population continues to expand.
• Higher fossil energy use is helping agriculture spread, further modifying the environment.
• Humans continue to fish (12 of the 13 major fisheries on the planet are now considered severely depleted) and harvest timber for building materials and just plain fuel, pollution, and soil erosion from agriculture creates dead zones in fisheries (as in the Gulf of Mexico)
• While the human Diaspora has meant the spread, as well, of alien species that more often than not thrive at the detriment of native species. For example, invasive species have contributed to 42% of all threatened and endangered species in the U.S.'

source: http://www.actionbioscience.org/newfrontiers/eldredge2.html