Notes on The Sixth Extinction

It took until the late 1700s for scientists to discover and debate the concept of mass extinctions. Before that, the idea simply did not exist.
Even the concept of extinctions was novel. The concept of a species going extinct ties closely with the concepts of evolution — the creation of species. Europe’s religious history and reliance on Creationism needed to be overcome for evolution to hit the mainstream.
There have been five mass extinctions in the past, and we are currently in the midst of a sixth extinction, caused by humans in several ways: (1) overexploitation of resources; (2) transporting invasive species, fungi, and disease which kill local species; (3) climate change and ocean acidification; and (4) modification or destruction of natural habitats.

Megafauna were massive animals — sometimes big versions of species we see today.
These includes 10 foot kangaroos, giant land sloths, mastodons, mammoths, and more.
Humans likely drove them extinct slowly, over hundreds of years. Since megafauna reproduce slowly (modern elephants spend some ~22 months in pregnancy), humans only needed to kill them slightly faster than their reproduction rate. This would lead to a gradual population decline. Over several hundred years, this decline could kill off an entire population.
An important facet of this gradual-extinction theory is that it takes longer than a single human lifespan to kill the species. Even though a ~thousand years is just about nothing on the geological timescale, it is dozens of human generations long — long enough that a single human would ever see a meaningful decline in the megafauna population during their lifetime. During the span of an individual human life, the population decline of megafauna would be imperceptible, so they would not be able to realize that they were driving the species to extinction.
The fossil record backs up this claim, showing that within a few hundred years of homo sapien arrival in a new area, megafauna would disappear from the fossil record.

Our increasingly connected world makes it easy for invasive species, fungi, and disease to travel from place to place by piggybacking on our cargo ships and planes.
Similar to how diseases brought by Europeans killed hundreds of thousands of Native Americans, invasive species, fungi, and disease are moving to new areas and killing local species that aren’t adapted to them
Examples include Panamanian golden frogs and North American bats, both of which are on a rapid march to extinction

“Saving the rainforest” doesn’t do much if the environment is split up by cities, roads, etc. These obstacles can prevent members of a species from (1) moving out of a deteriorating environment to a better nearby area and (2) meeting and mating with other members of their species who are separated by a road or town. This breakup of ecological communities is referred to as “islands on dry land”, as each area is disconnected from others. Each island being too small to hold a stable population makes it difficult for endangered species to repopulate or relocate, and can drive them closer to extinction.
Mass media generally shows animals at the poles suffering due to loss of habitat, but the impact is felt everywhere, including near the equator, where habitat destruction is huge.

Our current age is called the Anthropocene
Once a species drops below a certain population count, extinction is all but inevitable. This is because the species’ tolerance to disasters/accidents (resulting in the inability to breed or the death of newborn members) has a significantly higher impact, and if such disasters occur in multiple successive generations, it can drive the species to extinction
All humans have some Neanderthal DNA; between 1-4%
“Latitudinal diversity gradient” or LDG: there is more biodiversity at the equator than the poles. This is likely because the area by the poles was covered by ice during the Ice Age more recently and thus has had less time to evolve

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