What is a background rate of extinction and what is mass extinction?

Answer 1

The background rate of extinction is the number of extinctions that would be occurring naturally in the absence of human influence. Estimates range from one to ten species per year for the past 600 million years. It is difficult to estimate this rate, in part because the number of species in existence is not known. The background rate of extinctions establishes a baseline from which the severity of the current extinctions crisis can be measured. The current rate of extinction appears to be hundreds, or perhaps even thousands, of times higher than the background rate. It is difficult to be precise because most of the disappearing species today have never been identified by scientists.The background rate of extinction has been interrupted periodically in Earth's history by episodes of mass extinction, periods in which a large percentage of the existing species become extinct in a geologically short amount of time. Mass extinction episodes represent major collapses of biodiversity and ecosystems, and they lead to fundamental changes in the make-up and distribution of life on Earth. The species that are most likely to survive mass extinctions are widespread generalists such as cockroaches and weeds.There are five widely recognized major mass extinction episodes in the Earth's history, and many scientists believe that we have now entered the sixth. However, there is a fundamental difference. In the past, mass extinctions have been caused by climate change, extreme geological activity, huge meteors colliding with the Earth or other natural factors. These changes in the environment took tens of thousands or even millions of years to occur. The sixth great extinction episode has been precipitated by human activities, and it appears to be happening very quickly.


A mass extinction is a relatively sudden, global decrease in the diversity of life forms. Mass extinctions have occurred periodically throughout the existence of life on Earth. To be a mass extinction, the following must occur:

* Extinctions occur all over the world.
* A large number of species go extinct.
* Many types of species go extinct.
* The extinctions are clustered in a short amount of geological time (a few million years is very short in terms of geological time).

The five largest mass extinctions in Earth's history occurred during:

* The late Ordovician period (about 438 million years ago) - 100 families extinct - more than half of the bryozoan and brachiopod species extinct.
* The late Devonian (about 360 mya) - 30% of animal families extinct.
* At the end of the Permian period (about 245 mya) - Trilobites go extinct. 50% of all animal families, 95% of all marine species, and many trees die out.
* The late Triassic (208 mya) - 35% of all animal families die out. Most early dinosaur families went extinct, and most synapsids died out (except for the mammals).
* At the Cretaceous-Tertiary (K-T) boundary (about 65 mya) - about half of all life forms died out, including the dinosaurs , pterosaurs, plesiosaurs, mosasaurs, ammonites, many families of fishes, clams, snails, sponges, sea urchins and many others.

Many minor extinctions have occurred through Earth's history.

Answer 2

Background Extinction

Answer 3

1

Answer 4

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What is a background rate of extinction and what is mass extinction?