PHY - Extinction [LESSON]

Extinction

Types of Extinction

Species go extinct all the time. Scientists estimate that at least 99.9 percent of all species of plants and animals that ever lived are now extinct. So the demise of dinosaurs like T. rex and Triceratops some 65 million years ago wouldn't be especially noteworthy—except for the fact that around 50 percent of all plants and animals alive at the same time also died out in what scientists call a mass extinction.

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Time runs from left to right in millions of years ago. The y-axis is the percentage of marine animal genera (plural of genus) becoming extinct. The figure shows the fraction of genera that are present in each interval but do not exist in the following interval.

Mass extinctions—when at least half of all species die out in a relatively short time—have happened a handful of times over the course of our planet's history. The largest mass extinction event occurred around 250 million years ago when perhaps 95 percent of all species went extinct.

Earth’s fossil record shows that periodically, conditions change so rapidly that many species cannot adapt quickly enough. This results in many thousands of species going extinct almost at the same time. Evidence indicates that these periods of mass extinction are followed by several million years of rapid evolution. This is because the mass extinctions leave ecological niches suddenly available. Existing species might split up into different populations, each taking over one of the newly available niches. This is in opposition to background extinction.

Background extinction is an ongoing, gradual extinction of individual species due to environmental or ecological factors such as climate change, loss of habitat, disease, or competitive disadvantage in relation to other species. It has been occurring since the origin of life on Earth at a steady rate over time and is the result of normal evolutionary processes. If a species cannot succeed in adapting to its surroundings, it eventually becomes extinct. This type of slow and natural extinction does not include extinctions caused by human destruction of species or ecosystems.

Geologic Record

The geologic time scale is a timeline that uses certain divisions to represent evolutionary time. The divisions are marked with major changes found in the fossil record. Times in years were assigned to these divisions: the Archaean and the Proterozoic eons are the first two eons, estimated to have lasted about 4 billion years. The Phanerozoic eon has lasted the remaining half billion years. This eon is divided into three eras:

The Paleozoic, Mesozoic, and Cenozoic. Eras are further divided into periods, which range in length from tens of millions of years to less than 2 million years.

  • Paleozoic (ancient time) -- the vertebrates and invertebrates proliferated.
  • Mesozoic (middle time) -- the age of dinosaurs.
  • Cenozoic (recent time) -- the age of mammals.

Review the diagram of the geological record below.

geologic_time.jpg

Scientists currently have evidence for five mass extinctions throughout Earth’s history, and many scientists would argue that we are currently in the middle of our sixth. This sixth extinction, also known as the Holocene or Anthropocene extinction, is an ongoing event and is due to human impact on Earth’s systems. The rate of extinction is thought to be as high now as it was at the end of the Cretaceous Period 65 million years ago.

  • Ordovician-Silurian Extinction: Small marine organisms died out. (440 mya)
  • Devonian Extinction: Many tropical marine species went extinct. (365 mya)
  • Permian-Triassic Extinction: The largest mass extinction event in Earth's history affected a range of species, including many vertebrates. (250 mya)
  • Triassic-Jurassic Extinction: The extinction of other vertebrate species on land allowed dinosaurs to flourish. (210 mya)
  • Cretaceous-Tertiary (K-T) Extinction: (65.5 mya)

But the cause of this mass extinction is not a single catastrophic natural event or even a suite of a few catastrophic events. Instead, human activities have rapidly destroyed or degraded the resources that many species need to survive. Adaptation and speciation cannot occur at a rate high enough to keep pace with current extinction rates. When niches are left unfilled, whole ecosystems can collapse.

Try the Extinction activity below to check your knowledge.

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