Biodiversity Change through Geological Time
The number of species on the planet, or in any geographical area, is the result of an equilibrium of two evolutionary processes that are ongoing:
Both are natural “birth” and “death” processes of macroevolution. When speciation rates begin to outstrip extinction rates, the number of species will increase; likewise, the numbers will decrease when extinction rates begin to overtake speciation rates. Throughout Earth’s history, these two processes have fluctuated.
Paleontologists have identified five strata in the fossil record that appear to show sudden and dramatic (greater than half of all extant species disappearing from the fossil record) losses in biodiversity. These are called mass extinctions.
There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research. An argument can be made that the five mass extinctions are only the five most extreme events in a continuous series of large extinction events throughout the Phanerozoic (since 542 million years ago).
In most cases, the hypothesized causes are still controversial; however, the most recent event seems clear.
The fossil record of the mass extinction events was the basis
for defining periods of
They typically occur at the
transition point between
geological periods. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species. These transitions can be seen in the rock strata.
The Ordovician-Silurian extinction event is the first recorded mass extinction and the second largest.
During this period, about 85 percent of marine species (few species lived outside the oceans) became extinct. The main hypothesis for its cause is a period of glaciation and then warming. Although not as diverse as the other two, deep ocean ecosystems contain a wide variety of marine organisms. Such ecosystems exist even at the bottom of the ocean where light is unable to penetrate through the water.
The extinction event actually consists of two extinction events separated by about 1 million years.
The first event was caused by cooling, and the second event was due to the subsequent warming. The climate changes affected temperatures and sea levels. Some researchers have suggested that a gamma-ray burst, caused by a nearby supernova, is also a possible cause of the Ordovician-Silurian extinction.
A gamma- ray burst would have stripped away the Earth’s ozone layer causing intense ultraviolet radiation from the sun and may account for climate changes observed at the time.
The hypothesis is speculative, but extraterrestrial influences on Earth’s history are an active line of research. Recovery of biodiversity after the mass extinction took from 5 to 20 million years, depending on the location.
The late Devonian extinction may have occurred over a relatively long period of time.
It appears to have affected marine species and not the plants or animals inhabiting terrestrial habitats. The causes of this extinction are poorly understood. The extinction affected marine life including, trilobites and reef-building organisms. The causes of the extinctions are unclear but theories range from changes in sea level possibly triggered by global cooling or the impact of a comet or meteor.
The end-Permian extinction was the largest in the history of life. Indeed, an argument could be made that Earth nearly became devoid of life during this extinction event.
The planet looked very different before and after this event. Estimates are that 96 percent of all marine species and 70 percent of all terrestrial species were lost.
It was at this time, that the trilobites, a group that survived the Ordovician-Silurian extinction, became extinct. The causes for this mass extinction are not clear, but the leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event.
The oceans became largely anoxic, suffocating marine life. Terrestrial tetrapod diversity took 30 million years to recover after the end-Permian extinction.
The causes of the Triassic-Jurassic extinction event are not clear and hypotheses of climate change, asteroid impact, and volcanic eruptions have been argued.
The extinction event occurred just before the breakup of the supercontinent Pangaea.
The causes of the end-Cretaceous extinction event are the ones that are best understood.
It was during this extinction event about 65 million years ago that the dinosaurs, the dominant vertebrate group for millions of years, disappeared from the planet (with the exception of a theropod clade that gave rise to birds). Indeed, every land animal that weighed more then 25 kg became extinct.
The hypothesis for the end-Cretaceous extinction event, first proposed in 1980, was a radical explanation based on a sharp spike in the levels of iridium.
Iridium rains down from space in meteors at a fairly constant rate but is otherwise absent on Earth’s surface. The sharp iridium spike was noticed at the rock stratum that marks the boundary between the Cretaceous and Paleogene periods. This boundary marked the disappearance of the dinosaurs in fossils as well as many other taxa.
The researchers who discovered the iridium spike interpreted it as a rapid influx of iridium from space to the atmosphere (in the form of a large asteroid) rather than a slowing in the deposition of sediments during that period.
It was a radical explanation, but the report of an appropriately aged and sized impact crater in 1991 made the hypothesis more believable.
Now an abundance of geological evidence supports the theory. Recovery times for biodiversity after the end-Cretaceous extinction are shorter, in geological time, than for the end-Permian extinction, on the order of 10 million years.
adaptive radiation rapid branching through speciation of a phylogenetic tree into many closely related species
biodiversity hotspot concept originated by Norman Myers to describe a geographical region with a large number of endemic species and a large percentage of degraded habitat
biodiversity variety of a biological system, typically conceived as the number of species, but also applying to genes, biochemistry, and ecosystems
bush meat wild-caught animal used as food (typically mammals, birds, and reptiles); usually referring to hunting in the tropics of sub-Saharan Africa, Asia, and the Americas
chemical diversity variety of metabolic compounds in an ecosystem
chytridiomycosis disease of amphibians caused by the fungus Batrachochytrium dendrobatidis; thought to be a major cause of the global amphibian decline
DNA barcoding molecular genetic method for identifying a unique genetic sequence to associate with a species
Log in to save your progress and obtain a certificate in Alison’s free Ecology Studies - Conservation Biology and Biodiversity online course
Sign up to save your progress and obtain a certificate in Alison’s free Ecology Studies - Conservation Biology and Biodiversity online course
Please enter you email address and we will mail you a link to reset your password.