Throughout the history of the Earth, there have been numerous mass extinctions in which large numbers of species have been completely wiped out. Each of these events has coincided with a change in the geologic evolution of the planet. These geologic changes have precipitated changes in the terrestrial environment, and therefore the type of life that could be supported during that period.
The key to understanding mass extinctions is to understand that they happen with a domino effect and can span periods of thousands, or even hundreds of thousands, of years. These periods may seem long, but they are very short in geologic terms. As the environment changes during these periods certain organisms such as plants and microbes, which serve as food sources for other larger organisms, begin to die off. With their food sources obliterated, the larger species begin to die off. Whatever feeds on those species may die off, as well. Some species of life, however, have been able to survive the changes. With nothing to feed on them, these species are able to grow stronger and even adapt to take advantage of the new environmental conditions. Essentially, nature has always been able to recover from these devastating events.
The exact cause of a mass extinction is not always clear. These global events are generally attributed to meteorite impacts, which are capable of raising massive dust clouds that block out the sun and prevent plants from growing. This is the case made for the Cretaceous extinction, which killed off the dinosaurs along with about half of all the other species on the planet. The meteorite-impact theory is supported by a huge, 65 million-year-old crater located in Mexico’s Yucatán Peninsula. However, it is possible that the impact occurred at the end of the extinction period and that its initial cause was global warming from volcanic activity. Volcanoes release large amounts of carbon dioxide and other gases that trap heat in the atmosphere.
The largest and worst extinction in history is the Permian-Triassic extinction, which occurred approximately 260 million years ago. This event marked the largest die-off ever, with over 90 percent of all species disappearing from the Earth. The cause of this extinction is thought to be a meteorite, but no crater has been found to support this theory. Another more likely cause of this extinction is a flood of volcanic activity from the Siberian Traps. The volcanoes would have released heat trapping gases into the atmosphere, causing global temperatures to rise and resulting in the deaths of terrestrial plants and runaway global warming. The increased carbon dioxide levels also would have resulted in higher ocean temperatures and increased acidity of water in equatorial regions.
An analysis of the fossilized shells of marine organisms from this period reveals a concentration of oxygen isotopes, which tend to form at warmer temperatures. Researchers estimate that oceanic temperatures near the equator exceeded 100 degrees Fahrenheit during this era. Increased ocean temperatures may have driven many species toward the poles, resulting in a “dead zone” in which survival would have been impossible. Flood volcanism is also a likely cause of the Triassic-Jurassic extinction, which occurred approximately 200 million years ago. In this event, warmer waters destroyed methane crystals, releasing large amounts of methane into the oceans. Methane is extremely toxic to marine life.
Global warming is not the only environmental factor that can result in a mass extinction. Any type of environmental change can precipitate such an event. An example of a mass extinction with a terrestrial cause other than global warming is the Ordovician-Silurian extinction. This event, which occurred approximately 440 million years ago, involved glaciation, in which most of the world’s water became locked up in ice as global temperatures dropped. Glaciation caused sea levels to decline dramatically and destroyed many of the marine species that existed during that time.
Mass extinctions can give rise to new types of life, but they can also have detrimental effects on evolution. An example of this would be the Devonian extinction, which occurred approximately 360 million years ago. This was about the time that the first proto-amphibians started to make their way onto land. The Devonian extinction mainly wiped out marine species; however, it did severely set back the evolution of land-based species, as well, reducing some species of plants and insects.
The Devonian extinction also completely destroyed reef ecosystems, with no new ones emerging for 100 million years. The causes of this extinction remain unclear, although it seems to be linked with global cooling. Possible causes for global cooling are the emergence of terrestrial forests and an increase in the weathering of silicate rocks, both of which would reduce the amount of carbon dioxide in the atmosphere.
Though there have been several extinctions in the natural history of the Earth, there are only five that can truly be called mass extinctions. Each has wiped out 50 to 90 percent of the species on the planet. Scientists now believe that there is a sixth mass extinction on the horizon. This time, the cause will almost certainly be humans. Activities such as poaching, overfishing, deforestation, introduction of non-indigenous species and environmental pollution are all contributing factors to the kind of environmental change that causes mass extinction.
In fact, at least 50 species have gone extinct in the last hundred years alone. It is a list that is continuously growing. Scientists estimate that at least half of the species alive today could be extinct by the year 2100 if nothing is done to stop the destruction. Worse yet, because humans are constantly altering the environment to suit their needs, nature might not be able to recover from another mass extinction event as it has in the past.
Extinctions are happening all the time, and there is probably no way to stop them from occurring. However, the ability of nature to recover can still be preserved. Fossil fuels like oil and natural gas could be forsaken for natural, renewable energy sources like solar power and wind energy. The use of non-biodegradable materials like Styrofoam could be eliminated in favor of reusable containers. Pollution could be reduced with the use of recyclable materials. Stricter limits on hunting and fishing, along with more severe penalties for breaching those limits, could be imposed. Companies could be presented with incentives to use more effective waste management programs. These are just a few examples of things that can improve the ability of nature to recover from a mass extinction. It is not a simple plan to put into action, but it is an effective one.
