What Is the Definition of Mass Extinction

Although biologists are still debating the extent to which the current extinction rate exceeds the base rate, even the most conservative estimates show an exceptionally rapid loss of biodiversity typical of a mass extinction. More than 99% of all organisms that ever lived on Earth are extinct. As new species evolve to adapt to ever-changing ecological niches, older species disappear. But the extinction rate is far from constant. At least a handful of times in the last 500 million years, 75 to more than 90 percent of all species on Earth have disappeared in the blink of an eye geologically in what we call mass extinctions. Mass extinctions are defined as „any substantial increase in extinction (ancestry termination) that more than one geographically widespread higher taxon experiences during a relatively short geological interval, resulting in at least a temporary decline in its permanent diversity” (Sepkoski, 1986, p. 278). Even taking into account a conservative background rate of two extinctions per million species-years, the number of species that have become extinct in the last century would otherwise have taken between 800 and 10,000 years to disappear if only they had succumbed to the expected accidental extinction. This alone supports the idea that the Earth is experiencing at least many more extinctions than predicted by the background rate. At least since the Cambrian, which began about 540 million years ago, when the diversity of life first exploded in various forms, only five extinction events have definitively met these criteria for mass extinction. But these two processes are not always linked. If the loss of species quickly exceeds the formation of new species, this balance may be tilted enough to trigger so-called „mass extinctions.” Figure 6.27.

The great mass extinction during the Phanerozoic era shows the number of families as a measure of biodiversity. „The Cretaceous-Paleogene extinction is the most recent and probably the most studied mass extinction,” Katie adds. „We should understand the Cretaceous event fairly well, but many aspects, including introduction, cause and recovery, are still areas of active research.” While we generally view mass extinctions as historical events, many scientists argue that we are currently at the beginning of another mass extinction. In fact, based on the species we can measure and observe as they disappear, we can estimate the overall rate of extinction. This rate is much higher than in most periods of history. Moreover, this 6th mass extinction can be entirely caused by human action. If we use the same approach to estimate the current extinction per million years of species, we arrive at a rate that is between ten and 10,000 times higher than the background rate. Food systems at sea and on land have not collapsed above GRT. There has been no mass extinction of ocean plankton throughout GRT, so there has been no disruption of marine food chains either. Similarly, the absence of mass extinction of terrestrial plants makes it difficult to imagine a collapse of the metazoic trophic structure that depended on plants as primary food sources.

As more and more species fall victim to extinction, the food web is collapsing and needs to be rebuilt from the bottom up. Often, a change in the Earth like weather patterns will cause the extinction event. At other times, a species or group of species will alter the environment and lead to extinction. In the next section, we look at the major mass extinction events that have occurred in Earth`s past. This extinction event, one of the oldest mass extinctions, occurred nearly 450 million years ago. At that time, many forms of multicellular life roamed the ocean. Shortly before this extinction event, many changes took place. For example, land plants had emerged and probably changed the composition of the atmosphere. In doing so, they shifted the balance from a carbon-dioxide-rich atmosphere to an oxygen-rich atmosphere. Theoretically, this could have cooled the planet dramatically. As can be seen in the graph below, these extinction events underscore the fossil record.

The graph below shows the intensity of extinction over time, which is a gradual and constant process. Peaks represent significant extinction events. This background rate indicates how quickly species would disappear without human effort, and it is primarily measured by the fossil record to count how many species went extinct between mass extinctions. Mass extinctions are considered here: (1) biodiversity crises, which are mainly determined by significantly increased extinction rates, and (2) ecological (or biotic) crises, where the ecosystem consequences of biospheric disturbances were disproportionate to the prolonged/progressive loss of biodiversity alone. Only the mass extinctions of the late Permian and Final Cretaceous were clearly mass extinctions in the narrow sense. The global event at the end of the Ordovician was only a major biodiversity crisis, while the extinctions of the late Devonian and late Triassic were major ecological (or biotic) crises. In the causal context, the late Cretaceous catastrophe could have been caused by the impact of a giant meteorite, but it was probably only a final step that led to the collapse of the biosphere, previously influenced by the volcanism of the Deccan trap. Four other mass extinctions are more (Mesozoic) or less (Paleozoic) certainly associated with land-related destructive factors, with large magmatic provinces as the main proposed trigger. The volcanic/ice greenhouse scenario has been updated and is supported by recently discovered mercury anomalies.

A wide range of deadly factors related to volcanic cataclysms and amplified by non-volcanic factors worked in a completely different time scale.