Paleontologists know that there are important moments in Earth's history when the rate of extinction is accelerating. For example, scientists have identified five large mass extinctions: five events in the last half a billion years, when more than three-quarters of the planet's species died out in a short time. Unfortunately, today we are witnessing another mass extinction, as the rate of extinction has increased dramatically over the past century.
Death is the inevitable end of life, says University of Kansas paleontologist Luke Strotz. And this is true for all species. According to various estimates, 99.99% of all species that have ever existed have become extinct. All species that exist today - including humans - will also inevitably die out at some not very beautiful moment.
But what factors make this or that species more or less vulnerable to extinction? Extinction rates vary between different groups of animals and over time, so not all species are equally susceptible to this. Scientists have done a great job documenting extinction, but identifying the processes that lead to extinction has proven to be much more difficult.
By examining contemporary examples, we find some obvious breaks in history that have led to the extinction of species. One of these factors is the decline in the number of the species. As the number of individuals in a species decreases, genetic diversity decreases, and the species becomes more susceptible to random catastrophic events. If the remaining population of a species is small enough, one wildfire or even random variations in sex ratio could eventually lead to extinction.
Why are species dying out?
Extinctions that have happened in the past are getting more attention - everyone is sad about dodo, thylacin, or the wandering pigeon. But the vast majority of extinctions happened long before humans appeared. Thus, the fossil record is the main source of extinction data.
When paleontologists view fossils in the context of what we know about the past state of the world, a clearer picture emerges of the beginning that leads to the extinction of the species. Today, the likelihood of extinction of the species is associated with several factors.
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We definitely know that temperature is one of the important elements. Nearly every major rise or fall in global temperatures in Earth's history has resulted in the extinction of various organisms.
The size of the geographic area occupied by a species is also important. Species that are widespread are less likely to disappear than those that occupy a small area or whose habitat is isolated.
There are also random events that lead to extinction. The meteorite that led to the extinction of 75% of life at the end of the Cretaceous, including flightless dinosaurs, is the best example of this. This random aspect of extinction results in the one who is fortunate enough to survive rather than the fittest.
Recently, paleobiologists have discovered a physiological component of extinction. It found that the typical metabolic rate for both fossil and living species of molluscs strongly predicts the likelihood of extinction. Metabolic rate is defined as the average rate of absorption and distribution of energy in individuals of a species. Shellfish with a higher metabolic rate are more prone to extinction than those with a lower one.
Returning to the metaphor of "survival of the fittest / most fortunate", we can assume that sometimes the laziest survives. Higher metabolic levels correlate with higher mortality in both mammals and fruit flies, so metabolism may represent an important control of mortality at different biological levels. Since metabolic rate is associated with a number of characteristics, including growth rate, maturation time, maximum lifespan and maximum population size, it seems likely that the nature of any or all of these traits plays a role in how vulnerable a species is to extinction.
And no matter how much scientists know about the drivers of extinction, there are also many unknowns. For example, some species are dying out regardless of any serious ecological or biological shocks. This is called the background rate of extinction. Since paleontologists pay more attention to mass extinctions, the background extinction rate is poorly determined. How much or how little this indicator varies is not well known. And in general, most of the extinctions probably fall into this category.
Another problem is determining how important changes in biological interactions are in explaining extinction. For example, extinction of a species can occur when competition increases or a predator proliferates, or when a species's critical prey disappears. The fossil record, however, rarely records this information.
Even the number of extinct species can be a mystery. We know very little about the current or past biological diversity of microorganisms such as bacteria or archaea, let alone any facts of extinction of these groups.
The biggest mistake we could make in evaluating and explaining extinction may have to do with an approach that tries to fit everything into the same box. The vulnerability of one particular species to extinction changes over time, and different biological groups respond differently to changes in the environment. While major changes in the global climate led to the extinction of some biological groups, the same events eventually led to the emergence of many new species.
So whether the vulnerability of one particular species is the result of human activities or climate change remains an open question.
It is clear that the current rate of extinction is well above anything that can be called the background level, and we are on the verge of a sixth mass extinction. Therefore, the question of the vulnerability of any particular species - including our own - needs to be answered quickly if we are to preserve future biodiversity.
Ilya Khel