Picture Source - Wikipedia

Hi everyone,

In the last article, all of you (readers) have been introduced to the field of conservation biology, a discipline which examined the potential cause of why specific organisms started to decline and ways to treat it. If every single living things on the planet will eventually be replaced by a new species which would assume the same role in the ecosystem, then the course of extinction event is one of the most critical factors to be considered. Any losses of living organisms in an ecosystem would be replaced by other creatures which would assume the roles of an extinct species, but what if the rate of disappearance is quicker than what nature could replace?

Background Extinction Rate

If we would like to see some data which shows the effect of human activity on the disappearance rate of a particular species, we must analyse first, how quick a species being obliterated without human's activity. A study which is conducted in 1995 has concluded that assuming the progression of extinction is gradual; a species would be expected to live for one to ten million years before they were obliterated by the natural extinction process. This is not what happened with dinosaurs since they were demolished by multiple catastrophic events at the end of the Mesozoic era.

Assuming the progression of extinction is natural, then among one million species of living things, we would expect the extinction of approximately 0.1 to 1 species per year which is equivalent to the disappearance of 1 species in every one to 10 years. This would, of course, depends on the group of animal. I mean mammals will have a different extinction rate compared to marine mammals; so does others.

Let's take human lifespan as an example. The life expectancy of a human in 2017 was 71.5 years so let's make it 72 years. If people were expected to live for 72 years and there is no other factor which would affect the rate of mortality among these humans, then we can predict for each year, 1 out of 72 people would die. However, this is an ideal figure because we knew that, in actuality, the rate of mortality was low in infancy and elderly which means, most of the people die prematurely. So if the number of people dying was more than 1 out of 72 people, we would think, there must be something or any factors which would affect mortality such as epidemics, disease and fatal motor-vehicle accident.

Picture Source - Pxhere

Now let's used the "one disappearance per one million of species each year" as the benchmark value that would determine the normal rate of extinction of every existing species. In the last article, I've mentioned, there are 10,000 species of birds, currently which are increasing by one to two new species each and every year. Let's use the 10,000 as the relative figure to calculate the benchmark extinction rate for bird species. If we considered one out of one million species would be obliterated each year, then, it would take 100 years for one species of bird to disappear naturally. By using this concept, we can measure the relative value of extinction by comparing it with the past actual extinction rate.

Oh, by the way, the benchmark extinction rate was also known as the background extinction. My bad, I should have mentioned that first. This background extinction can be determined by examining various fossil records from different species. However, there is some risk of underestimation of the background rate of extinction. That's why this estimation should be supplemented with the knowledge of speciation rate.

Speciation can be defined as the formation of a distinct, new species in the line of evolution. Speciation would split a single evolutionary lineage into two or more genetically lineages.

Source - Encyclopaedia Britannica

The rate of speciation of species is important. If the rates were much lower than the rate of extinction, then there is a high probability for a species to be obliterated. I think we can use some analogy to illustrate the concept of speciation rate. Let's use human as an example. Species can be thought as siblings or family. In the family, let say, your father was around 70 years old, and the youngest member of your family is around 4 months, then the average age calculated would yield an age in between them; about 35 years give and take a few months/years. If we use mice, then the answer would be months, and if we consider some trees in the rainforest, it could be a millennium.

Just a while ago, I wrote, "species can be thought as siblings". If we considered ourselves living in an evolutionary tree, siblings are the closest individual who possessed an almost similar structure of DNA compared to the other. If we want to compare the similarity between the DNA of a human with the other creature, then our closest relative would be Pan paniscus or bonobo which has chimpanzees as one of their closest relatives. If we compare the differences between the structure of DNA of a chimpanzee, a human and a bonobo, then chimpanzee and a bonobo have a more significant similarity in DNA composition. The degree of differences would determine the duration of splits between two organisms from a common ancestor.

The bonobo and the chimpanzee appear to have split one million years ago, and humans split from the line containing the bonobo and chimpanzee about six million years ago.

Source - Encyclopaedia Britannica

The concept of estimation of durations and splits were known as the molecular clock. The fact that the molecular clock of bonobo and chimpanzee are one million years, it's a proof that the lifespan of a species without any human's activities involved, were on a million-year scales, at least. However, this figure is not accurate since we're only used one kind of species which could not represent the other, but you do get the idea, right?

The notion of using one extinction per million of species is not entirely practical, but it is a useful figure which could act as a benchmark to determine the appropriateness of the current rate of extinction with the ancient (where the effect of human's activity is negligible).

Recent Rates Of Extinction

A lot of information has been conveyed to know about the background extinction better. Now let's take a look a few examples to illustrate the way human affect the rate of extinction.

Birds Species In Pacific Island

Picture By Anthony Quintano - Flickr

2000 years ago, a few islands such as Hawaiian, Easter and New Zealand were inhabited by Polynesians. A few investigations and studies which are conducted a few decades ago have revealed some definite proofs which pointing towards the Polynesian as the primary suspect of causing thousands of bird species to go extinct.

Polynesians often hunt flightless bird species which are usually unwary of the danger or any implication that a human can inflict on them. Apart from that, the introduction of some pigs and rats brought by the ship from the mainland changes the ecological behaviour. The pig could have destroyed the forest's ground cover, and the rats could have preyed on the small bird's offspring or any bird's eggs that they could find.

Imagine, even without having any guns or sophisticated weapon for hunting activities, Polynesians were able to demolish up to 2000 bird species which make up for 17% of global species of birds. They could be killing whatever they found for food or just for amusement.

All of the discovery of potentially extinct bird species came from identifying the bird's bone which has been left out. As most of the birds have quite fragile and destructible bones, we can conclude that the number of extinct species might be higher than what we have found. Even though the other bird species which are yet to be discovered could never be found due to the loss of biological evidence (bones), the unknown figures can still be estimated.

There are a few ways to determine the relative total number of bird species on that island:

• Since 1800, there are a few naturalists who collected all of the Hawaiian birds. If we assume, that was, in fact, true and all of that specimen available today in the form of bones, then the documentation of bird's bones could be completed.

• The second ways involve the examination of modern bird species to determine which species could be explained by the bones. If, let say, only about a half of the total bird species can be identified by the bone at hand, then we know the list was actually half-completed. In Hawaiian island, they found 43 bones related to the extinct bird species, so the actual number of extinct species could be around 80-90 bird species.

In 1778, when James Cook, a British explorer found Hawaiian island along with the residing Polynesian, he introduced a few dairy farms animal to increase the island's food source. This particular animal destroyed most of the native plants available on the island which in turns killed some of the natural habitats for 18 bird species which became extinct due to this onslaught. The number of species of bird could be higher than expected which make the number of extinct bird species, at least 101 in Hawaiian island.

---

As you can see, the natural progression of extinction among living things in an ecosystem is a slow and steady process. Catastrophic events such as extreme climate changes and asteroid which caused the extinction of dinosaur 65 million years ago accelerated the extinction process making it unnatural. After the event, the earth took quite sometimes, to recondition in order to make it livable again. Of course, some of the living creatures survived that cataclysmic event but barely. Human activity could be catastrophic. Here are some other examples which can be used to illustrate the effect of human activity on the progression of extinction:

• Agriculture has caused 36 out of 8500 species of plants in Africa to become extinct for the past 1000 years. 618 from the remaining species are on the IUCN's Red List, marked "Endangered".

• Out of 297 species of mollusc (mussels and clams) residing in the St. Lawrence and Mississippi river, 21 are extinct, and 120 were critically endangered for the past 1000 years due to the construction of dams which change the direction of flow of the river. Chemical pesticides and sewage system altered the water quality which caused the mollusc to die.

• Due to extensive damming and water pollution, 40 out of 950 species of freshwater fish which resides in North America's river has become extinct for the past 1000 years.

---

That's all for this article. For next section, I would be writing about the future rates of extinction and factors that could accelerate the progress of extinction.

References and further reading materials

• Prüfer, K., Munch, K., Hellmann, I., Akagi, K., Miller, J. R., Walenz, B., … Pääbo, S. (2012). The bonobo genome compared with the chimpanzee and human genomes. Nature, 486(7404), 527–531.
• Stuart L. Pimm. Encyclopaedia Britannica. Conservation. Retrieved March 20, 2018, from https://www.britannica.com/science/conservation-ecology/Calculating-background-extinction-rates
• Wikipedia. Background extinction rate. Retrieved March 20, 2018, from https://en.wikipedia.org/wiki/Background_extinction_rate