Biodiversity is the total diversity of living systems and is composed of species, habitat diversity and genetic diversity. 

Watch this video (c. 5.5 min) from the Royal Society and David Attenborough about the importance of biodiversity.

Species diversity is the diversity of species in a given unit of area for a given period of time. It is a product of two variables, the number of species (richness) and their relative proportions (evenness).

Habitat diversity refers to the range of different habitats in a given area. 

It's important to understand the impact of habitat diversity on overall biodiversity as this will help understand the arguments about species vs habitat conservation and also help analyse biodiversity data.

There was an old exam question that gave biodiversity figures for different areas of the Amazon Rainforest. The flat Brazilian lowlands had much lower diversity than the mountainous regions in Venezuela and the question was "Why?". Habitat diversity of course! In mountainous areas, there are valleys and cliffs and zonation each with different microclimates and different habitats and niches. This habitat diversity provides more opportunities for competition, adaptation and evolution of different species (diversification).

Genetic diversity refers to the range of genetic material present in a population of a species.

Genetic diversity is very important when considering the conservation status of a population or species. The gene pool of species indicates how resilient a species may be to change. The greater the diversity of different versions of genes, the greater the chance that there will be a version of a gene that will allow a species to adapt to change.

Some species have very low levels of genetic diversity although their population size might be large. This is usually a genetic clue to the history of a species. Cheetahs are a well-known example of this. Their population crashed during the last ice age and although their population recovered (until the 20th century) their genetic diversity did not^[1]. Now with the increasing pressures of habitat loss, their populations have decreased and the species is classed as "vulnerable" during an IUCN Red List assessment of 2021^[2]. You can learn more about the conservation genetics of cheetahs in this article from Heredity by S. O'Brien et al. (2017) or this article by A. Schmidt-Künzel (2018). For a simpler overview try this National Geographic article.

Californian Sea Lions were hunted heavily and in 1965 their population was estimated to be 35,000 but today stands at about 180,000. Recently their genetic diversity has been found to be high and did not go through a population bottleneck event^[3]. They are listed as Least Concern in the Red List.^[4] 

Conversely, the Hawaiian Monk Seal was hunted to near extinction in the late 19th century. They have extremely low levels of genetic diversity. Their populations are still very low and decreasing^[5]. They are listed as Endangered in the Red List.^[6] You can read more about the conservation of the Hawaiian Monk seal in this article from NOAA.

When scientists are studying a population's risks and chances of survival they look at the genetic diversity in different populations. They may even use this to plan any re-introductions. The larger the genetic diversity, the better and this may be related to possible gene flow between populations (habitat corridors), the size of a population's range and the actual population size. The smaller the population, the more likely the population's gene pools are likely to suffer from random events such as genetic drift, population bottlenecks and inbreeding depression where rare deleterious (bad) alleles come together to cause genetic problems.

There is a nice question in the May 2007 P3 for Environmental Systems about how information about genetic diversity can help in decision-making during conservation programmes.

Watch this video (7 min) from Ecotasia about the importance of genetic diversity for conservation.

Resilience is the ability of a system to resist change and to return to an equilibrium despite inputs that could push the system away from that stable state. An ecosystem’s capacity to survive change may depend on its diversity and resilience. The components of diversity contribute to the resilience of ecological systems. High biodiversity leads to complexity in that system. High productivity, resulting from the combination of insolation (solar radiation) and precipitation (water availability) will contribute to the 

Complexity means that there are more connections in a food web and this leads to stability as consumers can switch to other food sources. High productivity from high solar radiation combined with high precipitation will enable high levels of habitat and niche diversity which leads to greater species diversity and more connections in the food web. Greater genetic diversity also provides more resilience and therefore stability when change happens in a system. More complex systems have more negative feedback loops which help return the system to the equilibrium and resist change. During succession, a newly formed pioneer community will have very simple communities with short food chains and limited resilience to change. A more mature climax community may have more complex communities with longer food chains and more species interactions and therefore greater resilience. Ancient climax communities in a harsh environment, however, may have simpler communities and lower resilience.

Human activity can reduce the resilience of a system by applying inputs (change) too quickly that do not allow the negative feedback loops time to act. Removing one or more species results in shorter food chains and can disturb the whole food web. By reducing the productivity and biodiversity of the system this also reduces the ability of the system to resist any change. Humans generally simplify ecosystems, shortening food chains, lowering species interactions and biodiversity, resulting in less complexity and less resilience so the ecosystems are less stable.

Human activity can increase the resilience of a system by applying inputs that trigger positive feedback that results in a trophic cascade such as in rewilding projects such as the reintroduction of wolves to Yellowstone National Park which increases system resilience and carbon stores in the park. Keystone species provide a bigger than expected impact on their habitats, e.g. the sea otters of the Pacific Northwest (USA). By protecting these species, the resilience of the whole ecosystem improves.

To conserve biodiversity, decision-makers, via scientists, need to know what exists where and how these species interact in their ecosystems. This can sound a little imperialistic and it's very important to recognise that indigenous and traditional knowledge keepers are some of the most effective actors in conservation. In fact, in the COP15 Convention on Biodiversity, indigenous people were finally recognised for this role.

Gorongosa National Park is a collaboration between the government of Mozambique and the Carr Foundation ^[7]. This project has been documented by the HHMI Biointeractive team^[8]. In this video (c. 8 min) we see how scientists, in collaboration with local experts document the biodiversity in the park using multiple different sampling techniques.

This video (c. 4 min) from the Threatened Species Recovery Hub explains how citizen science is contributing to the conservation of mammals in Australia.

This video (c. 2 min) shows how indigenous people are being empowered to protect ecosystems that document the biodiversity present.

Evolution

Throughout the Earth's history of 4.5 billion years, the evolution of life and all the biodiversity observed in life is due to evolutionary processes. The origin of the raw material of diversity is random in nature and natural selection results in the evolution of this diversity. This process is continuous.  Evolution is the cumulative change in the heritable characteristics of a population or species.

Biological Variation arises from random mutations to DNA, that is, genetic diversity. These changes to the DNA can be harmless, have no effect on the individual, have negative consequences, even being fatal to the development of an organism, or provide the organism with benefits in its evolution. So natural selection contributes to biodiversity over time.

Natural Selection

Evolution by natural selection can be summarised in these steps:

1. Variation exists within a species – all individuals are not the same. This variation exists as genetic variation, that is differences in the DNA sequence of the individuals and is therefore heritable. The genetic differences (variation) may mean that an individual or individuals are better adapted (fitter) to an environment or change in an environment than other individuals.

2. Overproduction: Populations of species generally produce more offspring than are required to replace the parents.

3. Competition exists for limited resources: As the ecosystem has a limited carrying capacity and populations normally remain stable, this excess production results in competition for resources. The "fitter" individuals have a competitive advantage. These individuals have adaptations to an environment and therefore are more likely to survive (survival of the fittest) and reproduce than less well-adapted individuals.

4. Differences in adaptation affect rates of survival and reproduction. The survivors' genes (and their genetic variation) are thus more likely to be passed onto the offspring and they are then more likely to survive due to their competitive advantage. The frequency of these advantageous genes will increase over time with each generation although this is dependent upon a complex set of associate factors.

Speciation

This process of natural selection and may result in the formation of a new species. This process is normally slow and takes place over many generations. Speciation is the generation of new species through evolution.

This is the basis of Charles Darwin’s theory of evolution. His observations of finches on the Galapagos Islands were an important cornerstone of the development of this theory. He observed 14 different species of finch on the islands, each with a morphological adaptation that allowed it to feed on a different food type – small seeds, large seeds, nectar etc. Darwin concluded that the finches had evolved from an initial single species that had migrated to the islands from the mainland. He was able to suggest this because the Galapagos are volcanic in origin and must have been colonised from other sources. This is an example of adaptive radiation.

Environmental change gives new challenges to species: those that are suited will survive, and those that are not suited will not survive. Isolation of populations can be caused by environmental changes forming barriers such as mountain formation, changes in rivers, sea level change, climatic change or plate movements. Speciation is the evolutionary process whereby populations of a single species separate and, through natural selection by environmental pressures, gradually evolve into distinct species. Two populations of the same species can become geographically (physically) isolated and then different environmental pressures may act upon the two populations giving rise to different selection pressures on the two populations.

In this short HHMI video (c. 2.5 min) we learn how geographical isolation can lead to reproductive isolation and thus speciation when the populations are no longer able to interbreed to yield fertile offspring, in this case in anoles in the Caribbean.

Evolution in Foxes

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Theoretically, ancestral foxes at one time were distributed widely but when a population migrated into the Arctic, different environmental pressures applied to them and thicker fur, shorter tails, legs, ears and nose and white colour would all have been favoured.

In desert areas, different selection pressures again apply and animals have evolved thinner coats, grey coats, large ears, big eyes, and longer legs and noses.  

From the ancestral species, several species, with different genotypes, have evolved through this geographical isolation. Geographical isolation and consequential speciation can lead to different species being produced that are unable to interbreed to yield fertile offspring. 

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Evolution of Ratites

Emu from Australia	Ostrich from South Africa
Rhea from Brazil	Kiwi from New Zealand

All these birds are in a group called the Ratites. Ratites all had a common ancestor that was present in Gondwanaland, the southern supercontinent. When Gondwanaland separated, the population of ratites that were present in Australia evolved into the Emu, in New Zealand -  there was the Moa (now extinct), Elephant Birds (now extinct) and the emblem of New Zealand, the Kiwi. In Papua New Guinea you will find the Cassowary, in Africa, there is the Ostrich and in South America, the Rhea. From one common ancestor, this divergence led to an increase in biodiversity and all these bird species.

Here is a video about speciation from HHMI. It is about 16 minutes. It features Peter and Rosemary Grant and gives an excellent insight into the Grant's work, scientific methods and how natural selection can work very quickly when conditions change.

When cells are duplicated for the growth or maintenance of an organism, the DNA in the cells also has to be duplicated. DNA (deoxyribonucleic acid) is the genetic material of life and has to be passed from one generation to the next, meaning it is the hereditary material carrying the instructions for that organism.

DNA contains the genetic code for life made up of the four nitrogenous (nitrogen-containing) bases, adenine, thymine, cytosine and guanine. The combination of these bases in codons of three, codes for the amino acids which build proteins. 

Mutations change the sequence of these bases. Mutations can happen by errors during the replication (copying) of DNA when new cells are made but they can also be induced by environmental mutagens (chemicals which cause mutations) or high energy wavelengths like ultraviolet radiation (this is why you should use sun cream, x-rays and MRI scans (this is why your doctor or dentist asks if you are pregnant or had other recent x-rays or scans), or even nuclear radiation.

DNA is stored in chromosomes in the nucleus of cells. Every eukaryotic organism (all organisms which have a clearly defined nucleus) has a pair of each of its chromosomes. Along the chromosomes are the loci (locations) of genes that code for different proteins that lead to the characteristics of that organism. The same genes are on the matching chromosomes (homologous chromosomes) but there can be differences in these genes (variants) and these are called alleles.

Mutations can lead to these different variants and therefore genetic diversity.

During cell replication, the chromosomes are duplicated and form sister chromatids. The sister chromatids will be separated during cell division into the two dividing cells so that there is an exact copy of the chromosome in each cell. 

If mutations happen during this process then the chromosomes may vary.

During sexual reproduction, at the cellular level, one of the pairs of chromosomes will come from the mother and one of the pairs from the father. This means that there may be different combinations of alleles in the offspring. This is a source of genetic variation.

Another source of genetic variation is that during the pairing process of these chromosomes in sexual reproduction (this is called meiosis), the sister chromatids crossover and form new combinations of genes.

Sexual reproduction, is, therefore, a very important source of genetic variation.

Watch this short video (c. 4 min) from Ted-Ed about where genes come from. It doesn't cover sexual reproduction but does give a good overview of the evolutionary perspective on genetic variation.

Try this video (12 min) from the California Academy of Sciences that provides an overview of the origin of genetic variation in species, does cover sexual reproduction and goes slightly further and explains gene flow and speciation.

Species richness is the number of species in a community and is a useful comparative measure. It is also a useful quick measure to estimate the number of samples needed in a diversity study.

Species diversity is a function of the number of species (their richness) and their evenness (the similarity of population sizes for each species).

Simpson's reciprocal index can be used to quantify species diversity in an ecosystem and allow the comparison of this ecosystem to others but also how it changes over time. There are other diversity indices.

Similar habitats can be compared using D; a lower value in one habitat may indicate human impact. Low values of D in the Arctic tundra, however, may represent stable and ancient sites. The value of D will be higher where there is greater richness (number of species) and evenness (similar abundance), with 1 being the lowest possible value.

Using this formula, the higher the result (D), the greater the species diversity. 

D is Simpson's reciprocal index

N is the total number of individuals of all species found

n is the number of individuals of each species

Sampling Strategies to compare diversity require comprehensive counts of the number of different species in a given area and the abundance of each species. It is usually not possible or efficient to count all species and all individuals in your ecosystem and so a sampling strategy is required. This usually involves random sampling within your ecosystem from larger quadrats or along transects, if you are interested in studying change between different zones.

These strategies can be used in terrestrial ecosystems or aquatic ecosystems.

In a meadow, you might compare two areas for their flowering plant diversity using random sampling with open 50cm² quadrats within a 10 m² quadrat. For insect diversity within the same 10 m² quadrat, you will likely use sweepnets and walk systematically backwards and forwards across the whole area. Ground living insects can be sampled using pitfall traps.

In a woodland, you could also look at flowering plant diversity as already mentioned but you could also make a 100 m² quadrat and count tree species diversity, counting every tree in the area.

In a stream, you could look at the macroinvertebrate diversity using the same method as the meadow and kick sampling within your small quadrats.

In a lake, you might look at plankton diversity at different depths or distances from the lake edge. This would require specialist equipment.

Bird diversity is usually done by mist-netting an area or staying in one area and making bird sightings or bird song recordings.

Small mammals can be sampled, with a license, using small mammal traps. Large mammals are sampled through sightings or scat (droppings) collections.

Speciation can occur allopatrically (by geographical isolation) or sympatrically (without geographical separation). There is also parapatric speciation (where speciation occurs along a hybridisation zone between geographically abutting areas). Sympatric speciation can occur for a variety of reasons including ecological or behavioural differences.

Allopatric Speciation of Bonobos from Chimpanzees

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Today Bonobos and Chimpanzees, Homo sapiens closest genetic relatives, are separated by the Congo River in West Central Africa. Bonobos occur to the south of the Congo River and Chimpanzees to the north.

Genetic sequencing of the DNA of both species, combined with submarine Congo river sediments and palaeotopography mapping suggest that the common ancestor of a chimpanzee and bonobo crossed the Congo basin during dry periods and then the bonobo evolved, geographically isolated from the chimpanzee.^{[9][10][11][12]} 

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Sympatric speciation in the Apple Maggot (Rhagoletis pomonella)

The apple maggot fly is originally from the east of the USA and bred in the fruit of hawthorn trees. Apples were imported to America in the 17th century by European colonisers. In 1864 these flies were first noted as pests of apple trees in the North East of America (the Hudson Valley).

The apple and hawthorn trees have fruit at slightly different times and this acts as a barrier to hybridisation between the flies infesting the apples and those in the hawthorn trees.

Morphologically the flies are almost identical but they can now be distinguished genetically through this 300 - 400 year period of sympatric speciation.^[13] The two flies are considered races and not yet species but this is speciation in process.^[14] In this article, you can learn more about the chromosomal changes that are leading to this sympatric speciation.

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Speciation in Caribbean Anoles

Watch this video (18 min) from HHMI Biointeractive to learn about how resource partitioning and behavioural changes have led to the evolution of the same body types multiple times on the Caribbean Islands.

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The Caribbean anoles are examples that illustrate the evolution of high rates of endemism on isolated islands. This means that as islands are colonised following their formation, or as they raft away from the mainland that they were originally attached to, there are lots of empty niches. Resources may be abundant and so it is selectively advantageous for populations that can exploit these empty niches. There are many examples of islands with many closely related species that have evolved to fill these empty niches.

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An extreme example is New Zealand.  When New Zealand broke away from Gondwanaland, there was no common ancestor of mammals in the land mass. This means that there are now no native land-living mammals in New Zealand, only bats which must have arrived more recently by air.

The niches that are held by mammals in other land masses are held by the diversification (adaptive radiation) of birds. This is also why there are so many ground-living birds (flightless) in New Zealand which makes them very vulnerable to imported mammals such as cats and rats.

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These birds are Hawaiian Honeycreepers and like Darwin's finches are a group of closely related species from one ancestral species that have evolved on Hawaii's islands. This is an example of adaptive radiation where one species adapts to become many different species and to fill new and empty niches.

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Author: Conservation International (conservation.org) 

Creative Commons Attribution-Share Alike 4.0 International

Thirty-six areas of the world have been identified as biodiversity hotspots. They comprise 2.5% of Earth’s land surface but account for 35% of the “ecosystem services” that vulnerable human populations depend on^[15]. These biodiversity hotspots are also deeply threatened. To qualify as a biodiversity hotspot, a region must meet two strict criteria^[16]:

• It must have at least 1,500 vascular plants as endemics.
• It must have 30% or less of its original natural vegetation.

Although tropical areas contain a lot of biodiversity, it is important to think about why certain areas have particularly high levels of endemism (species that are found nowhere else). Islands are often hotspots as isolation is a factor that leads to adaptive radiation and endemism. Other factors are often related to the geological history of an area and the high habitat diversity of an area; compare the Andean mountain regions to the Brazilian basin.

Humans can and are providing powerful selection pressures on species in the wild which are resulting in evolutionary change in these species. In several locations throughout Africa, poaching of Elephants for their tusks has led to natural selection for tuskless elephants. This video from HHMI Biointeractive (6 min) explains how this selection has applied to populations of elephants in heavily poached areas.

Climate change is providing some of the strongest selection pressures on species and scientists are concerned that the rapid change induced by the changing climate is too fast for the majority of species and that it will result in a mass extinction event (the 6th extinction). A paper in Nature, published in 2019, explained that there are two types of observations that might be made on species' response to climate change; morphological, geographical distributions, and phenological (changes in timings of events). They showed that, at least, bird species are responding phenologically, e.g. the timing of nesting, but urged caution in their findings as only a few common species were studied. The title of the paper tells their story, Adaptive responses of animals to climate change are most likely insufficient^[17].

Another paper published in 2021 in Trends in Ecology and Evolution by Ryding et al.^[18] and summarised in the NHM online news stories^[19] provides examples of species which are evolving morphologically but again the warning is that this can't keep on happening at this pace. There are trade-offs in these changes and eventually the cost of the change may become too great. An example is "Australian parrots which have seen up to a 10% increase in their bill surface area since 1871, with the amount of increase predicted from the average summer temperatures in the years before the specimens were collected"^[19]. In this Smithsonian article, there are ten examples of species adapting to climate change. These include table corals where populations growing in small pools with hotter temperatures can withstand bleaching better; Mediterranean thyme, which is producing more phenols as winter temperatures increase, Pink salmon which are migrating earlier, Tawny owls in Finland are becoming browner, Pitcher-Plant Mosquitoes are delaying hibernation, Sockeye Salmon are migrating earlier and Red Squirrels in the Yukon are birthing earlier with a correlation to bigger pine cones on white spruce trees.^[20] Another article from the Sierra Club provides more examples.

As climate change takes effect, the geographic range of species can change. A paper published in 2017 in PLOS ONE by Riley and Griffen looked at the changes associated with a Mangrove tree crab, Aratus pisonii moving into new, novel salt marsh habitats. They found that "consistent with traditional biogeographic concepts, size at maturity and mean body size of reproductive females increased with latitude within the native habitat, however, they decreased significantly in novel habitats at the highest latitudes of the species’ range, which was consistent with habitat-specific differences in both biotic (predation) and abiotic (temperature) selection pressures"^[21]. 

Artificial selection is the process by which humans select for particular characteristics in a species and increase its frequency through selective breeding. We can see an explanation of this process in this short video (2 min) from HHMI on dog breeding.

During artificial selection, however, there is a reduction in genetic diversity and this is becoming extreme in the breeding of industrialised food production. The risks of a species becoming vulnerable to one disease are increased during this process, reducing a species' resilience. Corn or Maize is an extreme example of this process. This TED-Ed talk (5 min) explains, briefly, how corn was domesticated in Southern Mexico and how pressures of industrialisation (the ability to move it easily by railroad (train)) have increased the narrowing of the types of corn grown so that 90% is one variety of corn.

To understand how corn has been domesticated from a variety of grass found in Southern Mexico, this longer video from HHMI Biointeractive (18 min) is an interesting watch. It explains how selection of just four or five genes can results in such a difference from the wild species and even suggests the origin of popcorn.

In this video from HHMI Biointeractive we zoom into the story of the genetic variation of maize. Scientists are search for this diversity, checking seeds and storing the variation in seed banks but they are also searching for the genetic variation associated with particular environmental pressures and growing varieties based on this research, for example, search for varieties with greater drought tolerance.

Today there is a growing awareness of the importance of saving this genetic diversity in our food crops. Watch any gardening programme or go to a farmers' market and you will see talk of heritage varieties, heirloom crops and, here in Switzerland, varieties labelled as Pro Specie Rara (link in German).

The Earth was formed around 4.6 billion years ago. Scientists have broken this time down into chunks of time with different scales. In order of scale, we have Eons, Eras, Periods and Epochs. These geological time units are based on the classification of layers of rock (stratigraphy) and are identified by the fossils and/or minerals found in them. Often the boundaries between these epochs are marked by mass extinctions that have led to the end of certain lineages of organisms the diversification of other groups of organisms and the evolution of new species. These are characterised by massive environmental changes caused by geological events such as meteorite hits on the Earth, changes in the atmosphere due to the evolution of life, massive volcanism and climate changes to Hothouse Earth or Snowball Earth. You can explore more about geological time with the Deep Time activity.

Watch this video from PBS (12 min) to understand how these geological timescales have been described and some of the important events that happened in particular epochs.

There have been five mass extinction events and these have been followed by rapid rates of speciation as new opportunities for resource partitioning and new niches become available. Extinction is the complete disappearance of a species and it is forever; once the genetic resource of that species is gone it cannot be replaced naturally. Extinction has always occurred and scientists can calculate the background rate of extinction. This can then be compared to current rates of extinction and this has led scientists to say that we are in a current 6th mass extinction event. 

This HHMI Biointeractive video (9 min) explains how a background extinction rate is calculated (for mammals) and compares this to the current rate of extinctions in mammals.

The fossil record shows one species becoming extinct every year (this is called the background rate of extinction)^[22]. During the mass extinctions, more than half (and up to 90%), of all existing species, disappeared from the earth in a relatively short period.

Mass Extinction Event 1

End of Ordovician c. 440 MYA about 85% of species were wiped out

Possible Cause: Massive glaciations^[23]; Snowball Earth^[24]

Type of Organisms Living: First Vertebrates in Sea

When Gondwana passed over the north pole in the Ordovician, global climatic cooling occurred to such a degree that there was global large-scale continental glaciation resulting in widespread glaciation.

This glaciation event also caused a lowering of sea level worldwide as large amounts of water became tied up in ice sheets^[25]. Carbon dioxide is removed from the atmosphere all the time by plants during the process of photosynthesis, but a far more efficient removal process is needed to plunge the earth into icehouse conditions.

Another common cause of cooling is ice ages, thought to be caused by astronomical forcing, related to the earth’s orbit. A snowball earth-type mechanism is currently blamed for the End Ordovician extinction^[24].

Mass Extinction Event 2

Late Devonian c. 365 MYA two waves of extinction, a million years apart; marine species were particularly hard hit  75%

Possible Causes: Global cooling

Type of Organism Living: First forests and land animals; first amphibians; the age of fish - the image is a coelocanth known as a living fossil as it still exists now.

Glaciations events on Gondwanaland. There is inconclusive evidence of meteor impacts^[26].

Mass Extinction Event 3

End of Permian, c. 251 MYA the largest mass extinction of all – 96% of all species

Possible Cause: Flood volcanism^[27] or comet activity^[23]. Greenhouse Earth^[28]

Type of Organism Living: First reptiles and insects; forests formed coal; expansion of reptiles

Global widespread cooling and/or worldwide lowering of sea level^[29]. Toxic gases emitted by giant salt lakes^[30].

Pnatholossa the giant world ocean may well have stopped circulating after the late Palaeozoic glaciations, the lack of ice at the poles would have meant there was no influx of cold water to drive the oceanic heat engine, and since there were no isolated basin seas there was no hypersaline water to drive a thermohaline circulation current. All that needed to occur then was the overturn of water to release huge volumes of carbon dioxide (Greenhouse Earth), which may have triggered the End Permian extinction^[28][27].

Mass Extinction Event 4

End of Triassic, c. 205 MYA, an estimated 76% of species were lost, mainly marine species

Possible Cause: Volcanic Action and Meteor Impact

Type of Organism Living: First mammals and early dinosaurs

Massive floods of lava erupting from the central Atlantic magmatic province^[23]. 

At least 2 craters have been found of about the right age, the first is in Western Australia, where scientists have discovered a 75 mile (120km) wide crater. Another has been found in Quebec Canada, surrounding the Manicouagan Reservoir, with an age of 210 million years old. This seems to add weight to the theory that mass extinctions may well also be in some way assisted by impacts^[31].

Mass Extinction Event 5

End of Cretaceous (K-T boundary), c. 65 MYA, probably 75-80% of all species went extinct. This is the most famous mass extinction because it signalled the end of the dinosaurs.

Cause: Meteor Impact

Type of Organism Living: Giant dinosaurs; first birds; end of dinosaurs; spread of flowering plants

The Chicxulub crater from a meteor impact on the coast of Mexico is widely accepted as evidence of a major impact that had some hand in causing death and destruction^[31]. An international panel of experts has strongly endorsed evidence that a space impact was behind the mass extinction event that killed off the dinosaurs^[32].

This HHMI Biointeractive video (16 min) explains how scientists are reconstructing what happened after the 5th extinction when the dinosaurs became extinct (the K-T extinction). This is the time when the mammals started to radiate, showing how the extinction event was followed by a rapid period of speciation due to the increased niche availability. If you want to learn more about the discovery of the K-T extinction cause then watch this video (30 min) from HHMI Biointeractive.

The epoch of the Anthropocene is likely to be announced in August 2024. In July 2023, Crawford Lake, Ontario, Canada was announced as the location for the golden spike which will mark the  Global Boundary Stratotype Section and Point (GSSP) that is required for the citation of a new Epoch. According to this article from Yale Environment 360, "a GSSP site needs to meet a long list of criteria. Most importantly, it needs to preserve indefinitely the most typical changes in the chemical composition of sediments and rocks, or in the organisms that have turned into fossils. Sites with a high risk of being washed away or disturbed by either animals or humans are not suitable. Also, a site must be adequately thick and accessible, so it can be examined by scientists."

The Anthropocene marks a geological epoch that is characterised by extreme environmental change brought about by human activity. Various candidates for the start of the Anthropocene have been considered including:

• The period 5-8000 years ago when human activity started converting forests to agricultural land
• The period in the 1600s when Western Europeans invaded the Americas and brought with them fatal diseases such as smallpox. The indigenous peoples of the Americas declined rapidly in numbers and this led to an increase in forest cover and a global dip in carbon dioxide levels as photosynthesis increased
• The period from the 1750s marked by the Industrial Revolution when the burning of fossil fuels led to the start of an increase in carbon dioxide

But scientists have narrowed down their focus to the 1950s when there was a spike in radioactive nucleotides and spherical fly ash from nuclear testing and an increase in mass consumerism, burning fossil fuels, the production of plastics, changes in the biotic composition marked in sediments and organic pollutants. This period is called The Great Acceleration. You can read a little more about this at the Geological Society.