There are two types of extinctions those that are at background levels and those that are characterised as mass extinctions. A mass extinction can be defined as a relatively rapid extinction of a geographically widespread and diverse group of organism. In the history of earth there have been about five major extinction events in the invertebrate record. These are known as the big five. They include the end Ordovician, late Devonian, end Permian, end Triassic and end Cretaceous. The end Permian was probably the most devastating extinctions, where 80-90% of marine species went extinct and on land there was major changes in plant assemblages (Futuyma 2009).
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A mass extinction is thought to occur due to the organisms failing to adapt to changes in the environment. Therefore a mass extinction can be seen as being selective. It can be trait selective where the cause of the mass extinction (extrinsic factor) is affecting a certain biological trait (intrinsic factors) of an organism or group of organisms, making it less or more prone to the mass extinction event. For example a plant will be more prone to a global warming event if it has narrow leaves preventing it from overheating. It can also be taxonomic selective, where a taxa is actively selected upon due to the cause of the mass extinction as maybe some taxonomic groups suffer more from the extinction then other groups, however taxonomic selectivity is minor with some exceptions such as the dinosaurs (Raup 1994). A mass extinction may also be geographical selective, acting on one or more parts of the world.
A lot of study has been done on the causes of mass extinction events and less on the causes it may have in term of evolution (Erwin 2001). The extinction of a widespread species will require an environmental shock , either physical or biological, which the species has not previously encountered or is so rapid that adaption via natural selection or migration is prevented (Raup 1994). Survivors of the mass extinction are those that are maybe pre-adapted to the unexpected stress of the extinction. An example of one is described in (Raup 1986) where if there was ionising radiation, it would kill all mammals exposed but would have less of an effect on most insects and plants. This means that extinctions are highly selective but does not contribute to the general success a species has in terms of normal times. However if this was a reoccurring event caused by the same crisis each time, then the evolution of a mechanism to cope would form through natural selection (Raup 1986).
Factors affecting survival
The diversity we see today are the survivors of mass extinctions. There have been suggestions to what can cause one species to survive while others go extinct during a mass extinction event. One is the generalisation of the species. It is more likely that less specific species will cope better under the stress then more adapted, specific species. For example if a species is highly adapted and specific to a certain food type or restricted to a location due to having narrow environmental tolerances, it will be impacted on more harshly then an species that is maybe more geographically widespread (i.e. environmental generalists), being able to adapt or survive in different locations, or has a wide range of suitable food sources (Erwin 1998). However this also depends on how long the extinction goes on for. If it is prolonged then ecological specialists will be removed and generalists will most likely survive .
Other factor to why a species might survive an event, could be due to its location at the time. Depending geographically where the species is located will determine it its survival , for example in the early Permian where max diversity at low latitudes, shifted to max diversity at middle to high latitudes by the late Triassic this is evidence of ice climate to a hot one (Rees 2002).This is because lower latitude taxa( in particular tropical ecosystems) might be under more pressure and at a higher risk of extinction then higher latitude taxa (Erwin 1998) during a global warming event. This is because a global warming event causes the tropical ever-wet biome to become narrower due to the expanding of precipitation globally.
Body size may also play its role on survival. It was proposed that large body size (or long generation times) would suffer greater in extinctions(Erwin 2001). Large body size can be seen as a type of specialization by some(McKinney 1997) and has lead to the agreement that the effect body size on extinction is different between taxa. For example small body size of freshwater fish promotes extinctions because they are inefficient at dispersal. Large body size on the other hand has two patterns; one is between closely related species where large body size makes the species less likely to go extinct. This is because maybe larger body size will give that species greater competitive advantages and so become more abundant. The other patter is seen between distantly related taxa this time large body size increases the risk of extinction because of the liabilities of having such a large body. (McKinney 1997). In terms of evolution, the survivor’s body sizes are those that will propagate after the extinction, potentially affecting future species size diversity.
Effect on Evolution
It is evident that almost all of the species that have lived on Earth have died out (Newman et al 1994). The cause of this is extinction, and so it plays an important role in the evolution of life (Raup 1986). After any extinction event, be it mass or background, there will always be a loss of populations and species resulting in an overall decrease is diversity. However during recovery there is a repopulation event, after which species become available to new open niches for testing and adaption. This is different than usual gradual adaption, but none the less has contributed greatly to the diversity of life we have on earth today. The evolution of one species evolving to the new niche can cause not only the extinction of the ancestor but also cause a dominos effect, where a number of other species will evolve in conjunction as a result. This causes an avalanche effect thought the ecosystem (Newman et al 1994).
Mass extinctions essentially reset the evolutionary clock, wiping the slate clean, they create new evolutionary opportunities and can even redirect the course of evolution (Erwin D 2001) (Futuyma 2009) but this consequentially affects the ecosystem, as both organisms and environment affect one another. Studies have shown that the mass extinction events are followed by a survival interval in which there is no diversification, followed by a recovery phase which has a rapid diversification phase- an interval of exponential growth (Erwin D 2001). When looking at the fossil record at the extinction horizon there is usually a gap in the sediments void of fossils, and above this a species poor assemblage of survivors (Erwin D1998). The survivors are often abundant and geographically widespread consisting of opportunistic taxa. Described below are two examples of such a case.
Example 1: The Cretaceous-Tertiary( K-T) extinction which occurred 65mya was associated with an impact of a large bolide. In the sediments it was characterised by a layer consisting of iridium and shocked quarts succeeded by a bed rich in fern spores. From new-Zealand sediments there is evidence of fungal spores across the boundary. The fungal rich interval provides evidence for a survival period of opportunistic taxa. Post impact the humidity was most likely high while there was reduced solar luminosity caused by the increase in atmospheric sulphur aerosols and dust. This kind of environment would have favoured saprophytic life (along with the availability of dead plant matter) leading to the dominance for fungal species for a few years (Vajda et al 2004).
The recovery phase usually ends with the reappearance of clades which would have appeared to have ‘disappeared’ due to the mass extinction crisis (Erwin D 1998). Example 2: is during the end Permian mass extinction (caused by an environmental change due to flood basal volcanism as Pangaea was moving northward) , where there was an initial disappearance of conifers during the crisis with lycopsids and mosses becoming dominant in the survival phase. Lycopsids could survive the crisis because they were capable of surviving the harsh conditions and UV-B radiation caused by the increased release of volcanic substances (Visscher 2004). None the less, the dominance of lycopsids was not permanent and after the survival period conifers were dominating once again at the recovery phase. The plants in this time did not go extinct but rather, the gymnosperm died back while the lycopides became abundant but were able to make a comeback after the event.
Interestingly is that mass extinction event at each of the boundaries is offset between fauna and flora(asynchronous), as they are both impacted differently both evolutionary and ecologically by mass extinction events.
It is clear that plants, unlike animals, rarely go extinct during a mass extinction but rather there is a change of dominance. Plants are unique to animals in that they are highly adapted to their environment, this is due to the fact that they are not mobile, and any movement for example through migration can take up to thousands of years. Owing to this they are linked and adapted supremely through their roots to the lithosphere and hydrosphere of the geosphere and via their aerial parts such as leaves, shoots and reproductive structures to the atmosphere. For this reason in all the mass extinction events we see hardly any extinction of plants and rather a turnover of dominance and this had two distinct effects on evolution and ecology. If the environmental change is slow plants can migrate however if the change is faster than the migration rate we see turnover, change in ecosystems and extinction.
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The Triassic -Jurassic mass extinction was the 3rd largest in the history of Earth and was caused most likely by flood basalt volcanism causing rises in CO 2 and consequently raising temperatures. There was a 30% extinction of marine genera’s, a massive disappearance of corals and reed organisms while on land we had 50% extinction of tetrapod species. However in contrast only one family of plants became extinct.- peltaspermaceae and instead we see a major turnover(95%) of fossil plants (McElwain et al 1999). By looking at presence absence data the full effect the crisis had on ecology would not be seen. For example in East Greenland, high diversity plants such as Podozamites and bennettites( Pterophyllum , Anomozamites) were replaced by the lower diversity Czekanowskia, Sphenobaiera and Todites in the Jurassic (McElwain et al 2007).The Triassic dominant plant here went from making up 75% of the relative abundance data of the plant community to less than 10% in the Jurassic (McElwain et al 2007).This completely altered the ecology-the die back of dominant species can lead to available space and suitable conditions for other species.
Animals on the other hand are not as uniquely linked to their environment as plants are and so they are highly affected to environmental chance, even shift in the ecological change of plant can have detrimental exponential effects on the animals as plants are the bottom of the food chain effecting most higher taxa. Both marine and terrestrial extinction are however synchronic showing high levels of global extinction of higher taxonomic groups during the mass extinction event (McElwain et al 2007).
Future events and conclusion
How does the future or current extinction threatening us have an effect on the ecology and evolution of current species? So far human activity has been causing dramatic decreases in diversity and aiding in the change of climate and ultimately ecosystems. Proposed by (Myers et al 2001) is that this extinction will not only alter the biological diversity but also the evolutionary processes which diversity is generated. From the past mass extinctions we can understand that following it, there is re-diversification (or recovery) and ecological reorganization (approximately taking up to 5 million years (Erwin 2001)). The current crisis is estimated to have the following effects on evolution :
1) Due to fragmentation of species ranges, gene flow will be disturbed,
2) With population decline there will also be a decrease in gene pools
3) Addition of invasive species into new areas will cause founder effects ( as we are already seeing today) causing imbalanced ecological interactions, such as competition for space and predation ultimately disrupting food chains. This all contributed to the loss of biodiversity globally.
Another concern is how this crisis will affect the recovery process. One concern is the depletion of the ‘evolutionary powerhouse’ i.e. the tropics which is one of the key biomes on earth which is notorious for it explosions of evolutionary processes and life. This will be lost due to the increasing global warming and human activity. So far the tropics have become drier and an estimated 10-25% of rainforest species (5-10% of earth species diversity!) will be extinct in the next 30 years (Futuyma 2009). This decline could cause consequence to the re-diversification after the extinction crisis. It is clear that what will probably survive this extinction will most likely be species that are adapted to or can cope with human environments.
More pressing is the effect of increasing of atmospheric CO2 and temperature levels. What kind of plants are likely to survive? How will this affect the food chain of other species, and indeed the final effect it will have on the human race? From the fossil record of the previous mass extinctions of global warming, leaves were shown to become more dissected to cope with the increasing temperatures. This is because an increased in atmospheric CO2 means less stomata required on the leafs surface but this also causes less transpiration leading to overheating of the leaves, a way to adapt to this is to have dissected leaves as those seen at the T-J boundary.
Atmospheric CO2 levels on plants is thought to cause a secondary effects which include reduced water recycling in the environment such as precipitation and water runoff. Since the 1960’s there has been an increase in global surface run off by about 30% and this can impact significantly on marine systems causing eutrophication (Gedney et al. 2006).
In conclusion, it is evident the pass mass extinction cause dramatic changes in evolution, setting out new path and space for new forms of life and niches, but consequently this leads to ecological changes also. From the past mass extinctions, we can learn how the current crisis will take hold and maybe ways to prevent it. Currently it has been estimated that survivors of this extinction based on fossil records would be r-strategists and opportunist species leading to a pest-weed ecology (Myers et al 2001).
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