The+Factors+of+The+Sixth+Mass+Extinction

Factors of the Sixth Mass extinction =** Introduction **=

toc The sixth mass extinction, or Holocene (current, during modern time of human expansion), or Anthropocene (Time period when human expansion has affected the world) extinction event, is a pandemic that is occurring faster than previously thought. Due to a myriad of stress concentrators and other factors, the decline in population have increased and have experienced an extinction rate of around 2 species per year [2]. Because the extinction event is current and has very little public concern at the moment, there is not much known about how to solve it or its true consequences but has also has sparked the interest of scientists. To be conservative, many analyses have put this on the level of [|The Big Five events] in which around 75%- 96% of all species have gone extinct [16]. The factors of extinction has been likened to the four horsemen of the apocalypse, where there are invasive species, habitat loss, overkill, and finally coextinction or cascade extinction [9].

= I**nvasive Species** =

[|Invasive species] are plants and animals that are introduced to a foreign area either intentionally or accidentally. The danger behind this is that, the native species do not have any way of protecting itself from the foreign invader. Some invasive species include: Lionfish, which eats anything in its path including coral reefs [22], Zebra mussels, in the Great Lakes, which are clogging up pipes and killing off local clam species [23], Rats, decimating Pacific island species [20].

Bottle Neck and Hybridization
Because invasive species typically have no natural predators, they spread at an exponential rate and gives the local species little time to adapt and evolve to counter the species. Usually it has been thought that the bottleneck effect would lower the genetic diversity of the species to lower its survivability in its new environment, however studies show that it promotes genetic variation due to the new environment and the adaption to this new location. This can be thought of how species on the Galapagos islands are genetically diverse although being trapped on a small island [17]. Hybridization also effects the invasive species spread. In plants, some plants can cross breed and gain traits or defense mechanisms that negatively affects the other local species. In one case a sunflower that was hybridized with a beach sunflower to develop “herbivore-resistant” t raits [17]. This integration then becomes really only beneficial for the species but harmful to the local environment. [[image:1-s2.0-S136013850800126X-gr1.jpg width="407" height="363" align="left" caption="Figure 1. Timeline of plant species invasion "]]

=** Habitat Loss & Fragmentation **=

Habitat loss is mainly driven by extraction of resources( deforestation, mining, farming etc..) or human settlements where fragmentation is caused by roads, or anything man made that prevents a species from migration. These factors are not inherently evil, however, due to large demands and greed, the cheaper practices, which are typically not sustainable, are used. In a study of 177 mammal species around the globe, it was shown that around 40% have range decline of 80% or more. The study also found that in areas of higher biodiversity actually experienced more population decline compared to areas of lower biodiversity [2]. This leads to lower amounts of individuals, which leads to inbreeding especially when ranges of populations continue to decline. Species that experience inbreeding are more susceptible to genetic issues that lower their chances of survival [2].

**Amphibians**
A study with amphibians have shown that the development of roads have a negative effect on the immigration of the species. These create landscape barriers and have increased mortality, lowered genetic diversity and spread of amphibians. When the road was built, the amphibians have been reluctant to cross them and those that do, have a higher risk of getting run over. Typically in amphibians, dispersal occurs mainly at the juvenile state, but typically have lower spread per generation than mammals or bids. Immigration is very important for the survival of a species. For example, a study shows that 18% of the California tiger salamander needs to be reproduced in order to maintain the local population, but only 5% reach maturity [18]. Thus, this species is doomed to extinction due to the lack of immigration in which salamander species cannot find each other. In spotted salamanders and wood frogs, it has been noticed that these species will not cross landscapes such as pastures, clear cuts or roads [18].

**Climate Change**
In some studies, there has been an negative interaction of climate change with habitat loss. In many cases, species can adapt to slight climate changes, however, some extreme climate changes can cause habitat losses. For example, coral reefs produce highly diverse ecosystems but due to the rapid increase in temperature, carbonic acid and pollution, coral reefs are starting to die off. If the corals start to die off, the fish species that depend on these regions end up losing their homes. Mass deforestation has been shown to cause drying and rainfall shifts, and in some cases flooding [19]. In some hotter areas, many species depend on forest coverage and the habitat to survive, but as temperatures increase, many species become more susceptible to habitat loss and Fragmentation. These have different effects on different families of course, Mammals and amphibians are more susceptible to habitat loss and fragmentation with a higher habitat amounts whereas reptiles and plants show lower loss and fragmentation [19].

=** Overkill **=

Overkill is another horseman of the apocalypse. The general idea is that species are hunted or fished faster than their reproduction rate and with every generation, there are continuously a declining population.

Fishing
One of the biggest case studies of overkill was conducted on bluefin tuna and has been an issue since the 1960’s. Because of the huge fishing industry, tuna was being fished at such a number that the reproduction rate of tuna was not replenishing. Most of the legal bluefin fishing is done in the Mediterranean sea; however, because they are an expensive delicacy especially in sushi and sashimi, fisheries are still illegally fishing for bluefin tuna causing their population to decline over time [21]. There has been international laws to illegalize bluefin fishing in certain areas, but they have been very unhelpful as illegal fishing has continued since. The [|WWF] are trying to combat illegal fishing of Atlantic bluefins by putting trackers on bluefin to make sure that fisheries will be fined for illegally fishing [21].

Protected Areas
Another overkill study can be conducted on illegal poaching. Since 1993, African Lion populations have declined by 43%. Giraffes have dropped from 117,000 individuals to around 97,000 individuals since 1985 and have been reduced to 4 species [2]. Some precautions have been made such as developing [|protected areas], in fact, around 12% of land around the world are PAs. Although these protected areas have different management and tactics, in Africa, the protected areas still face population decline. In a study from 1970 to 2005, there was a bout a 59% decline in population in the protected areas. This has generally been thought to be caused by undermanagement and underfunding of the areas which allows poachers to kill the large mammals. [5] Although many protected areas have shown a lower amount of habitat loss and population decline, they are thought to be rather inefficient [11].

Much of the overkill is still hard to document as many of the species that experience overkill are illegally hunted or fished.

=** Coextinction **=

Coextinction, the idea that species become extinct in groups and not individual species. This can be seen as collapse of a food web when one species becomes extinct and mutualist or symbiotic species that depend on those become extinct with them. In Madagascar, there is a specialized species of moth that pollinates a species of orchids. Darwin, observed that because these moths lived on these orchids, if one were to go extinct, so will the other. This proposed the idea of coextinction [9]. Many studies have been conducted but parasites show the greatest susceptibility to coextinction. Although it is rather difficult to document, parasites and lice species have shown to be an easier target for study. When California condors were put into captivity to combat their extinction, they were deloused. In studies the lice that were taken off are nowhere to be found even in their close relatives such as the black vulture [4]. Some species of parasites sometimes can change its diet to increase its survival. In California, the western fence lizards are the host of larva of ticks. When removed from an enclosure, around 5% increase of larva was found on other hosts [4]. Thus the survival of species really depends on the range of the hosts it can interact with.

The network between species and their survival heavily depends on how specialized the species are. In the case of the moth in Madagascar, due to how specialized the species are, coextinction can easily happen but, in the case of pierid butterflies, the amount of individuals extinct are lower due to the variety of host plants it can use [9]. Coextinction and climate change can also show a negative effect. Plant phenology and bird migrations are an example of how climate change has affected species. Networks that can uniformly adapt to climate change have almost no effect on coextinction but many species have shown to move its geography to areas that can support itself [9].

Due to the nature of coextinctions, they are very hard to index and document. That being said there is still a myriad of evidence that supports coextinction; however, much of the data is shown through models and calculations with assumed variables.

=** Conclusion **=

This sixth mass extinction is present and have many studies that can predict the consequences and the progression of this event. The four greatest factors: invasive species, overkill, habitat loss and fragmentation, and coextinction, all work together to increase the rate of extinctions and population decline. Coextinction and habitat loss have even shown results to synergize with climate change. Another difficulty with truly understanding the nature of these extinctions are that different families or different genetics of species are effective differently by the four horsemen.

Most of these factors have been shown to complement each other, and the synergies between the four horsemen and climate is also prevalent in most of the categories. Many species need to adapt in order to overcome many of these factors. Overkill on the other hand is more in the hands of the human race. Many of these factors, although well studied, are still inconclusive. Many of these studies use predictions, models to communicate their data because many of these species are hard to track and document extinction. Even when looking at extinctions, some elusive species have actually been found after being labeled extinct. Habitat loss, for example, can only use a finite sample, and most of the numbers can be based on those finite samplings. Coextinxtion data is mostly predominant on statistical analysis because coextinctions are so hard to actually document. Most of the research on extinctions are taken as a conservative manor as they are trying to predict the worst case scenario.

Overall the data shows that researches are still trying to evaluate when and where to use different tactics in order to fight these factors to slow down the progress of extinction. While some tactics do show some improvements, it is still not enough to combat this fast acting event. Much of the infrastructure at play as relatively gone underfunded and undermanaged and for the most part rely on human kindness. There is still room for improvements as long as public concern continues to increase.

= ** References ** =

[1] G. Ceballos, P. R. Ehrlich, A. D. Barnosky, A. García, R. M. Pringle, and T. M. Palmer, “Accelerated modern human–induced species losses: Entering the sixth mass extinction,” Science Advances, vol. 1, no. 5, p. e1400253, Jun. 2015.

[2] G. Ceballos, P. R. Ehrlich, and R. Dirzo, “Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines,” Proc Natl Acad Sci USA, vol. 114, no. 30, p. E6089, Jul. 2017.

[3] J. A. Estes, D. F. Doak, A. M. Springer, and T. M. Williams, “Causes and consequences of marine mammal population declines in southwest Alaska: a food-web perspective,” Philosophical Transactions of the Royal Society of London B: Biological Sciences, vol. 364, no. 1524, pp. 1647–1658, Jun. 2009.

[4] R. K. Colwell, R. R. Dunn, and N. C. Harris, “Coextinction and Persistence of Dependent Species in a Changing World,” Annual Review of Ecology, Evolution, and Systematics, vol. 43, pp. 183–203, 2012.

[5] I. D. Craigie et al., “Large mammal population declines in Africa’s protected areas,” Biological Conservation, vol. 143, no. 9, pp. 2221–2228, Sep. 2010.

[6] S. Meyer, B. C. Robertson, B. L. Chilvers, and M. Krkošek, “Marine mammal population decline linked to obscured by-catch,” PNAS, vol. 114, no. 44, pp. 11781–11786, Oct. 2017.

[7] E. C. Fricke, J. J. Tewksbury, E. M. Wandrag, and H. S. Rogers, “Mutualistic strategies minimize coextinction in plant–disperser networks,” Proc. R. Soc. B, vol. 284, no. 1854, p. 20162302, May 2017.

[8] G. Ceballos and P. R. Ehrlich, “The misunderstood sixth mass extinction,” Science, vol. 360, no. 6393, p. 1080, Jun. 2018.

[9] R. R. Dunn, N. C. Harris, R. K. Colwell, L. P. Koh, and N. S. Sodhi, “The sixth mass coextinction: are most endangered species parasites and mutualists?,” Proc Biol Sci, vol. 276, no. 1670, p. 3037, Sep. 2009.

[10] T. Pievani, “The sixth mass extinction: Anthropocene and the human impact on biodiversity,” Rend. Fis. Acc. Lincei, vol. 25, no. 1, pp. 85–93, Mar. 2014

[11] J. Geldmann, M. Barnes, L. Coad, I. D. Craigie, M. Hockings, and N. D. Burgess, “Effectiveness of terrestrial protected areas in reducing habitat loss and population declines,” Biological Conservation, vol. 161, pp. 230–238, May 2013.

[12] P. Scholte, “Towards Understanding Large Mammal Population Declines in Africa’s Protected Areas: A West-Central African Perspective,” Tropical Conservation Science, vol. 4, no. 1, pp. 1–11, Mar. 201 [13] D. Sanders, E. Thébault, R. Kehoe, and F. J. F. van Veen, “Trophic redundancy reduces vulnerability to extinction cascades,” PNAS, vol. 115, no. 10, pp. 2419–2424, Mar. 2018.

[14] O. of A. and R. US EPA, “Carbon Footprint Calculator | Climate Change | US EPA.” [Online]. Available: []. [Accessed: 01-Jul-2018].

[15] D. Sanders, E. Thébault, R. Kehoe, and F. J. F. van Veen, “Trophic redundancy reduces vulnerability to extinction cascades,” PNAS, vol. 115, no. 10, pp. 2419–2424, Mar. 2018.

[16]F. He and S. P. Hubbell, “Species-area relationships always overestimate extinction rates from habitat loss,” Nature; London, vol. 473, no. 7347, pp. 368–71, May 2011.

[17]P. J. Prentis, J. R. U. Wilson, E. E. Dormontt, D. M. Richardson, and A. J. Lowe, “Adaptive evolution in invasive species,” Trends in Plant Science, vol. 13, no. 6, pp. 288–294, Jun. 2008.

[18]S. A. Cushman, “Effects of habitat loss and fragmentation on amphibians: A review and prospectus,” Biological Conservation, vol. 128, no. 2, pp. 231–240, Mar. 2006.

[19]Mantyka-pringle Chrystal S., Martin Tara G., and Rhodes Jonathan R., “Interactions between climate and habitat loss effects on biodiversity: a systematic review and meta-analysis,” Global Change Biology, vol. 18, no. 4, pp. 1239–1252, Nov. 2011.

[20]G. A. Harper and N. Bunbury, “Invasive rats on tropical islands: Their population biology and impacts on native species,” Global Ecology and Conservation, vol. 3, pp. 607–627, Jan. 2015.

[21]“The Lionfish Invasion,” Sailors for the Sea, 01-Sep-2011. [Online]. Available: []. [Accessed: 07-Jul-2018].

[22]“Bluefin Tuna | Species | WWF,” World Wildlife Fund. [Online]. Available: []. [Accessed: 07-Jul-2018].

[23]“Zebra mussel - Invasive species - Minnesota DNR - MN Department of Natural Resources.” [Online]. Available: []. [Accessed: 08-Jul-2018].

[24]“The big five mass extinctions,” Cosmos Magazine. [Online]. Available: []. [Accessed: 07-Jul-2018].

[25]“Invasive and Exotic Species Profiles.” [Online]. Available: []. [Accessed: 07-Jul-2018].

[26] “United Nations List Of Protected Areas - UNEP-WCMC,” UNEP-WCMC’s official website - United Nations List Of Protected Areas. [Online]. Available: []. [Accessed: 01-Jul-2018].

[27] “BBC Nature - Big Five mass extinction events.” [Online]. Available: []. [Accessed: 01-Jul-2018].