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Current Extinction Rates Versus Mass Extinction Events 1
Current Extinction Rates Versus Mass Extinction Events
Jack Boynton
University of Rhode Island
Current Extinction Rates Versus Mass Extinction Events 2
Abstract
News articles have emerged claiming that Earth is entering its sixth mass extinction event,
sometimes called the Holocene or Anthropocene extinction event. An extinction event is when
many observable organisms go extinct over a relatively short period of time, causing between
50% and 90% of species to go extinct (Dictionary 2015, Whitty 2007). This mass extinction
event seems to be rapidly occurring and articles seem to correlate on the underlying cause:
humans. Humans are causing extinction rates to skyrocket through our practices like hunting and
traveling (which spreads disease and introduces species) and the consequences of them including
climate change, habitat fragmentation, and pollution. This extinction period will reduce species
diversity exponentially and promote homogeneity in populations (Vince 2012). This is partially
due to many species being few in number (compared with their prior status) and divided between
communities due to fragmentation (Whitty 2007). Even formerly resilient species, such as
amphibians, are rapidly declining due to a human introduced fungus that they have no defenses
against. Some feel that that our actions will soon lead to our own demise (Younker 2015). Some
people still debate that this mass extinction event is even a real concern, pitting it up to ecoactivists replacing climate change with loss of diversity so they can keep people paranoid. This is
defended with paleontological articles about former mass extinction events determining that
climate change is a continuous factor in Earth’s history and mass extinctions have never been
caused by climate change alone. Usually a huge natural disaster happens, such as asteroid
collisions. They say the extinctions we are causing will certainly change evolutionary history,
but they are incomparable to gigantic natural disasters (Hoffman 2010).
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In order to determine the real status of extinction scientists have turned to studying
population dynamics, species abundance, and ecosystem functioning to determine how pressing
this matter actually is. Typically ecologists seem to focus their studies on flora, probably due to
their role as the primary producers for ecosystems and their sessile nature. A study done in 2001
in Victoria, Australia surveyed thirty locations along a 225 kilometer gradient to understand
comparative extinction rates between rural and urban areas. The locations ranged from suburbs
to agricultural districts and were labeled as urban, peri-urban, or rural. This area was ideal for
studying the effects of urbanization because the once dominant grasslands have been fragmented
and reduced to less the 0.5% of their original area. The grasslands were characterized by the
grass Themeda triandra, which forms tussock spaces where other small herbaceous species took
residence. A list of previously identified flora was compiled from reports dating from 1979 to
1991. Variables were included in the calculations for false identifications and failure to observe.
Species that were difficult to identify were excluded and all areas were surveyed twice to make
sure absence wasn’t due to different flowering seasons. The research teams were surveying for
166 species in 1104 populations and any species not found was considered extinct. Traits of the
different plants were also examined because of their role in survival. Traits examined were,
“lifeform, dispersal mechanism, presence or absence of vegetative reproduction, seed mass,
presence or absence of persistent soil seed bank, and prevalence of the species.” In this method,
they can exam what traits will help a plant survive in urban and rural landscapes as well. By
2001, 289 populations of 126 species were absent and thus declared locally extinct. In urban
areas 37% of indigenous species found in that landscape went extinct, comparatively to periurban(27%) and rural(20%). Species of all but one lifeform and all dispersal methods had a
higher rate of extinction in urban areas. Some traits of the plants resilient enough to occupy
Current Extinction Rates Versus Mass Extinction Events 4
urban areas were vegetative reproduction, and high prevalence. They predict that part of the
extinction pattern is grass productivity increasing from lack of fires and nitrogen inputs from
run-off, causing less water and nutrients for struggling organisms. This will give way to less
diversity in ecosystems over time and promote homogeneity. The study predicts that human
disturbance will only increase and with it, rates of plant extinction (Williams 2005).
Some scientists disagreed with the prediction rates of others claiming their calculations
were off. They said that when they subtract the proportion of the population survival and the
detection probability from 1 they should have only been subtracting it from the population
survival and then multiplying by the detection probability. They also took issue with some
researchers (mostly in older studies) only revisiting once. This would create inflated extinction
rates and give higher readings for vulnerability. They also said detection probability’s role in the
factors associated with survival, such as habitat, can falsely link those factors because they are
affected by the incorrect calculation. To compare using their own calculations they did their own
study with 80-100 year old records from north-east Switzerland, targeting 11 species over 367
sites. The study took place in the Swiss Alps among a variety of diverse ecosystems. They made
6 of the species easy to find based on leaf color, size, form, prevalence and other such visibility
factors, while the other five were supposedly harder to find. Visits occurred during the flowering
season of the individual plants. There wound up being around a 13% difference between the
calculations used the original way and the amended calculation used in this study. This study has
however been criticized for its small species count and improper use of their “Jackknife”
calculator (Kéry 2006).
In a study from 2003 that uses an entirely different extinction calculation, the number of
extinct past species plus the number of presently recorded species then multiplied by the extinct
Current Extinction Rates Versus Mass Extinction Events 5
past species, 446 species were searched for in the Swiss lowlands in Küsnacht. Küsnacht used to
have many variable ecosystems caused by the glacial formation of the landscape. This created
many different landforms, soil types, and niches to cause the multitude of habitats. Due to recent
urbanization these ecosystems have plummeted in the percentages of land they still occupy,
especially vineyards with only two individual vineyards left. Currently eutrophication of the soil
and habitat management seem to be persistent causes of extinction. The factors taken into
account for each species were “habitat affiliation, life-form, resource requirements, and
extinction risks.” Habitat affiliation was broken up into forests, meadows, wetlands, rocks, and
disturbed sites. Some factors showed positive or negative reactions to certain habitats, for
instance medium soil moisture requirements worked best for plants in meadows and disturbed
sites. Of the environmental factors, soil requirements were most heavily tied in with extinction
rates. A high demand for light was also a large contributor. Extinction rates also varied greatly in
different habitats. Rocky and forest sites had the lowest rates at 9% and 13% respectively.
Meadows had 20%, disturbed sites 23%, and at the top of the extinction rates were wetlands at
28%. Of all lifeforms, aquatic species went extinct at the fastest rate of 61%. Overall 127 species
went locally extinct leaving the extinction total between 17% and 28% with a higher probability
towards the prior (Stehlik 2007).
One study also viewed the species and functional group dispersion in order to see how
the extinction of functional groups occurred in relation to species. Four hypothetical
communities were utilized to compare results. All species were compared by species richness,
functional group richness, and functional group evenness. Community 1 was used as the standard
where community 2 differed in species richness, community 3 in functional group evenness, and
community 4 in functional group richness. Species richness was a major attribute for
Current Extinction Rates Versus Mass Extinction Events 6
communities, creating better functional redundancy and allowing functional groups to maintain
their order with 80% of species within going extinct. This was higher than community 2 which
had less species rich environments and lost a functional group 16% of species before community
1. In addition to a healthy diversity, functional richness improves chances of survival be 24%
and evenly distributed species were preferred. Overall, 75% of species could typically go extinct
before the functional group itself did. This is a positive message because ecosystems are much
more highly impacted by functional group extinction than species extinctions (Fonseca 2001).
So in conclusion, especially since these studies are only comparing local extinctions, we
are not quite near the standards necessary for a mass extinction event. Even in local regions with
relatively high urbanization species extinctions still don’t typically exceed the necessary 50%
marker. However, we are able to see that urbanization is increasing species extinction at an
alarming rate (Williams 2005). Even with possible biases factored in, extinction rates are still
much higher than average (Kéry 2006). Extinction rates can vary depending on ecosystem type,
but are rising independent of the ecosystem and diversity is falling correspondingly (Stehlik
2007). Finally, functional group extinctions will not start occurring until a good portion of the
species within have gone extinct (Fonseca 2001). So even though we are not currently in a mass
extinction event, it seems like our effect on the planet is certainly gearing us up to experience
one a lot sooner than we might think. The only question left is what do we do about it? It has
been suggested by some to stop trying to revert ecosystems and upkeep them as they are now,
even going as far as to suggest manipulating them to our desire and then preserving them. Some
have hopes that measures will be taken, or that science will find a way to counteract extinction
rates with cloning (Vince 2012). Nobody truly knows, but as species diversity continues to fall
our ability to save them will only fall with it (Fonseca 2001).
Current Extinction Rates Versus Mass Extinction Events 7
References
Dictionary.com Unabridged. (July 18, 2015), Dictionary.com website. Retrieved from
http://dictionary.reference.com/browse/mass+extinction
Fonseca, C. R. and Ganade, G. (2001), Species functional redundancy, random extinctions and
the stability of ecosystems. Journal of Ecology, 89: 118–125. Retrieved from
http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2745.2001.00528.x/full
Hoffman, L., Doug. (September 12, 2010). The Developing Diversity Scam. The Resilient Earth.
Retrieved from http://theresilientearth.com/?q=content/developing-diversity-scam
Kéry, M., Spillmann, J. H., Truong, C. and Holderegger, R. (2006), How biased are estimates of
extinction probability in revisitation studies?. Journal of Ecology, 94: 980–986. Retrieved
from http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2745.2006.01151.x/full
Stehlik, I., Caspersen, J. P., Wirth, L. and Holderegger, R. (2007), Floral free fall in the Swiss
lowlands: environmental determinants of local plant extinction in a peri-urban landscape.
Journal of Ecology, 95: 734–744. Retrieved from
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2745.2007.01246.x/full
Vince, Gaia. (November 1, 2012), A looming mass extinction caused by humans. BBC News.
Retrieved from http://www.bbc.com/future/story/20121101-a-looming-mass-extinction
Whitty, Julia. (April 30, 2007) Animal Extinction - the greatest threat to mankind. The
Independent. Retrieved from http://www.independent.co.uk/environment/animalextinction--the-greatest-threat-to-mankind-397939.html
Current Extinction Rates Versus Mass Extinction Events 8
Williams, N. S. G., Morgan, J. W., McDonnell, M. J. and McCarthy, M. A. (2005), Plant traits
and local extinctions in natural grasslands along an urban–rural gradient. Journal of
Ecology, 93: 1203–1213. Retrieved from
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Younker, Nick. (June 21, 2015) Holocene Extinction: Scientists Say the World Has Entered Into
its 6th Mass-Extinction Event. Latin Post. Retrieved from
http://www.latinpost.com/articles/61058/20150621/scientists-claim-the-world-hasentered-into-its-6th-mass-extinction-event.htm