Background:
Extinction is as important in biology as mortality rate. There are two kinds of extinction that have been identified in biological history, mass extinction and background extinction. Usually the focus of studies is mass extinction; however ecologists are more concerned about background extinction that is an ongoing process. There are several questions in extinction that is needed to be explored in conservation of species. Is extinction selective or not? What aspects of biological spheres are under selective pressure of extinction? And why do species extinct? There are several kinds of extinction selections like introduction of invasive species, Habitat loss, over-exploitation, climate change and pollution. Here in this knol, I have selected Climate change as it is affecting all biomes and its rates are rapidly increasing. I have highlighted the areas that are currently under risks of extinction due to climate change pressures ranging from Polar Bears, Penguins, Amphibians, Royal Tiger and Pollinators.
I. Introduction:
It seems that the two processes of speciation and extinction are balancing each other, however when we look at numbers of both processes, it creates doubts. Though fossil record is not complete but it is grossly estimated that there have been 4 billion speciation, in evolutionary history of life, out of which only 4 millions species are alive today. It means 3.996 X 109 species have extinct in evolutionary history of life (D. Raup, 1991). In 1994, Raup in his famous paper, “The role of extinction in evolution”, which is published in National Academy of Science, argued that ignoring extinction for an evolutionary biologist is as for a demographer to ignore mortality rate. Whenever there are discussions on extinction, people usually refer and think to mass extinctions, especially big five mass extinctions. Table. 1, compares big five mass extinctions.
The figures of extinctions in big five mass extinctions seem impressive as they are occurring in short time intervals and eliminating form 75% to 95% of existing species. Though extinctions in these events are impressive but they are collectively making only 4% of the total extinction over past 600 million years.
David Raup (D. Raup et al, 1994) has calculated the extinction rate and found that the extinction rate is 25% per million and mean species duration is 4 million years. The comparison of extinction by mass extinction and background extinction is important because mass extinction are usually considered as a result of sudden changes in environment but the background extinction is the result of constant changes in environment. Hence background extinction is important for ecologist to understand.
Some published estimates of mass extinction (Different authors differs in their estimates based on their subjects)
(1) 15 million years : Raup, D.M. and J.J. SEPKOSKI, JR. 1982. Mass extinctions in the marine fossil record. Science 215:1501-1503
(2) 10 million years : HOUSE, M.R. 1967. Fluctuations in evolution of paleozoic invertebrates. Pp. 41-54. In HARLAND, W.B., C.H. HOLLAND, M.R. HOUSE, N.F. HUGHES, A.B. REYNOLDSS, M.J.S. RUDWICK. G.E. SATTERTHWAITE, L.B.H. TARLO, AND E.G. WILEY, (eds.), The fossil record. The Geological Society of London: london.
(3) 8 million years: FARSAN, N.M. 1986. Fransian mass extinction - a single catastrophic event or cumulative? Pp. 198-197. In Walliser, O.H. (ed.), Global Bio-Events: a critical appraoch. Lecture Notes in Earth Sciences 8. Springer-Verag: Berlin.
(4) 7 million years: McGHEE, G. R., JR. 1982 The Fransian-Famennian extinction event: a preliminary analysis of Appalachian marine ecosystems. Geological Society of America Special Paper 190:491-500.
(5) 3 to 5 million years: COPPER, P. 1984. Cold water oceans and the Fransian-Famennian extinction crisis. Geological Society of America Abstracts with program 16:10.
(6) 1 million year: ZIEGLER, W. 1984. Conodonts and Frasnian/Famennian crisis. Geological Society of America Abstracts with program 16: 73.
(7) 0.5 million year: SORAUF, J.E. AND A.E.H. PEDDER. 1984. Rugose corals and the Frasnian- Famennian boundary. Geological Society of America America Abstracts with program 16:64.
After D. Raup, et al. 1994
Once it is established that background extinction is more important than mass extinction then the second question arises that is extinction indifferent or selective? If it is selective then which group of organisms is at greater risks of extinction?
Paleobiologists have tried to establish a link between selectivity of extinction over taxonomic, specific traits and species levels. There is quiet small difference in selectivity across taxonomic level, however there is strong selectivity for specific traits like, widespread geographical distributions, body size along with long generation size. Jablonski, (Jablonski, 1986) showed that in background marine mollusks with planktonic larvae survive longer than those develop from egg. Mollusks with planktonic larvae have greater dispersal abilities and as a result higher survival capabilities to stresses than species with limited dispersal capabilities. Similarly there are enough evidence exists of extinction of organisms with large body sizes like, Dinosaurs, ammonites, eurypterids, mammoths, mastodons and rudist clams(LaBarbera et al, 1986). Simpson have also suggests that mammals with long generation times suffered greater extinction in latest Pleistocene because natural selection could not operate quickly enough for adaptation to changing climatic conditions.
Now that it is established that extinction is somehow selective then, a third and more related question come arise and that is why do species become extinct? There are three points of views among evolutionary biologists mostly based on the philosophical predisposition,
i. Extinction occur because of inferior adaptations(Invasive species replace native species)
ii. Bad luck (Habitat loss due to volcanism, continental drift, climate change and sea level rise)
iii. The red queen hypothesis of L.Van Valen, “It takes all the running you can do just to stay in place” Or antagonistic interaction: If one organism improves adaptation, it forces other related organisms to improve adaptations. For example, if one organisms improve a trait that make it able to successfully avoid predators, then the natural selection pressure increases on others and forces them to improve their traits.
Now, that we have established from life history that constant extinction is more important than sudden extinctions, there is some selectivity in extinction and there are different causes that organisms go extinct so let’s see the current trends of extinction under anthropogenic climate change.
II. Climate Change?
We can be skeptic about the climate change controversies that arise from time to time but still there are certain areas that there is a common census. Vitousek has identified three such areas that are well documented and is beyond the doubt. The first is increasing concentration of the carbon dioxide in the atmosphere, second is alteration in the biogeochemistry of nitrogen cycle and third that is ongoing land use/land cover change (
Vitousek et al,. 1994). Though Vitousek didn't include the human in his list but the human population growth is beyond doubt both in numbers and in effect on the climate so we can include it as fourth factor.
Though climate change is not new for current biological populations and they have seen the fluctuating glacial, interglacial and little ice age for last since Pleistocene period. However the current anthropogenic climate change is different from several perspectives. In past climate changes, human weren't in direct competitions for same resources to natural world, the rate of climate changes were not rapid as today, species had space for movements in response to climate change. Species had not lost genetic diversities to the rates they are losing due to massive hunting, introduction of invasive species, habitat loss, introductions of pests, herbicides and pollution and human barriers in their routes of migration, etc.
III. Why we should care about global Change
The lists of impacts on human by the human caused climate change are very long. I would like to mention a few of them so it make us realize that, why anthropogenic climate change should be a matter of concern for us. Climate change may affect the ecosystems which provide essential services to human like, key nutrient cycles (Carbon cycle, Nitrogen cycle), maintaining air quality, food, fuel, raw resources for numerous industries, medicines, new compounds, pests and diseases, etc. The ecosystem services is not limited only to physical, chemical and biological services but also provide aesthetic, tourisms, cultural and spiritual services. Following we give some reports from UN and WWF, so we could sense, how much the issue on hands is serious.
A. Millennium Ecosystem Report (2005);
Few important points related to biodiversity that is notable in the report of Millennium Ecosystem Report (2005) under title of, "Ecosystems and human well-being" is as follows,
1. Figure 3.10 under title of "Main direct drivers", has compared the impacts of habitat change, climate change, invasive species, over-exploitation and pollution (nitrogen, phosphorus) on biodiversity. This comparison reveals that the impact in each biome including forest (boreal, temperate, and tropical), dry land (temperate grassland, Mediterranean, tropical grassland and savanna, desert), inland water, coastal, marine, island, mountain, island, mountain and polar environments are compared. It is interesting to note that the impact of climate change vary across the biomes from high to low but there is very rapid increase of the climate change on all biomes.
2. Figure 1, of this report shows that the rate of extinction is increasing with passing times. Figure 1, under title of, "Species extinction per thousand species per millennium" the rate of extinction of almost all major organism have compared based on fossil record (for distant past), known extinction (for recent past) and modeled prediction for future extinction. Fossil record shows for every thousand mammal species, less than one went extinct every millennium. Current extinction rate is up to thousand times higher than the fossil record. Projected future extinction rate is more than ten times higher than current rate.
3. Table 2.2. Trends in the Human Use of Ecosystem Services and Enhancement or Degradation of the Service around the Year 2000,
a. Global: In the middle of last century terrestrial ecosystems were turned from a net source of Co2 into a net sink. The land cover use increasing albedo effect, which contrary to Co2 emissions caused the world temperature do not increase much due to greenhouse effect.
b. Regional: Though land cover use have affect local and regional climate in both positive and negative directions, however the negative impact is higher like tropical deforestation and desertification have overall reduced the rainfall (humidity and temperatures are directly proportional).
c. Pollination: Globally the population of pollinators has declined except of Antarctica, which lack pollinators. The decrease of pollinators have caused decline in number of seed and fruit productions. The decline of pollinators has also affected the reproduction of some rare plants.
B. Living Planet Report 2008:
Several points that World Wildlife Fund in its 2008, "Living Planet Report 2008" point out is eye opening. The living planet index shows that over the past 35 years alone the Earth's wildlife populations have declined by a third. Human global footprint now exceeds the world's capacity to regenerate by about 30 per cent. For the first time in recorded history, this past summer (2008) the Arctic ice cap was surrounded by open water-literally disappearing under the impact of our carbon footprint. The Living Planet Report 2008 tells us that more than three quarters of the world's people live in nations that are ecological debtors- their national consumptions has outstripped their country's biocapacity.
IV. Risks of extinctions:
According to The International Union for Conservation of Nature (IUCN)’S 2004 Red List, 15,589 species of animals and plants are threatened to extinction in near future due to human activity – one of these human activities is human caused climate change. The order of threatened species is amphibian species 32% (one in three), mammal species 24% (one in four), bird species 12% (one in eight), Conifer species 25% (one in four) and Cycads 52% (IUCN Red List,2004). This high rate of extinction as also compared to five mass extinction in geological times and it is why sometimes referred as 6th mass extinction but it is some sort of overstatement as human is aware of their impacts and working for the conservation of threatened species which is though controversial but is progressing.
A. Arctic Polar Bears
Cold regions are very sensitive to temperature rise. Arctic is believed to be twice and thrice sensitive to climate change than other parts of the world. Organisms that have adapted to these cold regions are also sensitive to temperature rise. In Arctic region, Polar bears is thought to be threatened by climatic warming, esp of warming of air in late spring (April-June). As the sea ice melts in this time period, it limits the movements of bears and hence declines the chances of mating and hunting. Derocher has concluded that polar bears are unlikely to survive (Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar bears in a warming climate. Integr. Comp. Biol. 44, 163–176) however, Dyck and co-authors contradict with this view and they think that there is no climatic warming since last 70 years around Hudson bay area and what others have considered as warming is part of arctic variability (Dyck et al, 2007).
Image source:http://legalplanet.files.wordpress.com/2009/05/polar-bear.jpg
B. Antarctic Penguins:
Like Polar bears of arctic, the Penguins of the Antarctica is also threatened to extinction. According WWF Antarctic Climate Change Focal Project (ACCFP)'s report, "impacts of 2C global warming on Antarctica penguins", 50% of the colonies of the iconic Emperor penguin and 75% Adelie penguin colonies face marked decline or disappearance if global temperature is allowed to rise 2°C above preindustrial levels.
C. Amphibians:
The hotspot group is amphibians that are concerned about mostly due to their current rate of extinction. Pound reports extinction and declines of many cloud-forest amphibians on a mountain in Costa Rica (Pounds et al.1999,2005). Malcolm has compared the current rate of mass extinction of amphibians with their background rate of extinction and found that Amphibian’s current rate of extinction is 211 times of their background extinction (Malcolm et al., 2007). Stuart estimates that almost a third of amphibians are threatened with extinction (Stuart et al., 2004) and Pounds considers one of the causes among others is climate change (Pounds and Crump, 1994; Pounds et al., 1999).
D. Sundarban Mangroves and Royal Tigers:
Panthera Tigris is the only tiger species that have adapted to live in Mangrove forest. The Mangrove forest of Sundarbans is the home of this Mangrove tigers. Over 10,000 Km2 of Sundarban, 6000 Km2 is in Bangladesh and the rest is in India. This mongrove forest which lies on Southern fringe of Ganges Delta supports a rich fauna and flora. Rahman (Rahman et al., 2000) reported that the Mangrove forest which is composed of 401,600 ha in land and remaining 175,400 ha, under the water in the forms of river, canals and creeks of width varying from a few meters to several kilometers supports different species of about 334 plants, 120 fishes, 35 reptiles, 270 birds and 42 mammals. The Sundarbans is the only habitat of the famous Royal Bengal Tiger and estuarine crocodile.
Loucks and colleagues (Louks et al,. 2010) of World Wildlife Fund estimated that with a 28 cm rise above 2000 sea levels, remaining tiger habitat in Bangladesh’s Sundarbans would decline by 96% and the number of breeding individuals would be reduced to less than 20. They Assumed that current sea-level rise predictions and local conditions do not change, a 28 cm sea level rise is likely to occur in the next 50–90 years. If actions to both limit green house gas emissions and increase resilience of the Sundarbans are not initiated soon, the tigers of the Sundarbans may join the Arctic’s polar bears (Ursus maritimus) as early victims of climate change-induced habitat loss. It is noticeable that globally, sea level has increased by 1.8 ± 0.5 mm year−1 from 1961 to 2003, but 3.1 ± 0.7 mm year−1 from 1993 to 2003 (Bindoff et al. 2007).
Image Source:http://www.earthweek.com/online/ew070511/ew070511c.jpg
E. North Western Hawaiian endemic and endangered species:
Hawaiian Monk Seal is one of the rarest marine mammals in the world. There are only 1300 individuals mainly in the 6 NWHI subpopulations (Carretta et al, 2006). Hawaiian Monk Seal is listed as endangered species under US Endangered Species Act 1973.
Another threatened species that is listed in US Endangered Species Act is Hawaiian green sea turtles. Although the range of Hawaiian green turtle covers the entire Hawaiian archipelago but French Frigate Shoals, one of the NWHI atolls is the home for the 90% breeding female nest. The sandy beaches of these islands provide protection from waves and sharks and an easy access to ocean (Westlake and Gil martin, 1990).
Adding to list are the vulnerable Laysan (phoebastria immutabilis) and endangered black-footed albatross (P. nigrispes), according to IUCN, whose nesting occur entirely in NWHI (Harrison, 1990). The NWHI are the habitat for some 14 million sea birds of 18 species (Harrison, 1990). NWHI also have endemic species including 4 land bird species, 3 terrestrial species, 12 plant species and over 60 species of terrestrial arthropods (Canont et al, 1984).
As we mentioned earlier the sea level is rising. Sea level rose up around 15 cm in twentieth century (Ruddiman, 2001) because of melting of ice at poles and glaciers and also thermal expansion of the ocean.
F. Coral Reefs:
The most evident effects of the rapidly increasing levels of Co2 in the atmosphere is increase in average global temperature, rise in sea levels and acidification of oceans. The Co2 is dissolvable in water and oceans have dissolved 25% (2.2 Pg C/year) of Co2 that is emitted as a result of human activity. The Co2 dissolved in ocean makes carbonic acid which dissolve the coral reefs (Coral reefs are made of up carbonate shelled organisms and their deposits and carbonate fizz out in dilute acid). This reef degradation as a result of acidification along with decline in herbivorous fish and nutrient loading are causing coral reefs shift to algal dominated systems.
G. Global vegetation distribution:
In 1967 the botanist, R. L. Holdridge sketched the global distribution of plant communities solely based on the climate considering that temperature and rainfall is the dominant factors relative to soil composition and altitudes, acidity etc (though these factors are important determinants in local distribution of vegetation). Holdridge's scheme is known as Holdridge life zone model and it is still widely used especially in comparison with other models to determine or predict the effects of climate change on the vegetative distribution.
After Holdridge 1967
Image Source: http://en.wikipedia.org/wiki/File:Lifezones_Pengo.svg#filelinks
Though the effect of the climate change on the distribution is not fully understood but still several factors are evident. The current global warming is largely caused by green house gases emitted by human activity. The most important green house gas is CO2. Vegetation is not only sensitive to warming caused by increasing concentrations of CO2 in atmosphere but also to CO2 itself as CO2 is one of the important nutrients. Concentrations of CO2 are especailly important for aquatic photosynthetic species and may cause eutrophication in lakes (if Nitrogen and phosphorus are sufficient).
H. Plant and insect interactions:
In 1998, Coviella and Trumble wrote a review paper under title "effects of elevated atmospheric carbon dioxide on insect-plant interactions" and they have listed the researches have done on specific plant-insect interactions. They have argued that there are limited researches available on this issue however it is certain that increased level of CO2 and temperature is species specific so it is expected to vary a lot across species. However it is evident that increased levels of CO2 favor C3 photosynthetic plants over C4 photosynthetic plants. This selective favorability of increased CO2 also affects insects’ dependant on the specific plants.
I. Phenological Changes and decline in Pollinators:
Plants, insects and birds are sensitive in certain times of their life history to temperature and humidity and also depend on each other’s life histories. Due climate change the plants tend to bloom earlier. This earlier blooming causes a mismatch between appearances of pollinators. The late arrival of pollinators (compared to early blooms of plants) cost pollinators with less food and plants with less seed. Decline in number of seed and pollinator insect which are food sources for birds, affect bird populations.
D. Inouye (Inouye et al, 2008) have linked several points to Phenological changes in Perennial herbaceous wildflower species at Rocky Mountain Biological Laboratory (Colorado, USA), a. timing of snow melt, b. snow cap size, and c. elevation. Date of snow melts are important to number of flowers in summer. Small snow caps lead to early blooming and risks of frost damage. He also found that difference of 12 meters in elevation resulted in 2 degree of Celsius difference and 37% frost damage to buds. The number of flowers and seed is very important as less flowers and seed results decline in populations.
J. Paleoecological studies;
Margaret Davis of University of Minnesota and her colleague, in a review paper in 2005 argued that Paleoecologists looked to evolution as a mismatch to climate change. "Evolution is too slow as compared to climate change so primary biotic response to climate change is not "adaptation", but instead (1) persistence in situ if climate remains within species tolerance limits, (2) range shifts (migration) to regions where climate is currently within the species' tolerance limits, or (3) extinction".
She argues that, all these processes are evolutionary responses which are accompanied by shifts in biotic ranges during climate change. The evolutionary responses of plants differs greatly as herbs response in decades and trees respond in centuries and millennia (Margaret et al,2005).
A series of populations of Lodgepole pine (pinus contorta), were progressively lossing alleles when migrating northward in Holocene in Western Canada as a response to climate warming. This allele loss is determined by help of allozymes (Cwynar and MacDonald 1987).
Trees show different levels of adaptation as a response to climate change, Scots pine (Pinus sylvestris) have adapted to diverse elevations and altitudes of Finland over past several thousand years. This area was covered with continental ice until mid-Holocene (Hurme et al.1997). While Introduced herbaceous species in North America and Europe show genetic variation in response to climate within decades or a century for example, Verbascum thapsus, Reinartz 1984, Daucus carota, Lacey 1988: Solidago sp, (Weber and Schmid 1998).
Short tailed albatross is listed as a vulnerable species according to ICUN red list because it is it still has a very small breeding range, limited to Torishima and Minami-kojima (Senkaku Islands) with approximate populations of 2500 individuals. Oslon and Hearty (2003) have reported based on their conclusion of possible extinction of short-tailed albatross (Phoebastria albatrus) colony during Pleistocene sea level rise in Atlantic ocean.
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