Examples of the multicellular biodiversity of the Earth.
Global biodiversity is the measure of biodiversity on planet Earth and is defined as the total variability of life forms. More than 99 percent of all species[1] that ever lived on Earth are estimated to be extinct.[2][3] Estimates on the number of Earth's current species range from 2 million to 1 trillion, but most estimates are around 11 million species or fewer.[4] About 1.74 million species were databased as of 2018,[5] and over 80 percent have not yet been described.[6] The total amount of DNA base pairs on Earth, as a possible approximation of global biodiversity, is estimated at 5.0 x 1037, and weighs 50 billion tonnes.[7] In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon).[8]
In other related studies, around 1.9 million extant species are believed to have been described currently,[9] but some scientists believe 20% are synonyms, reducing the total valid described species to 1.5 million. In 2013, a study published in Science estimated there to be 5 ± 3 million extant species on Earth although that is disputed.[10] Another study, published in 2011 by PLoS Biology, estimated there to be 8.7 million ± 1.3 million eukaryotic species on Earth.[11] Some 250,000 valid fossil species have been described, but this is believed to be a small proportion of all species that have ever lived.[12]
Global biodiversity is affected by extinction and speciation. The background extinction rate varies among taxa but it is estimated that there is approximately one extinction per million species years. Mammal species, for example, typically persist for 1 million years. Biodiversity has grown and shrunk in earth's past due to (presumably) abiotic factors such as extinction events caused by geologically rapid changes in climate. Climate change 299 million years ago was one such event. A cooling and drying resulted in catastrophic rainforest collapse and subsequently a great loss of diversity, especially of amphibians.[13]
Known species
Insects make up the vast majority of animal species.[14]
Chapman, 2005 and 2009[9] has attempted to compile perhaps the most comprehensive recent statistics on numbers of extant species, drawing on a range of published and unpublished sources, and has come up with a figure of approximately 1.9 million estimated described taxa, as against possibly a total of between 11 and 12 million anticipated species overall (described plus undescribed), though other reported values for the latter vary widely. In many cases, the values given for "Described" species are an estimate only (sometimes a mean of reported figures in the literature) since for many of the larger groups in particular, comprehensive lists of valid species names do not currently exist. For fossil species, exact or even approximate numbers are harder to find; Raup, 1986[15] includes data based on a compilation of 250,000 fossil species so the true number is undoubtedly somewhat higher than this. The number of described species is increasing by around 18,000–19,000 extant, and approaching 2,000 fossil species each year, as of 2012.[16][17][18] The number of published species names is higher than the number of described species, sometimes considerably so, on account of the publication, through time, of multiple names (synonyms) for the same accepted taxon in many cases.
Based on Chapman's (2009) report,[9] the estimated numbers of described extant species as of 2009 can be broken down as follows:
The distribution of numbers of known and undescribed (estimated) species on Earth, grouped by major taxonomic groups; according to Chapman 2009. Absolute number of species on the left (orange = estimated number of yet to be described species, blue = already described). Right: percentage of species already described (green) and estimated to be not yet known (yellow).
Estimates of total number of species
However the total number of species for some taxa may be much higher.
In 1982, Terry Erwin published an estimate of global species richness of 30 million, by extrapolating from the numbers of beetles found in a species of tropical tree. In one species of tree, Erwin identified 1200 beetle species, of which he estimated 163 were found only in that type of tree.[26] Given the 50,000 described tropical tree species, Erwin suggested that there are almost 10 million beetle species in the tropics.[27] In 2011 a study published in PLoS Biology estimated there to be 8.7 million ± 1.3 million eukaryotic species on Earth.[11]
By 2017, most estimates projected there to be around 11 million species or fewer on Earth.[4] A 2017 study estimated there are around at least 1 to 6 billion species, 70-90% of which are bacteria.[4] A May 2016 study based on scaling laws estimated that 1 trillion species (overwhelmingly microbes) are on Earth currently with only one-thousandth of one percent described,[28][29] though this has been controversial and a 2019 study of varied environmental samples of 16S ribosomal RNA estimated that there exist 0.8-1.6 million species of prokaryotes.[30]
Indices to describe trends
After the Convention on Biological Diversity was signed in 1992, biological conservation became a priority for the international community. There are several indicators used that describe trends in global biodiversity. However, there is no single indicator for all extant species as not all have been described and measured over time. There are different ways to measure changes in biodiversity. The Living Planet Index (LPI) is a population-based indicator that combines data from individual populations of many vertebrate species to create a single index.[31] The Global LPI for 2012 decreased by 28%. There are also indices that separate temperate and tropical species for marine and terrestrial species.
The Red List Index is based on the IUCN Red List of Threatened Species and measures changes in conservation status over time and currently includes taxa that have been completely categorized: mammals, birds, amphibians and corals.[32] The Global Wild Bird Index is another indicator that shows trends in population of wild bird groups on a regional scale from data collected in formal surveys.[33] Challenges to these indices due to data availability are taxonomic gaps and the length of time of each index.
The Biodiversity Indicators Partnership was established in 2006 to assist biodiversity indicator development, advancement and to increase the availability of indicators.
Summary of major environmental-change categories that cause biodiversity loss. The data is expressed as a percentage of human-driven change (in red) relative to baseline (blue), as of 2021. Red indicates the percentage of the category that is damaged, lost, or otherwise affected, whereas blue indicates the percentage that is intact, remaining, or otherwise unaffected.[34]
Climate change is another threat to global biodiversity.[47][48] For example, coral reefs—which are biodiversity hotspots—will be lost by the year 2100 if global warming continues at the current rate.[49][50] Still, it is the general habitat destruction (often for expansion of agriculture), not climate change, that is currently the bigger driver of biodiversity loss.[51][52] Invasive species and other disturbances have become more common in forests in the last several decades. These tend to be directly or indirectly connected to climate change and can cause a deterioration of forest ecosystems.[53][54]
Groups that care about the environment have been working for many years to stop the decrease in biodiversity. Nowadays, many global policies include activities to stop biodiversity loss. For example, the UN Convention on Biological Diversity aims to prevent biodiversity loss and to conserve wilderness areas. However, a 2020 United Nations Environment Programme report found that most of these efforts had failed to meet their goals.[55] For example, of the 20 biodiversity goals laid out by the Aichi Biodiversity Targets in 2010, only six were "partially achieved" by 2020.[56][57]
This ongoing global extinction is also called the holocene extinction or sixth mass extinction.
^IISE (2010). SOS 2009: State of Observed Species. Arizona State University: International Institute for Species Exploration. pp. 1–8. Archived from the original(PDF) on 2013-09-22. Retrieved 2013-09-16.
^IISE (2011). SOS 2010: State of Observed Species. Arizona State University: International Institute for Species Exploration. pp. 1–10. Archived from the original(PDF) on 2013-09-22. Retrieved 2013-09-16.
^Proceedings of the National Academy of Sciences, Census of Marine Life (CoML) BBC News
^David L. Hawksworth, "The magnitude of fungal diversity: the 1•5 million species estimate revisited" Mycological Research (2001), 105: 1422-1432 Cambridge University Press Abstract
^Pawlowski, J. et al. (2012). CBOL Protist Working Group: Barcoding Eukaryotic Richness beyond the Animal, Plant, and Fungal Kingdoms. PLoS Biol 10(11): e1001419. doi:10.1371/journal.pbio.1001419, [1].
^Adl, S. M. et al. (2007). Diversity, nomenclature, and taxonomy of protists. Systematic Biology 56(4), 684-689, [2].
^Erwin, Terry L. (March 1982). The Coleopterists Society (ed.). "Tropical Forests: Their Richness in Coleoptera and Other Arthropod Species". The Coleopterists Bulletin. 36 (1): 74–75. ISSN0010-065X. JSTOR4007977.
^ abBradshaw, Corey J. A.; Ehrlich, Paul R.; Beattie, Andrew; Ceballos, Gerardo; Crist, Eileen; Diamond, Joan; Dirzo, Rodolfo; Ehrlich, Anne H.; Harte, John; Harte, Mary Ellen; Pyke, Graham; Raven, Peter H.; Ripple, William J.; Saltré, Frédérik; Turnbull, Christine; Wackernagel, Mathis; Blumstein, Daniel T. (2021). "Underestimating the Challenges of Avoiding a Ghastly Future". Frontiers in Conservation Science. 1. doi:10.3389/fcosc.2020.615419.
^Stokstad, Erik (6 May 2019). "Landmark analysis documents the alarming global decline of nature". Science. doi:10.1126/science.aax9287. For the first time at a global scale, the report has ranked the causes of damage. Topping the list, changes in land use—principally agriculture—that have destroyed habitat. Second, hunting and other kinds of exploitation. These are followed by climate change, pollution, and invasive species, which are being spread by trade and other activities. Climate change will likely overtake the other threats in the next decades, the authors note. Driving these threats are the growing human population, which has doubled since 1970 to 7.6 billion, and consumption. (Per capita of use of materials is up 15% over the past 5 decades.)
^Pimm SL, Jenkins CN, Abell R, Brooks TM, Gittleman JL, Joppa LN, Raven PH, Roberts CM, Sexton JO (May 2014). "The biodiversity of species and their rates of extinction, distribution, and protection". Science. 344 (6187): 1246752. doi:10.1126/science.1246752. PMID24876501. S2CID206552746. The overarching driver of species extinction is human population growth and increasing per capita consumption.
^Cafaro, Philip; Hansson, Pernilla; Götmark, Frank (August 2022). "Overpopulation is a major cause of biodiversity loss and smaller human populations are necessary to preserve what is left"(PDF). Biological Conservation. 272. 109646. Bibcode:2022BCons.27209646C. doi:10.1016/j.biocon.2022.109646. ISSN0006-3207. S2CID250185617. Archived(PDF) from the original on December 8, 2023. Retrieved December 25, 2022. Conservation biologists standardly list five main direct drivers of biodiversity loss: habitat loss, overexploitation of species, pollution, invasive species, and climate change. The Global Assessment Report on Biodiversity and Ecosystem Services found that in recent decades habitat loss was the leading cause of terrestrial biodiversity loss, while overexploitation (overfishing) was the most important cause of marine losses (IPBES, 2019). All five direct drivers are important, on land and at sea, and all are made worse by larger and denser human populations.
^Crist, Eileen; Mora, Camilo; Engelman, Robert (21 April 2017). "The interaction of human population, food production, and biodiversity protection". Science. 356 (6335): 260–264. Bibcode:2017Sci...356..260C. doi:10.1126/science.aal2011. PMID28428391. S2CID12770178. Retrieved 2 January 2023. Research suggests that the scale of human population and the current pace of its growth contribute substantially to the loss of biological diversity. Although technological change and unequal consumption inextricably mingle with demographic impacts on the environment, the needs of all human beings—especially for food—imply that projected population growth will undermine protection of the natural world.
^Hughes, Alice C.; Tougeron, Kévin; Martin, Dominic A.; Menga, Filippo; Rosado, Bruno H. P.; Villasante, Sebastian; Madgulkar, Shweta; Gonçalves, Fernando; Geneletti, Davide; Diele-Viegas, Luisa Maria; Berger, Sebastian; Colla, Sheila R.; de Andrade Kamimura, Vitor; Caggiano, Holly; Melo, Felipe (2023-01-01). "Smaller human populations are neither a necessary nor sufficient condition for biodiversity conservation". Biological Conservation. 277: 109841. Bibcode:2023BCons.27709841H. doi:10.1016/j.biocon.2022.109841. ISSN0006-3207. Through examining the drivers of biodiversity loss in highly biodiverse countries, we show that it is not population driving the loss of habitats, but rather the growth of commodities for export, particularly soybean and oil-palm, primarily for livestock feed or biofuel consumption in higher income economies.
^Finch, Deborah M.; Butler, Jack L.; Runyon, Justin B.; Fettig, Christopher J.; Kilkenny, Francis F.; Jose, Shibu; Frankel, Susan J.; Cushman, Samuel A.; Cobb, Richard C. (2021). "Effects of Climate Change on Invasive Species". In Poland, Therese M.; Patel-Weynand, Toral; Finch, Deborah M.; Miniat, Chelcy Ford (eds.). Invasive Species in Forests and Rangelands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Cham: Springer International Publishing. pp. 57–83. doi:10.1007/978-3-030-45367-1_4. ISBN978-3-030-45367-1. S2CID234260720.
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