question: How do scientifics estimate the remaining population of certain species on earth and how accurate is it?
comment: Most such species have been studied for years. Annual population counts, perhaps of one habitat, feed models that predict the total population. Estimates are Ok, often within 10%.
passages: Lists of organisms by population
  
  This is a collection of lists of organisms by their population. While most of the numbers are estimates, they have been made by the experts in their fields. Species population is a science falling under the purview of population ecology and biogeography. Individuals are counted by census, as carried out for the piping plover; using the transect method, as done for the mountain plover; and beginning in 2012 by satellite, with the emperor penguin being first subject counted in this manner.
  
  Section::::Number of species.
   Based on Chapman's (2009) report, the estimated numbers of described extant species as of 2009 can be broken down as follows:
  
  Section::::Estimates of total number of species.
  
  However the "total" number of species for some taxa may be much higher. 
  
  BULLET::::- 10–30 million insects;
  
  BULLET::::- 5–10 million bacteria;
  
  BULLET::::- 1.5 million fungi;
  
  BULLET::::- ~1 million mites
  
  BULLET::::- ~1 million protists
   Lists of mammals by population
  
  This is a collection of lists of mammal species by the estimated global population, divided by orders. (Lists only exist for some orders; e.g. the most diverse order - rodents - is missing.) Much of the data in these lists were created by the IUCN's Global Mammal Assessment Team, which consists of 1700 mammalogists from over 130 countries. They recognize 5488 species in the class.
   These lists are not comprehensive, as not all mammals have had their numbers estimated. For example, a live specimen of the spade-toothed whale was first observed in December 2010, and the event only recognized as such in November 2012; no estimate yet exists for the global population. The accuracy of the quote numbers may only be an order of magnitude.
  
  It is estimated that the total number of wild mammals in the world is about 130 billion.
  
  Section::::Lists by taxonomic order.
   This list is incomplete, because experts have not estimated all bird numbers. For example, the spectacled flowerpecker was only discovered in 2010, and has yet to be classified with a Linnean name, but would add to the other 73 new bird species described by ornithologists from 2000 – 2009. Global population estimates for many of these at this time would lack accuracy.
   List of birds by population
  
  This is a list of bird species by global population, divided by bird classification. While numbers are estimates, they have been made by the experts in their fields. For more information on how these estimates were ascertained, see Wikipedia's articles on population biology and population ecology. Contributing organizations include the IUCN, BirdLife International, and Partners in Flight.
   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. 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. 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. Challenges to these indices due to data availability are taxonomic gaps and the length of time of each index. 
   The measure of ongoing species loss is made more complex by the fact that most of the Earth's species have not been described or evaluated. Estimates vary greatly on how many species actually exist (estimated range: 3,600,000-111,700,000) to how many have received a species binomial (estimated range: 1.5-8 million). Less than 1% of all species that have been described beyond simply noting its existence. From these figures, the IUCN reports that 23% of vertebrates, 5% of invertebrates and 70% of plants that have been evaluated are designated as endangered or threatened. Better knowledge is being constructed by The Plant List for actual numbers of species.
   List of Charadriiformes by population
  
  This is a list of Charadriiformes species by global population. While numbers are estimates, they have been made by the experts in their fields.
   As of 2014, the Living Planet Database (LPD) is maintained by ZSL, and contains more than 20,000 population trends for more than 4,200 species of fish, amphibians, reptiles, birds and mammals.
  
  The global LPI is calculated using over 14,000 of these population time-series which are gathered from a variety of sources such as journals, online databases and government reports.
  
  A generalized additive modelling framework is used to determine the underlying trend in each population time-series. Average rates of change are calculated and aggregated to the species level.
   A variety of methods are used for counting penguins, and April 2012 saw their first census from space, when imagery from Ikonos, QuickBird-2, and WorldView-2 satellites were used to count Antarctican emperors. This is a similar technique to that used by the UNHCR to count humans in Somalia. Most maritime surveys use strip transect and distance sampling to measure density; this is then extrapolated over the animal's range. The Galapagos has been counted annually since 1961 by the Galápagos National Park Service. By land and sea, they carry out a full census in ten areas and partial census in four. The 2012 observation of 721 birds showed that levels have remained the same over recent years, and the current full estimate need not be changed. For more information on how these estimates were ascertained, see Wikipedia's articles on population biology and population ecology.
   List of Anseriformes by population
  
  This is a list of Anseriformes species by global population. While numbers are estimates, they have been made by the experts in their fields.
   List of Ciconiiformes by population
  
  This is a list of Ciconiiformes species by global population. While numbers are estimates, they have been made by the experts in their fields. For more information on how these estimates were ascertained, see Wikipedia's articles on population biology and population ecology.
  
  This list is incomprehensive, as not all Ciconiiformes have had their numbers quantified. The classification of this order of birds is currently in flux; the classification for this list is aligned with the IUCN's current position, but may change in the future.
   Living Planet Index
  
  The Living Planet Index (LPI) is an indicator of the state of global biological diversity, based on trends in vertebrate populations of species from around the world. The Zoological Society of London (ZSL) manages the index in cooperation with the World Wide Fund for Nature (WWF) a.k.a. the World Wildlife Federation. 
  
  As of 2018, the index is statistically created from journal studies, online databases and government reports for 16,704 populations of 4,005 species of mammal, bird, reptile, amphibian and fish, or approximately six percent of the world's vertebrate species.
  
  Section::::Results.
   In other related studies, around 1.9 million extant species are believed to have been described currently, 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. Another study, published in 2011 by PLoS Biology, estimated there to be 8.7 million ± 1.3 million eukaryotic species on Earth. 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.
   A variety of sampling methods are used to measure abundance. For larger animals, these may include spotlight counts, track counts and roadkill counts, as well as presence at monitoring stations. In many plant communities the abundances of plant species are measured by plant cover, i.e. the relative area
   The number of species missing from the data set (the missing area to the left of the veil line) is simply "N" minus the number of species sampled. Preston did this for two lepidopteran data sets, predicting that, even after 22 years of collection, only 72% and 88% of the species present had been sampled.
  
  Section::::Understanding relative species abundance patterns.:Descriptive approaches.:Yule model (Nee 2003).
   While the World Conservation Union (IUCN) mostly only categorizes whole species or subspecies, assessing the global risk of extinction, in some cases it also assesses the risks to stocks and populations, especially to preserve genetic diversity. In all, 119 stocks or subpopulations across 69 species have been assessed by the IUCN in 2006.
  
  Examples of stocks and populations assessed by the IUCN for the threat of local extinction:
  
  BULLET::::- Marsh deer (three subpopulations assessed)
  
  BULLET::::- Blue whale, North Pacific stock and North Atlantic stock
  
  BULLET::::- Bowhead whale, "Balaena mysticetus" (five subpopulations assessed), from Critically Endangered to LR/cd
   More than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. According to another study, the number of described species has been estimated at 1,899,587. 2000–2009 saw approximately 17,000 species described per year. The total number of undescribed organisms is unknown, but marine microbial species alone could number 20,000,000. The number of quantified species will "ipso facto" always lag behind the number of described species, and species contained in these lists tend to be on the K side of the r/K selection continuum. More recently, in May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described. The total number of related DNA base pairs on Earth is estimated at 5.0 x 10 and weighs 50 billion tonnes. In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC (trillion [million million] tonnes of carbon). In July 2016, scientists reported identifying a set of 355 genes from the Last Universal Common Ancestor (LUCA) of all organisms living on Earth.
   A variety of methods are used for counting Charadriiformes. For example, the piping plover is subject to the quinquennial "Piping Plover International Census", which is carried out in 9 Canadian provinces, 32 US states, Mexico, Central America, and the Caribbean. In the 2006 survey, Saskatchewan alone had 159 volunteers scour 294 waterbodies. The mountain plover has had its nests counted through the drive transect method. Once density has been calculated, the numbers are extrapolated over a bird's range. For more information on how these estimates were ascertained, see Wikipedia's articles on population biology and population ecology.
   covered by different plant species in a small plot. Abundance is in simplest terms usually measured by identifying and counting every individual of every species in a given sector. It is common for the distribution of species to be skewed so that a few species take up the bulk of individuals collected. 
  
  Relative species abundance is calculated by dividing the number of species from one group by the total number of species from all groups. 
  
  Section::::Community ecology.
   If species richness of the obtained sample is taken to represent species richness of the underlying habitat or other larger unit, values are only comparable if sampling efforts are standardised in an appropriate way. Resampling methods can be used to bring samples of different sizes to a common footing. Properties of the sample, especially the number of species only represented by one or a few individuals, can be used to help estimating the species richness in the population from which the sample was drawn.
  
  Section::::Trends in species richness.
   HBW and BirdLife International Illustrated Checklist of the Birds of the World
  
  The HBW and BirdLife International Illustrated Checklist of the Birds of the World is a checklist of the birds of the world published by Lynx Edicions in association with BirdLife International in two volumes in 2014 and 2016. This list follows the 16-volume "Handbook of the Birds of the World" and is used as a base for the birds in the IUCN Red List of Threatened Species and many other organizations.
   According to NASA in 2005, there were over 400 billion trees on our globe. However, more recently, in 2015, using better methods, the global tree count has been estimated at about 3 trillion. Other studies show that the Amazonian forest alone yields approximately 430 billion trees. Extrapolations from data compiled over a period of 10 years suggest that greater Amazonia, which includes the Amazon Basin and the Guiana Shield, harbors around 390 billion individual trees.
  
  Section::::See also.
  
  BULLET::::- Largest organisms
  
  BULLET::::- List of longest-living organisms
  
  BULLET::::- List of organisms by chromosome count
  
  BULLET::::- Lists of animals
   BULLET::::- "Mammals on Roads" (2001–present): an annual survey collecting records of mammal road casualties ('roadkill') on journeys along single carriageways. Changes in counts (over a given distance) from year to year are used to estimate changes in the wider population.
  
  BULLET::::- "The Traditional Orchard Survey of England and Wales":" "(2006 – present): orchards and fruit trees provide a highly biodiverse habitat and are listed as a Priority Habitat under the UK Biodiversity Action Plan (BAP). PTES is creating a UK inventory and map of the habitat.
  