Recently extinct mammals are defined by the International Union for Conservation of Nature (IUCN) as any mammals that have become extinct since the year 1500 CE. Since then, roughly 80 mammal species have become extinct. Extinction of taxa is difficult to confirm, as a long gap without a sighting is not definitive, but before 1995 a threshold of 50 years without a sighting was used to declare extinction. One study found that extinction from habitat loss is the hardest to detect, as this might only fragment populations to the point of concealment from humans. Some mammals declared as extinct may very well reappear. For example, a study found that 36% of purported mammalian extinction had been resolved, while the rest either had validity issues (insufficient evidence) or had been rediscovered. As of December 2015, the IUCN listed 30 mammalian species as "critically endangered (possibly extinct)". Conventions All species listed as "Extinct" are classified as being extinct (no known remaining individuals left) by the International Union for Conservation of Nature (IUCN). All species listed as Extinct in the wild are classified as being extinct in the wild, meaning that all remaining individuals of the species reside in captivity. All species listed as "Possibly extinct" are classified as being critically endangered, as it is unknown whether or not these species are extinct. Extinct subspecies such as the Javan tiger (Panthera tigris sondaica) are not listed here as the species, in this case Panthera tigris, is still extant. The IUCN Redlist classification for each species serves as a citation, and the superscripted "IUCN" by the date is a link to that species' page. A range map is provided wherever available, and a description of their former or current range is given if a range map is not available. Causes of extinction Habitat degradation is currently the main anthropogenic cause of species extinctions. The main cause of habitat degradation worldwide is agriculture, with urban sprawl, logging, mining and some fishing practices close behind. The physical destruction of a habitat, both directly (deforestation for land development or lumber) and indirectly (burning fossil fuels), is an example of this. Also, increasing toxicity, through media such as pesticides, can kill off a species very rapidly, by killing all living members through contamination or sterilizing them. Persistent organic pollutants (POPs), for example, can bioaccumulate to hazardous levels, getting increasingly more dangerous further up the food chain. Disease can