Genetic erosion
Genetic erosion is a process whereby an already limited gene pool of an endangered species of plant or animal diminishes even more when individuals from the surviving population die off without getting a chance to meet and breed with others in their endangered low population (see: Small population size).
Genetic erosion occurs because each individual organism has many unique genes which get lost when it dies without getting a chance to breed. Low genetic diversity in a population of wild animals and plants leads to a further diminishing gene pool, inbreeding and a weakening immune system and fast tracks that species towards eventual extinction.
All the world's endangered species are plagued by varying degrees of Genetic Erosion and most need a human assisted breeding program to keep their population viable and to keep them from going extinct in the long run. The more critically endangered the specie is the more magnified the effect of genetic erosion gets when each surviving individual of the species is lost without getting a fair chance to breed.
Genetic erosion gets compounded and accelerated by habitat fragmentation, today most endangered species live in smaller and smaller chunks of fragmented habitat interspersed with human settlements and farmland making it impossible for them to naturally meet and breed with others of their kind, many die off without getting a fair chance to breed and pass on their genes in the living population.
The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population.  A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see inbreeding and population bottlenecks) can cause reduced biological fitness and an increased chance of extinction.

# Processes and consequences
A population bottleneck creates a shrinking gene pool that leaves fewer and fewer mating partners. What are the genetic implications? The animals become part of a high stakes poker game -- with a crooked dealer. After beginning with a 52-card deck, the players wind up with, say, five cards that they are dealt over and over. As they begin to inbreed, congenital effects appear, both physical and reproductive. Often abnormal sperm increase; infertility rises; the birthrate falls. Most perilous in the long run, each animal's immune defense system is weakened. Thus, even if an endangered species in a bottleneck can withstand whatever human development may be eating away at its habitat, it still faces the threat of an epidemic that could well be fatal to the entire population.
SOURCE: GENETIC EROSION A Global Dilemma by Stephen J. O'Brien; Chief, Laboaratory of Viral Carcinogenesis, National Cancer Institute; National Geographic, April 1992, page 136; Accessed on October 20, 2007; Posted online by Oslo Cyclotron Laboratory at the Department of Physics, UiO; The University of Oslo in Norway

# Genetic erosion in agricultural and livestock biodiversity
Genetic erosion in agricultural and livestock biodiversity is the loss of genetic diversity, including the loss of individual genes, and the loss of particular combinants of genes (or gene complexes) such as those manifested in locally adapted landraces of domesticated animals or plants adapted to the natural environment in which they originated. The term genetic erosion is sometimes used in a narrow sense, such as for the loss of alleles or genes, as well as more broadly, referring to the loss of varieties or even species. The major driving forces behind genetic erosion in crops are: variety replacement, land clearing, overexploitation of species, population pressure, environmental degradation, overgrazing, policy and changing agricultural systems.
The main factor, however, is the replacement of local varieties of domestic plants and animals by high yielding or exotic varieties or species. A large number of varieties can also often be dramatically reduced when commercial varieties (including GMOs) are introduced into traditional farming systems. Many researchers believe that the main problem related to agro-ecosystem management is the general tendency towards genetic and ecological uniformity imposed by the development of modern agriculture.

# Human intervention, modern science and safeguards to guard against genetic erosion

## In-situ conservation
With advances in modern science several techniques and safeguards have emerged to check the relentless advance of genetic erosion and the resulting acceleration of endangered species towards extinction. However many of these techniques and safeguards are too expensive yet to be practical, the best way to protect species is to protect their habitat and to let them live in it naturally.
Wildlife sanctuaries and national parks have been created to preserve entire ecosystems with all the web of species which call them home. Wildlife corridors are created to join fragmented habitats (see Habitat fragmentation) to enable endangered species to travel, meet and breed with others of their kind. Scientific conservation and modern wildlife management techniques with the help of scientifically trained staff help manage these protected ecosystems and the wildlife found in them. Wild animals are also translocated and reintroduced to other locations physically when fragmented wildlife habitat is too far and isolated to be able to link it with a wildlife corridor or when local extinction has already occurred.

## Ex-situ conservation
Modern policies of the zoo associations and zoos around the world have changed to putting extreme importance on keeping and breeding wild sourced pure species and subspecies of animals and birds in their registered endangered species breeding programs which will have a chance to be reintroduced and survive in the wild. Main objectives of zoos today has changed to breed pure breed species and subspecies to assist conservation efforts in the wild. Zoos do this by maintaining extremely detailed scientific breeding records i.e. studbooks and loaning their pure breed wild animals and birds to other zoos around the country and indeed globally for breeding to safeguard against inbreeding and hybrids which are considered genetically compromised thus not fit for reintroduction in the wild and in the case of unnaturally found hybrids also to guard against genetic pollution in naturally evolved, region specific, pure wild stocks.
Costly and sometimes controversial ultra modern Ex-situ conservation techniques have emerged for saving the genetic biodiversity on our planet and the diversity in their gene pool by guarding against Genetic erosion through modern concepts like seedbanks, sperm banks, tissue banks, etc. Genetic diversity, DNA, Sperms, eggs, embryos can now be frozen and kept in special banks and laboratories which are sometimes called Modern Noha's Ark or Frozen Zoos where modern cryopreservation techniques are used to freeze these living materials and yet keep them alive by storing them submerged in liquid nitrogen tanks. Thus preserved material can then be used for Artificial insemination, in vitro fertilization, embryo transfer and cloning etc. to protect diversity in the gene pool of critically endangered species.
It is today possible to save Endangered species from Extinction by preserving parts like tissue, sperms, eggs etc. even after the death of a critically endangered animal or collected from one found freshly dead in captivity or from wild and resurrect it with the help of cloning and give it another chance to breed its genes into the living population of the respective species which is threatened with extinction. Resurrection of dead critically endangered wildlife with the help of cloning is still being perfected and is still too expensive to be practical but with time and advancement is science it may well become a routine procedure in the near future. However Modern Noha's Ark or Frozen Zoos and the use of modern cryopreservation techniques makes lot of sense to preserve living material cheaply which future generations of mankind may well use to diversify limited gene pools of endangered species.