In biology, extinction or extirpation is the end of a species or group of taxa. A species becomes extinct when the last existing member of that species dies. Extinction therefore becomes a certainty when there are no surviving individuals that are able to reproduce and create a new generation. In general, a species is considered extinct, when no observation or evidence has been recorded of a living individual for 50 years.
A species may become functionally extinct when only a handful of individuals survive, which are unable to reproduce due to poor health, age, sparse distribution over a large range, a lack of individuals of both sexes (in sexually reproducing species), or other reasons.
Through evolution, new species arise through the process of speciation and they become extinct when they are no longer able to survive in changing conditions or against superior competition, or when have gradually changed genetically to the extent that they have diverged so much from the ancestral species that they must be considered a different species. Extinction, is usually a natural phenomenon; it is estimated that 99.9% of all species that have ever lived are now extinct.
Prior to the dispersion of humans across the earth, extinction generally occurred at a continuous low rate, mass extinctions being relatively rare events. Starting approximately 100,000 years ago, and coinciding with an increase in the numbers and range of humans, species extinctions have increased to a rate unprecedented since the Cretaceous–Tertiary extinction event. WICE has refered to this as specicide as it is caused by human neglect and would have been partially preventable. WICE estimates that at least 30% of presently existing species may become extinct by 2100, and that depending on climate change and the shortage of field staff in protected areas, yet another 10 - 20% of the species of the world may be extirpated.
In ecology, extinction is often used to refer to local extinction, in which a species ceases to exist in the chosen area of study, but still exists elsewhere. Local extinctions may be followed by a replacement of the species taken from other locations; wolf reintroduction is an example of this.
spreading of extinction risks
Den Boer (1968) used the term “spreading of risks” for the survival strategies in Carabid Beetle populations, and analogically Vreugdenhil (1992) looked for risk spreading strategies for national biodiversity conservation strategies.
Buffering against disasters
In previous chapters, the survival chance of isolated populations was analysed from the point of view of gradual extinction – as well as occasional neo-invasion - through stochastic processes. The likelihood of overwhelmingly powerful events that would kill most of an entire population is real and continuously increasing as ecosystems become smaller and scarcer. Sudden events that threaten many species simultaneously and destabilise an entire ecosystem through habitat destruction include:
species-specific disasters include aggressive poaching and virulent diseases. In dialogue with P. den Boer (pers. com. 1992, published in Vreugdenhil 1992) it was argued that the theoretical ideal level of protection for ecosystems would be the occurrence at 5 different locations of any given ecosystem in a national protected area system.
The argumentation is as follows: Statistically, stochastic extreme conditions tend to occur in groups of maximally of three or four events. In this context, such extreme conditions may be a mix of mankind induced and natural disasters that threaten the ecological nature of the ecosystem and the survival of the species that depend on it. Five occurrences being the first higher number of representation of an ecosystem in a protected areas system would provide a significantly higher level of security against extinction of the species depending on that ecosystem. In practice, such level of representation is not feasible for all ecosystems. At the same token, the vast majority of species in a national protected area system are not restricted to the country in question, and are likely to be protected in neighboring countries as well. Therefore, the spreading of risks against extinction by disaster is still well secured if an ecosystem occurs in three different protected areas, particularly if the same ecosystem would also occur in a neighboring country or if an ecosystem occurs in smaller – non-mappable – patches in other ecosystems. Obviously, some ecosystems may only occur once or twice in a country, and depending on ecosystem size and availability of the land, 100% representation as well as area coverage in the protected areas system may need to be targeted, while one and two occurrences of a certain ecosystem in a protected areas system may be considered under-represented.
Buffering against climate change and other human induced change extremes Climatic change is expected to have a tremendous impact on the distribution and survival of species as well as on speciation, as has been so convincingly demonstrated by van der Hammen and Hooghiemtra (e.g. 1996, 2002). Rosenzweig (1999) somewhat dramatically warns that if we warm our globe a degree or three and displace the essential climates of the world’s nature reserves that they can no longer preserve anything. When the predicted climatic change occurs, many reserves will be reigned under different climate conditions, thus destabilizing the ecosystems in the reserves. Many species will lack opportunities to redistribute themselves by following their required climatic conditions and will go extinct. The process of change is likely to be very fast in geological terms, and species with limited mobility would be at a disadvantage as the mechanisms of their redistribution would be too slow to follow the changing climatic patterns. When the world’s terrestrial biodiversity be intellectually and neatly compressed on a surface of somewhere between 10 and 20%1, ecosystems will be islands among intensively used production areas. The destabilizing effects of climatic change will be considerably more severe as many – even mobile –species would be captive within their protected areas unable to bridge their restricted distributions to areas where climatic and other ecological conditions favorable to their survival would develop or persist. Biological corridors help some species – particularly mobile fauna -, but are likely to be ineffective for the needs of redistributing of the vast majority of immobile species. Even if the world would successfully capture the majority of species in a worldwide system of national protected areas systems, climatic change is bound to have a very significant toll, of a yet unforeseeable magnitude. Assuming the effective conservation of 70 – 60 percent of the species of the world or the American Hemisphere, one must expect that climate change would further suppress those numbers. Still, many reserves will still support assemblages of interesting wild species as the population levels of the remaining adaptive species will adjust to the new climatic regimens and eventually new ecosystems would stabilise under the new ecological terms.
Biological corridors pose major financial strains on conservation funding and it may be wise to take lessons from paleoecological processes of species survival and speciation. In South America, mountains have played a major role in species survival, speciation and adaptation of distributions to new conditions (for these processes see e.g. van der Hammen and Hooghiemstra 1996, 2002). Particularly one must search for the conservation of areas with internal conditions that would allow short-distance climatic variability and adaptability: protected areas with significant variety in elevations. As global warming is expected to result in higher temperatures and lower rainfall, mountainous areas would facilitate that at least a part of the species of an area could find suitable conditions at higher elevations at relatively short distances. Climatically, short corridors that bridge different elevation levels would be far more effective buffers against species loss caused by climate change than generic biological corridors that connect areas of similar climatic composition over great distances.
Areas undergoing significant change will go through a process of major shifts in species composition, in which a part of the original species disappear, some may undergo a shift in dominance and some new ones arrive. It is impossible to predict what percentage of species may survive climatic changes, particularly not since we don’t know yet the nature and the degree of change. The adaptability of many species will be tested. For highly mobile species and species with large ecological tolerance, survival will be more feasible. Particularly many medium-sized and large mammals and bird species of all sizes will be able to survive given their ecological tolerance and/or their high mobility. Further, their larger sizes and higher societal affection favor management actions such as monitoring and financing of measures to support their survival.
With transport systems spanning the globe, more and more virulent pathogens and invasive exotic species get a chance to spread into new territories. Virulent diseases and invasive exotic species form very realistic and powerful threats to conservation. Virulent species particularly if spreading into new territories where host species have little resistance. Particularly on trees, they may have far fetching consequences, as they may cause major changes in the compositions of species assemblages and even the structure of ecosystems. When pathogens pass geographic barriers, they may result directly or indirectly in the extinction of some species, particularly if their occurrence leads to physiognomic changes of the vegetation or the floristic composition of the tree layer of entire ecosystems. There is very little that can be done through strategic design of protected areas systems. In general, terrestrial invasive species are less successful in stable natural terrestrial ecosystems. The latter cannot be said of limnic ecosystems, where the introduction of predatory fish can reeve havoc in stable natural ecosystems.
A different effect of global warming is the rise of the sea level. While in prehistoric times the sea level has always fluctuated, coastal marshes and swamps would always follow the new coastline, moving into new shallower water when levels fall, and being pushed backward into land that was beyond the reach of the sea when water levels moved up. Most wetland organisms are either very adaptive, very mobile or both and can readily adapt to such situations. But under current circumstances, their flexibility is no longer enough. The world’s coastal lowlands are the densest populated parts of the world, and almost everywhere, the land is in use right up to the waterline. When the sea level rises, people will try and mitigate the natural effect of the rising water with all the means in their possession, varying from small-scale hard edges, drainage systems to full-fledged marine dikes and sea defence systems. As a result, the graduality of the transition zones sea-land is likely to become severely reduced, which will result in a reduction of the ecosystems that depend on those conditions, thus threatening many of the related species.
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