What is background extinction

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every species that have ever lived on Earth, 99 are no longer in existence. However, occurrences of species extinction can be split into two categories: mass extinctions, in which substantial percentages of the global diversity of life disappear within a geologically brief interval; and background extinctions, which represent a much lower rate of species loss that occurs routinely over geologically long intervals between mass extinctions see illustration.

Background extinctions are ongoing consequences of normal environmental changes, local catastrophes, or interspecies competition. On the basis of the fossil record, evolutionary paleontologists have generally estimated that the baseline level of background extinction is approximately 1 extinction per million species per year however, other analyses indicate that the rate might be as low as a tenth of that figure, so further investigations are necessary. Small mammals can be compared with short-lived, genetically homozygous plant species with high reproductive output, whose lifespans do not generally exceed a year.

Voles provide a good example. In Microtus subterraneous , genetic diversity is predictably low H e , 0. Attributing spontaneous embryonic abortion rates in both higher plants and animals to the same cause, i.

But consider that both groups are eukaryotes, have virtually identical genetic systems, as well as the same mode of gene action. In many ways small mammals might be more vulnerable to short-term extinction than short-lived plants because annual and short-lived perennials are able to survive decades of adverse environments as seeds. For example, the seed banks of many desert annuals are well known to remain dormant for decades before a favourable environmental event stimulates germination.

Although small mammals can survive adverse seasonal environments by hibernation, we are unaware that they can hibernate for more than a single season.

Thus, small mammals have no apparent multiple-year escapes from shifting environments, except emigration to more favourable habitats, if available. Such an emigration has occurred among small mammals in Yosemite National Park, California, where small mammals have expanded their distributions by migrating to higher elevations as climate has warmed Craig et al. In contrast, the pika, a small mammal endemic in the alpine ecosystems of the western USA, has no such migratory options.

An additional example is discussed under Habitat Dissolution. Thus, short-lived species sacrifice genetic variability for short-term reproductive success, but as a consequence are subject to rapid extinction if environments change.

In contrast, long-lived heterozygous organisms, because of their heterotic advantages, are not only able to survive relatively short-term environmental extremes, but can often persist for relatively long periods following environmental shifts while their populations slowly decline and ultimately succumb to background extinction. This is the most vital of issues because adaptations to environmental shifts especially climatic often occur as changes in the physio-biochemical functions that comprise the norm of reaction.

If environmental extremes approach lethal thresholds over multiple generations, and the norm of reaction is unable to adapt, reproductive fitness will necessarily be sacrificed in favour of selection for individual survival. Population decline will inevitably follow, as demonstrated in tropical lizards Sinervo et al. Likewise, the first response of higher plants subjected to water stress, or other resource limitations, will be to reduce or even eliminate reproductive growth Harper, Similarly, animals forego reproduction in resource-limited seasons.

However, results are rarely related to long-term studies of change in population size e. We are accustomed to thinking of selection as a positive force leading to increased reproductive fitness that is universally recognized as the mechanism of adaptation and speciation. But selection has a broader role in biology and both evolution and devolution can be thought of as the products of either positive or negative selection Raup, ; Grant, Evolution and devolution are merely subordinate categories of the more encompassing science of natural selection that is also applicable to other fields, e.

But we are unaware that he ever expanded the idea and also suspect that he was thinking largely in terms of competition. The loss of reproductive fitness as the mechanism of background extinction does not as might be supposed contradict Darwin and Wallace's tenet that excess production of progeny is a major feature of evolutionary theory.

This is largely a continuous, ongoing process in most biotic communities, where there are typically a few species that are expanding evolving and a few that are in decline devolving.

In many such cases, the devolving populations are, or will become, relicts. As an example, there are five species of flowering plants in the northern Mojave Desert of California, from a total number species listed by DeDecker , that have exceedingly limited, if any, reproductive potential Wiens et al. Of these five species, at least four can be considered relicts. In addition, three species from the California chaparral exhibit similar loss of reproductive capacity Wiens et al.

In a year empirical study of population demographics in Adenostoma sparsifolium , a rosaceous shrub endemic in the California chaparral, there is clear evidence of attritional, devolutionary population decline Wiens et al.

This loss is attributable to a climatic shift toward increasing aridity particularly the loss of summer rain during the mid-Holocene thermal optimum hypsithermal Axelrod, This species is characterized by either the virtual absence of seedlings following wildfires, or the inability of occasional seedlings to survive the extended summer drought typical of the present chaparral climate. In the geological short-term hundreds, thousands?

Additional mortality from prolonged 5-year drought and stochastic environmental disturbances will accelerate the process. Thus, in A. Discussion to this point has centred largely on climatic shifts in relation to background extinction. However, biological factors are also important. Typically these include coextinction, competition, disease, and predation. There is an immense literature on these subjects and we will only present a brief synopsis. With the exception of coextinction and predation, these appear to be of less importance in devolutionary decline and background extinction than climatic change.

Coextinction will probably become a critical factor in extinction as members of co-evolved systems e. Thus, coextinction, unlike the other factors, has the potential for initiating a cascading series of extinction events. Competition is perhaps more important among animals than plants, and the result is more often avoidance rather than the extinction of one of the competing species.

Disease and pandemics can result in dramatic population crashes, e. Inevitably, there are some resistant survivors in most populations.

The introduction of alien predators is the most important single biological cause of extinction and a number of such extinctions are now well documented. The changes necessary for adaptation by prey species to coexist with recently introduced predators are far greater than is possible over the time periods available.

The most important aspect of biological causes in terms of extinction is perhaps their interaction with other changes in the environment that produce the multiplicity of factors that typically result in extinction.

Background extinction is divisible into two intergrading stages: habitat attenuation and habitat dissolution. However, patterns do not necessarily explain the mechanism responsible for background extinctions. Such shifts can result in the reduction and fragmentation of species into a series of smaller, often relict, populations Oostermeijer et al.

The overall environment in these relict populations will probably remain amenable for adult survival, and reproductive output is not necessarily compromised. Thus, the effect is nonetheless manifest as the loss of reproductive fitness through devolutionary decline because replacement will not continue to equal adult mortality, even though there is an excess generation of progeny. There are many instances, worldwide, of individual species or suites of species that survive in what are essentially relict habitats.

The geographical distributions of species are essentially defined by the limits of their norms of reaction. As previously discussed, norms of reaction involve numerous integrated physiological processes. If the shift continues and emigration to suitable habitat is impossible, habitat attenuation will continue until populations are restricted to only the most favourable sites. We suspect that many environmental shifts occur either too rapidly for organisms to adapt their norms of reaction to the changing conditions e.

The process of habitat attenuation is substantiated by numerous, worldwide examples of populations, or small suites of species, that now occur in restricted habitats surrounded by species characteristic of wholly different ecosystems Dobrowski, One of the best known examples is the giant sequoia [ Sequoiadendron gigantea Lindley Buchholz], which had a much broader distribution in the Miocene Hartesveldt et al.

Animal examples of habitat attenuation show how quickly these changes can occur. Yet by it was observed that the habitat of the pika, a small mammal occurring in the alpine regions of the western United States, was gradually attenuating as a result of global warming Patterson, ; Beever et al.

Likewise, the habitat of the California alpine chipmonk Tamias alpinus is also in decline Craig et al. Species occupying extreme environments, e. The impending extinction crisis is largely the result of human-mediated habitat attenuation, and to a lesser extent hunting overkill, disease, introduction of alien species, and coextinction, in which species become extinct as a result of loss of another species upon which it depends Diamond, , This is the culmination of habitat attenuation and is the final stage of background extinction.

The original environment to which the species was adapted is now greatly altered or may even have disappeared. Adult individuals may persist, albeit at the margin of their norms of reaction. However, a devolutionary threshold has been crossed, after which self-perpetuating effects set the species on a path of irrevocable devolutionary decline and extinction Shaffer, ; Wiens et al. Such species are ecodisplaced, i. Populations of long-lived species suspected of being ecodisplaced might be identified by a population structure that is heavily skewed toward old individuals, as in populations of Juniperus deppeana Steud.

Cupressaceae in the south-western United States Patterson, , or the virtual absence of seedlings or juveniles in several Californian endemic plants, namely Adenostoma sparsifolium Rosaceae Wiens et al. Another example of climate change initiating biological effects that lead to population decline is found in tropical lizards. In this case, increasingly high ambient temperatures have exceeded the norm of reaction that allows lizards to reproduce normally Sinervo et al. Lizards must bask in the sun to raise body temperatures sufficiently to hunt effectively.

However, lizards must seek shade before physiological high temperature limits are exceeded. The increasing temperatures of tropical habitats now preclude adequate hunt-time before lizards must find shade. Thus, females are unable to acquire the nutritional levels necessary for successful embryonic development, thereby initiating population decline.

This example suggests that in cases of greatly accelerated environmental shifts, yet still requiring multiple generations, habitat attenuation is essentially circumvented and such populations can move directly into habitat dissolution. Such rapid devolutionary declines that result in extinction appear to be common, particularly among short-lived organisms. We posit that the mechanism of background extinction is a typically long-term, multi-generational loss of reproductive fitness that is generally the result of both environmental and genetic factors.

A lack of adequate vocabulary is largely responsible for our failure to recognize that the mechanism of background extinction is the loss of reproductive fitness and that it is a process that requires time and produces an effect. To provide a terminology to express background extinction as a process, we propose the term devolution.

Thus, evolution is a positive force leading to adaptation and speciation, while devolution is a negative process resulting in population decline and background extinction. Both are the products of natural selection. Only a small proportion of species are apt to be in the process of background extinction at any particular time, although the process will be faster during massive extinction events.

Nonetheless background extinction is responsible for the vast majority of all extinctions through biohistory. Under conditions of continuing adverse climatic change among long-lived species, selection will shift from maintaining populational reproductive fitness to individual survival. At least among higher plants the shift to individual survival was accomplished by selecting for heterozygous overdominance.

However, this had the unfortunate side effect of increasing the number of deleterious and lethal alleles expressed during embryogenesis, thereby decreasing fecundity, sometimes dramatically. Short-lived organisms, such as annual plants and small mammals occasionally long-lived organisms , generally maintain low levels of genetic variation and have high reproductive capacities. However, because of their low genetic variability such organisms have reduced adaptive potential during severe environmental shifts and are thus subject to rapid quasi-catastrophic?

In contrast, long-lived organisms tend to devolve and slowly succumb to background extinction. Habitat dissolution is the final stage of background extinction. The habitat to which the species is adapted has now essentially disappeared and it has few, if any, adaptations that permit successful reproduction in the transformed environment. We thank the following individuals for discussion and suggestions: L. Allphin, M. Harmon, B. Hearl, R. Johnson, D.

Mansfield, G. Thackray, E. Wakeman, D. Weatherby, C. Wiens, and E. The following individuals read the manuscript and provided constructive criticisms: D.

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Correspondence to Andrew E. Reprints and Permissions. Snyder-Beattie, A. An upper bound for the background rate of human extinction. Sci Rep 9, Download citation. Received : 21 February Accepted : 19 July Published : 30 July Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. By submitting a comment you agree to abide by our Terms and Community Guidelines.

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This article has been updated. Abstract We evaluate the total probability of human extinction from naturally occurring processes. Save This Word! The ongoing extinction of individual species due to environmental or ecological factors such as climate change, disease, loss of habitat, or competitive disadvantage in relation to other species. Background extinction occurs at a fairly steady rate over geological time and is the result of normal evolutionary processes, with only a limited number of species in an ecosystem being affected at any one time.

Compare mass extinction. We could talk until we're blue in the face about this quiz on words for the color "blue," but we think you should take the quiz and find out if you're a whiz at these colorful terms.