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8.2: Rates of Extinction - Biology

8.2: Rates of Extinction - Biology


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8.2: Rates of Extinction

DEFORESTATION AND EXTINCTION

The greatest loss with the longest-lasting effects from the ongoing destruction of wilderness will be the mass extinction of species that provide Earth with biodiversity. Although great extinctions have occurred in the past, none has occurred as rapidly or has been so much the result of the actions of a single species. The extinction rate of today may be 1,000 to 10,000 times the biological normal, or background, extinction rate of 1-10 species extinctions per year.

So far there is little evidence for the massive species extinctions predicted by the species-area curve in the chart below. However, many biologists believe that species extinction, like global warming, has a time lag, and the loss of forest species due to forest clearing in the past may not be apparent yet today. Ward (1997) uses the term "extinction debt" to describe such extinction of species and populations long after habitat alteration:

    Decades or centuries after a habitat perturbation, extinction related to the perturbation may still be taking place. This is perhaps the least understood and most insidious aspect of habitat destruction. We can clear-cut a forest and then point out that the attendant extinctions are low, when in reality a larger number of extinctions will take place in the future. We will have produced an extinction debt that has to be paid. We might curtail our hunting practices when some given population falls to very low numbers and think that we have succeeded in "saving" the species in question, when in reality we have produced an extinction debt that ultimately must be paid in full. Extinction debts are bad debts, and when they are eventually paid, the world is a poorer place.

For example, the disappearance of crucial pollinators will not cause the immediate extinction of tree species with life cycles measured in centuries. Similarly, a study of West African primates found an extinction debt of over 30 percent of the total primate fauna as a result of historic deforestation. This suggests that protection of remaining forests in these areas might not be enough to prevent extinctions caused by past habitat loss. While we may be able to predict the effects of the loss of some species, we know too little about the vast majority of species to make reasonable projections. The unanticipated loss of unknown species will have a magnified effect over time.

The process of extinction is enormously complex, resulting from perhaps hundreds or even thousands of factors, many of which scientists (let alone lay people) fail to grasp. The extinction of small populations, either endangered or isolated from the larger gene pool by fragmentation or natural barriers like water or mountain ranges, is the best modeled and understood form of extinction. Since the standard was set by MacArthur and Wilson in The Theory of Island Biogeography (1967), much work has been done modeling the effects of population size and land area on the survival of species.

The number of individuals in a given population is always fluctuating due to numerous influences, from extrinsic changes in the surrounding environment to intrinsic forces within a species' own genes. This population fluctuation is especially a problem for populations in isolated forest fragments and species that are critically endangered throughout their range. When a population falls below a certain number, known as the minimum viable population (MVP), it is unlikely to recover. Thus the minimum viable population is often considered the extinction threshold for a population or species. There are three common forces that can drive a species with a population under MVP to extinction: demographic stochasticity, environmental stochasticity, and reduced genetic diversity.

Demographic stochasticity involves birth and death rates of the individuals within a species. As the population size decreases, random quirks in mating, reproduction, and survival of young can have a significant outcome for a species. This is especially true in species with low birth rates (i.e. some primates, birds of prey, elephants), since their populations take a longer time to recover. Social dysfunction also plays an important role in a population's survival or demise. Once a population's size falls below a critical number, the social structure of a species may no longer function. For example many gregarious species live in herds or packs which enable the species to defend themselves from predators, find food, or choose mates. In these species, once the population is too small to sustain an effective herd or pack, the population may crash. Among species that are widely dispersed like large cats, finding a mate may be impossible once the population density falls below a certain point. Many insect species use chemical odors or pheromeres to communicate and attract mates. As population density falls, there is less probability that an individual's chemical message will reach a potential mate, and reproductive rates may decrease. Similarly, as plant species become rarer and more widely scattered, the distance between plants increases and pollination becomes less likely.

Environmental stochasticity is caused by randomly occurring changes in weather and food supply, and natural disasters like fire, flood, and drought. In populations confined to a small area, a single drought, bad winter, or fire can eliminate all individuals.

Reduced genetic diversity is a substantial obstacle blocking the recovery of small populations. Small populations have a smaller genetic base than larger populations. Without the influx of individuals from other populations, a population's genome stagnates and loses the genetic variability to adapt to changing conditions. Small populations are also prone to genetic drift where rare traits have a high probability of being lost with each successive generation.

The smaller the population, the more vulnerable it is to demographic stochasticity, environmental stochasticity, and reduced genetic diversity. These factors, often working in concert, tend to further reduce population size and drive the species toward extinction. This trend is known as the extinction vortex. See the box on the right for an example of an extinction vortex.

Some mathematical ecologists have suggested that population fluctuations may be governed by properties of chaos making the behavior of the system (the fluctuation of a species's population size) nearly impossible to predict due to the complex dynamics within a given ecosystem.


The Five Mass Extinctions

The fossil record of the mass extinctions was the basis for defining periods of geological history, so they typically occur at the transition point between geological periods. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species. These transitions can be seen in the rock strata. Table 2 provides data on the five mass extinctions.

Table 2. Mass Extinctions
Geological Period Mass Extinction Name Time (millions of years ago)
Ordovician–Silurian end-Ordovician O–S 450–440
Late Devonian end-Devonian 375–360
Permian–Triassic end-Permian 251
Triassic–Jurassic end-Triassic 205
Cretaceous–Paleogene end-Cretaceous K–Pg (K–T) 65.5

The Ordovician-Silurian extinction event is the first recorded mass extinction and the second largest. During this period, about 85 percent of marine species (few species lived outside the oceans) became extinct. The main hypothesis for its cause is a period of glaciation and then warming. The extinction event actually consists of two extinction events separated by about 1 million years. The first event was caused by cooling, and the second event was due to the subsequent warming. The climate changes affected temperatures and sea levels. Some researchers have suggested that a gamma-ray burst, caused by a nearby supernova, is a possible cause of the Ordovician-Silurian extinction. The gamma-ray burst would have stripped away the Earth’s ozone layer causing intense ultraviolet radiation from the sun and may account for climate changes observed at the time. The hypothesis is speculative, but extraterrestrial influences on Earth’s history are an active line of research. Recovery of biodiversity after the mass extinction took from 5 to 20 million years, depending on the location.

The late Devonian extinction may have occurred over a relatively long period of time. It appears to have affected marine species and not the plants or animals inhabiting terrestrial habitats. The causes of this extinction are poorly understood.

The end-Permian extinction was the largest in the history of life. Indeed, an argument could be made that Earth nearly became devoid of life during this extinction event. The planet looked very different before and after this event. Estimates are that 96 percent of all marine species and 70 percent of all terrestrial species were lost. It was at this time, for example, that the trilobites, a group that survived the Ordovician–Silurian extinction, became extinct. The causes for this mass extinction are not clear, but the leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event. The oceans became largely anoxic, suffocating marine life. Terrestrial tetrapod diversity took 30 million years to recover after the end-Permian extinction. The Permian extinction dramatically altered Earth’s biodiversity makeup and the course of evolution.

The causes of the Triassic–Jurassic extinction event are not clear and hypotheses of climate change, asteroid impact, and volcanic eruptions have been argued. The extinction event occurred just before the breakup of the supercontinent Pangaea, although recent scholarship suggests that the extinctions may have occurred more gradually throughout the Triassic.

The causes of the end-Cretaceous extinction event are the ones that are best understood. It was during this extinction event about 65 million years ago that the dinosaurs, the dominant vertebrate group for millions of years, disappeared from the planet (with the exception of a theropod clade that gave rise to birds). Indeed, every land animal that weighed more then 25 kg became extinct. The cause of this extinction is now understood to be the result of a cataclysmic impact of a large meteorite, or asteroid, off the coast of what is now the Yucatán Peninsula. This hypothesis, proposed first in 1980, was a radical explanation based on a sharp spike in the levels of iridium (which rains down from space in meteors at a fairly constant rate but is otherwise absent on Earth’s surface) at the rock stratum that marks the boundary between the Cretaceous and Paleogene periods (Figure 5). This boundary marked the disappearance of the dinosaurs in fossils as well as many other taxa. The researchers who discovered the iridium spike interpreted it as a rapid influx of iridium from space to the atmosphere (in the form of a large asteroid) rather than a slowing in the deposition of sediments during that period. It was a radical explanation, but the report of an appropriately aged and sized impact crater in 1991 made the hypothesis more believable. Now an abundance of geological evidence supports the theory. Recovery times for biodiversity after the end-Cretaceous extinction are shorter, in geological time, than for the end-Permian extinction, on the order of 10 million years.

Art Connection

Figure 5. Iridium band (credit: USGS)

In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michels discovered, across the world, a spike in the concentration of iridium within the sedimentary layer at the K–Pg boundary. These researchers hypothesized that this iridium spike was caused by an asteroid impact that resulted in the K–Pg mass extinction. In Figure 5, the iridium layer is the light band.

Scientists measured the relative abundance of fern spores above and below the K–Pg boundary in this rock sample. Which of the following statements most likely represents their findings?

  1. An abundance of fern spores from several species was found below the K–Pg boundary, but none was found above.
  2. An abundance of fern spores from several species was found above the K–Pg boundary, but none was found below.
  3. An abundance of fern spores was found both above and below the K–Pg boundary, but only one species was found below the boundary, and many species were found above the boundary.
  4. Many species of fern spores were found both above and below the boundary, but the total number of spores was greater below the boundary.

Link to Learnin

Explore this interactive website about mass extinctions.


Biodiversity devastation: Human-driven decline requires millions of years of recovery

Lake Volvi (Greece) temporarily dries up as a consequence of excessive irrigation for agriculture paired with climate change - one of many examples of a freshwater system under human impact. Credit: C. Albrecht (JLU)

A new study shows that the current rate of biodiversity decline in freshwater ecosystems outcompetes that at the end-Cretaceous extinction that killed the dinosaurs: damage now being done in decades to centuries may take millions of years to undo.

The current biodiversity crisis, often called the 6th mass extinction, is one of the critical challenges we face in the 21st century. Numerous species are threatened with extinction, mostly as a direct or indirect consequence of human impact. Habitat destruction, climate change, overexploitation, pollution and invasive species are among the main causes for Earth's biota to decline rapidly.

To investigate the tempo of extinction and predict recovery times, an international team of evolutionary biologists, paleontologists, geologists and modelers led by the Justus Liebig University Giessen compared today's crisis with the previous, 5th mass extinction event. That event was the result of an asteroid impact 66 million years ago, eradicating about 76% of all species on the planet, including entire animal groups such as the dinosaurs. Focusing on freshwater biota, which are among the World's most threatened, the research team gathered a large dataset containing 3,387 fossil and living snail species of Europe covering the past 200 million years. The scientists estimated rates of speciation and extinction to assess the speed at which species come and go and predict recovery times.

Microcolpia parreyssii (Philippi, 1847), a freshwater snail from a small thermal spring in Romania. The species is flagged as "critically endangered" by the IUCN Red List, but it has not been found living in the past few years and is probably extinct in the wild. Credit: Thomas A. Neubauer

The results of the study, which are recently published in the journal Communications Earth & Environment, are alarming. While already the extinction rate during the 5th mass extinction was considerably higher than previously believed for freshwater biota, it is drastically overshadowed by the predicted future extinction rate of the current 6th mass extinction event. On average the predicted rate was three orders of magnitudes higher than during the time the dinosaurs went extinct. Already by 2120 a third of the living freshwater species may have vanished.

Pyrgulifera matheronii, a freshwater snail common at the time of the dinosaurs and extinct along with them. Cretaceous, Hungary. Credit: Mathias Harzhauser, NHM Vienna.

The pace at which we lose species today is unprecedented and has not even been reached during major extinction crises in the past. "Losing species entails changes in species communities and, in the long run, this affects entire ecosystems. We rely on functioning freshwater environments to sustain human health, nutrition and fresh water supply," says the lead author of the study, Dr. Thomas A. Neubauer.

The trend the scientists revealed for the fifth mass extinction event has another, potentially even more dire prospect for the future. Although the cause for the rising extinction—an asteroid impact on the Yucatán Peninsula in Mexico—was a short event in geological time scales, the extinction rate remained high for approximately five million years. Afterwards followed an even longer period of recovery. It took altogether nearly 12 million years until the balance was restored between species originating and going extinct.

"Even if our impact on the world's biota stops today, the extinction rate will likely stay high for an extended period of time. Considering that the current biodiversity crisis advances much faster than the mass extinction event 66 million years ago, the recovery period may be even longer," says Neubauer. "Despite our short existence on Earth, we have assured that the effects of our actions will outlast us by millions of years."


3 RESULTS

3.1 Comparison of mating patterns and per female fitness

Under control abiotic conditions, the total reproductive output of small populations (five males and five females) did not differ significantly between those originating from monogamous and polyandrous sexual selection regimes (Poisson GLMM: χ 2 (1) = 2.54, p = .11). A significant but small difference was found in the reproductive output of pairs (Poisson GLMM: χ 2 (1) = 3.81, p = .05) with monogamous background showing higher reproductive output than polyandrous background under this mating system (Table 1). This difference could result from selection under monogamy encouraging the experimental evolution of adults that optimize reproduction as monogamous male–female pairs.

Calculation of per female offspring production (reproductive output/number of females) revealed significantly fewer offspring per female for small groups compared to pairs for both sexual selection backgrounds (Poisson GLMM: monogamy: χ 2 (1) = 221.09, p < .001, polyandry χ 2 (1) = 64.74, p < .001) which may have been caused by the quinone effect on group oviposition in T. castaneum (Khan et al., 2018 ) or cannibalism (Sokoloff, 1972 ). Thus, baseline fitness was equivalent for both mating regime backgrounds, and even slightly higher for monogamous pairs, before lines were assayed through the extinction vortex.

3.2 Extinction

Following 95 generations of experimental evolution under weak versus strong sexual selection, our simulated extinction vortex assay revealed that populations from the monogamous background were significantly more likely to go extinct than populations from the polyandrous regime (z = −6.86, p < .001, Figure 4). At the end of the experiment, after five cycles of three stress types across 15 generations, 100% of the 27 replicate populations from the monogamous background starting the vortex experiment had gone extinct, while 60% of the 27 polyandrous background populations were still surviving and reproducing. The AFT model predicted that polyandrous populations survived 2.65 times longer than monogamous populations, with the mean number of generations to extinction (accounting for censoring in the data) predicted as 16 (± 1 SE) for the polyandrous regime background, compared with only 6 (± 1 SE) for the monogamous background.

3.3 Population fitness decline

Across 15 generations of the experimental extinction vortex, the decline in mean population fitness occurred more rapidly, and to a greater degree, in populations from a background of monogamy compared to those from polyandrous backgrounds (Table 1 Figure 5). Fitness in small populations from the monogamous background declined rapidly to zero and 100% extinction. In contrast, small populations from the polyandrous background showed a more gradual fitness decline, losing

30% fitness over each cycle of three stresses relative to the previous cycle (Figure 5b). At the termination of the experiment, extant populations maintained

20% total reproductive fitness relative to total reproductive fitness under control conditions (Table 1).

Through the first complete cycle (generations 1–3) of the experimental extinction vortex, populations from both weak and strong sexual selection backgrounds showed a consistent response to nutritional, thermal and genetic stresses over consecutive generations. Overall, during the first cycle of the extinction vortex, mean population fitness of populations fell by 36% relative to mean population fitness under control conditions. However, during the second cycle of the vortex (generations 4–6), the response of populations from contrasting sexual selection backgrounds diverged. The fitness of populations from the monogamous background declined sharply in response to thermal stress (generation 5) and continued to decline throughout the remaining generations/cycles of the vortex until a population went extinct. In contrast, the fitness of populations from the polyandrous background showed declines in total reproductive fitness during bottleneck events, followed by an incomplete recovery which created an overall gradual decline in fitness over five complete cycles of three stresses (Figure 5).

3.4 Comparison of the relative effects of nutritional, thermal and genetic stress treatments

Within either sexual selection regime, there was no significant difference between the relative effects of nutritional, thermal and genetic stress on relative decline in population fitness (Figure 6 zero-inflated Poisson GLMM: monogamy: χ 2 (1) = 0.78, p = .67, polyandry χ 2 (1) = 5.19, p = .07).


Index

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    8.2: Rates of Extinction - Biology

    By the end of this section, you will be able to do the following:

    • Define biodiversity in terms of species diversity and abundance
    • Describe biodiversity as the equilibrium of naturally fluctuating rates of extinction and speciation
    • Identify historical causes of high extinction rates in Earth’s history

    Traditionally, ecologists have measured biodiversity, a general term for the number of species present in the biosphere, by taking into account both the number of species and their relative abundance to each other. Biodiversity can be estimated at a number of levels of organization of living organisms. These estimation indices, which came from information theory, are most useful as a first step in quantifying biodiversity between and within ecosystems they are less useful when the main concern among conservation biologists is simply the loss of biodiversity. However, biologists recognize that measures of biodiversity, in terms of species diversity, may help focus efforts to preserve the biologically or technologically important elements of biodiversity.

    The Lake Victoria cichlids provide an example with which we can begin to understand biodiversity. The biologists studying cichlids in the 1980s discovered hundreds of cichlid species representing a variety of specializations to specialized habitat types and specific feeding strategies: such as eating plankton floating in the water, scraping/eating algae from rocks, eating insect larvae from the lake bottom, and eating the eggs of other species of cichlid. The cichlids of Lake Victoria are the product of an complex adaptive radiation. An adaptive radiation is a rapid (less than three million years in the case of the Lake Victoria cichlids) branching through speciation of a phylogenetic clade into many closely related species. Typically, the species “radiate” into different habitats and niches. The Galápagos Island finches are an example of a modest adaptive radiation with 15 species. The cichlids of Lake Victoria are an example of a spectacular adaptive radiation that formerly included about 500 species.

    At the time biologists were making this discovery, some species began to quickly disappear. A culprit in these declines was the Nile perch, a species of large predatory fish that was introduced to Lake Victoria by fisheries to feed the people living around the lake. The Nile perch was introduced in 1963, but its populations did not begin to surge until the 1980s. The perch population grew by consuming cichlids, driving species after species to the point of extinction (the disappearance of a species). In fact, there were several factors that played a role in the extinction of perhaps 200 cichlid species in Lake Victoria: the Nile perch, declining lake water quality due to agriculture and land clearing on the shores of Lake Victoria, and increased fishing pressure. Scientists had not even catalogued all of the species present—so many were lost that were never named. The diversity is now a shadow of what it once was.

    The cichlids of Lake Victoria are a thumbnail sketch of contemporary rapid species loss that occurs all over Earth that is caused primarily by human activity. Extinction is a natural process of macroevolution that occurs at the rate of about one out of 1 million species becoming extinct per year. The fossil record reveals that there have been five periods of mass extinction in history with much higher rates of species loss, and the rate of species loss today is comparable to those periods of mass extinction. However, there is a major difference between the previous mass extinctions and the current extinction we are experiencing: human activity. Specifically, three human activities have a major impact: 1) destruction of habitat, 2) introduction of exotic species, and 3) over-harvesting. Predictions of species loss within the next century, a tiny amount of time on geological timescales, range from 10 percent to 50 percent. Extinctions on this scale have only happened five other times in the history of the planet, and these extinctions were caused by cataclysmic events that changed the course of the history of life in each instance.

    Types of Biodiversity

    Scientists generally accept that the term biodiversity describes the number and kinds of species and their abundance in a given location or on the planet. Species can be difficult to define, but most biologists still feel comfortable with the concept and are able to identify and count eukaryotic species in most contexts. Biologists have also identified alternate measures of biodiversity, some of which are important for planning how to preserve biodiversity.

    Genetic diversity is one of those alternate concepts. Genetic diversity, or genetic variation defines the raw material for evolution and adaptation in a species. A species’ future potential for adaptation depends on the genetic diversity held in the genomes of the individuals in populations that make up the species. The same is true for higher taxonomic categories. A genus with very different types of species will have more genetic diversity than a genus with species that are genetically similar and have similar ecologies. If there were a choice between one of these genera of species being preserved, the one with the greatest potential for subsequent evolution is the most genetically diverse one.

    Many genes code for proteins, which in turn carry out the metabolic processes that keep organisms alive and reproducing. Genetic diversity can be measured as chemical diversity in that different species produce a variety of chemicals in their cells, both the proteins as well as the products and byproducts of metabolism. This chemical diversity has potential benefit for humans as a source of pharmaceuticals, so it provides one way to measure diversity that is important to human health and welfare.

    Humans have generated diversity in domestic animals, plants, and fungi, among many other organisms. This diversity is also suffering losses because of migration, market forces, and increasing globalism in agriculture, especially in densely populated regions such as China, India, and Japan. The human population directly depends on this diversity as a stable food source, and its decline is troubling biologists and agricultural scientists.

    It is also useful to define ecosystem diversity, meaning the number of different ecosystems on the planet or within a given geographic area ((Figure)). Whole ecosystems can disappear even if some of the species might survive by adapting to other ecosystems. The loss of an ecosystem means the loss of interactions between species, the loss of unique features of coadaptation, and the loss of biological productivity that an ecosystem is able to create. An example of a largely extinct ecosystem in North America is the prairie ecosystem. Prairies once spanned central North America from the boreal forest in northern Canada down into Mexico. They are now all but gone, replaced by crop fields, pasture lands, and suburban sprawl. Many of the species survive elsewhere, but the hugely productive ecosystem that was responsible for creating the most productive agricultural soils in the United States is now gone. As a consequence, native soils are disappearing or must be maintained and enhanced at great expense.

    Figure 1. The variety of ecosystems on Earth—from (a) coral reef to (b) prairie—enables a great diversity of species to exist. (credit a: modification of work by Jim Maragos, USFWS credit b: modification of work by Jim Minnerath, USFWS)

    Current Species Diversity

    Despite considerable effort, knowledge of the species that inhabit the planet is limited and always will be because of a continuing lack of financial resources and political willpower. A recent estimate suggests that the eukaryote species for which science has names, about 1.5 million species, account for less than 20 percent of the total number of eukaryote species present on the planet (8.7 million species, by one estimate). Estimates of numbers of prokaryotic species are largely guesses, but biologists agree that science has only begun to catalog their diversity. Even with what is known, there is no central repository of names or samples of the described species therefore, there is no way to be sure that the 1.5 million descriptions is an accurate accounting. It is a best guess based on the opinions of experts in different taxonomic groups. Given that Earth is losing species at an accelerating pace, science is very much in the place it was with the Lake Victoria cichlids: knowing little about what is being lost. (Figure) presents recent estimates of biodiversity in different groups.

    Estimates of the Numbers of Described and Predicted Species by Taxonomic Group
    Mora et al. 2011 [1] Chapman 2009 [2] Groombridge & Jenkins 2002 [3]
    Described Predicted Described Predicted Described Predicted
    Animalia 1,124,516 9,920,000 1,424,153 6,836,330 1,225,500 10,820,000
    Chromista 17,892 34,900 25,044 200,500
    Fungi 44,368 616,320 98,998 1,500,000 72,000 1,500,000
    Plantae 224,244 314,600 310,129 390,800 270,000 320,000
    Protozoa 16,236 72,800 28,871 1,000,000 80,000 600,000
    Prokaryotes 10,307 1,000,000 10,175
    Total 1,438,769 10,960,000 1,897,502 10,897,630 1,657,675 13,240,000

    There are various initiatives to catalog described species in accessible ways, and the internet is facilitating that effort. Nevertheless, it has been pointed out that at the current rate of new species descriptions (which according to the State of Observed Species Report is 17,000 to 20,000 new species per year), it will take close to 500 years to finish describing life on this planet. [4] Over time, the task becomes both increasingly difficult and increasingly easier as extinction removes species from the planet.

    Naming and counting species may seem like an unimportant pursuit given the other needs of humanity, but determining biodiversity it is not simply an accounting of species. Describing a species is a complex process through which biologists determine an organism’s unique characteristics and whether or not that organism belongs to any other described species or genus. It allows biologists to find and recognize the species after the initial discovery, and allows them to follow up on questions about its biology. In addition, the unique characteristics of each species make it potentially valuable to humans or other species on which humans depend.

    Patterns of Biodiversity

    Biodiversity is not evenly distributed on Earth. Lake Victoria contained almost 500 species of cichlids alone, ignoring the other fish families present in the lake. All of these species were found only in Lake Victoria therefore, the 500 species of cichlids were endemic. Endemic species are found in only one location. Endemics with highly restricted distributions are particularly vulnerable to extinction. Higher taxonomic levels, such as genera and families, can also be endemic. Lake Michigan contains about 79 species of fish, many of which are found in other lakes in North America. What accounts for the difference in fish diversity in these two lakes? Lake Victoria is an ancient tropical lake, while Lake Michigan is a recently formed temperate lake. Lake Michigan in its present form is only about 7,000 years old, while Lake Victoria in its present form is about 15,000 years old, although its basin is about 400,000 years in age. Biogeographers have suggested these two factors, latitude and age, are two of several hypotheses to explain biodiversity patterns on the planet.

    Career Connection

    Biogeographer

    Biogeography is the study of the distribution of the world’s species—both in the past and in the present. The work of biogeographers is critical to understanding our physical environment, how the environment affects species, and how environmental changes impact the distribution of a species it has also been critical to developing modern evolutionary theory. Biogeographers need to understand both biology and ecology. They also need to be well-versed in evolutionary studies, soil science, and climatology.

    There are three main fields of study under the heading of biogeography: ecological biogeography, historical biogeography (called paleobiogeography), and conservation biogeography. Ecological biogeography studies the current factors affecting the distribution of plants and animals. Historical biogeography, as the name implies, studies the past distribution of species. Conservation biogeography, on the other hand, is focused on the protection and restoration of species based upon known historical and current ecological information. Each of these fields considers both zoogeography and phytogeography—the past and present distribution of animals and plants.

    One of the oldest observed patterns in ecology is that species biodiversity in almost every taxonomic group increases as latitude declines. In other words, biodiversity increases closer to the equator ((Figure)).

    Figure 2. This map illustrates the number of amphibian species across the globe and shows the trend toward higher biodiversity at lower latitudes. A similar pattern is observed for most taxonomic groups. The white areas indicate a lack of data in this particular study.

    It is not yet clear why biodiversity increases closer to the equator, but scientists have several hypotheses. One factor may be the greater age of the ecosystems in the tropics versus those in temperate regions the temperate regions were largely devoid of life or were drastically reduced during the last glaciation. The idea is that greater age provides more time for speciation. Another possible explanation is the increased direct energy the tropics receive from the sun versus the decreased intensity of the solar energy that temperate and polar regions receive. Tropical ecosystem complexity may promote speciation by increasing the heterogeneity, or number of ecological niches, in the tropics relative to higher latitudes. The greater heterogeneity provides more opportunities for coevolution, specialization, and perhaps greater selection pressures leading to population differentiation. However, this hypothesis suffers from some circularity—ecosystems with more species encourage speciation, but how did they get more species to begin with?

    The tropics have been perceived as being more stable than temperate regions, which have a pronounced climate and day-length seasonality. The tropics have their own forms of seasonality, such as rainfall, but they are generally assumed to be more stable environments and this stability might promote speciation into highly specialized niches.

    Regardless of the mechanisms, it is certainly true that all levels of biodiversity are greatest in the tropics. Additionally, the rate of endemism is highest, and there are more biodiversity “hotspots.” However, this richness of diversity also means that knowledge of species is unfortunately very low, and there is a high potential for biodiversity loss.

    Conservation of Biodiversity

    In 1988, British environmentalist Norman Myers developed a conservation concept to identify areas rich in species and at significant risk for species loss: biodiversity hotspots. Biodiversity hotspots are geographical areas that contain high numbers of endemic species. The purpose of the concept was to identify important locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots, governments are able to protect a larger number of species. The original criteria for a hotspot included the presence of 1500 or more endemic plant species and 70 percent of the area disturbed by human activity. There are now 34 biodiversity hotspots ((Figure)) containing large numbers of endemic species, which include half of Earth’s endemic plants.

    Figure 3. Conservation International has identified 34 biodiversity hotspots, which cover only 2.3 percent of the Earth’s surface but have endemic to them 42 percent of the terrestrial vertebrate species and 50 percent of the world’s plants.

    Biodiversity Change through Geological Time

    The number of species on the planet, or in any geographical area, is the result of an equilibrium of two evolutionary processes that are continuously ongoing: speciation and extinction. Both are natural “birth” and “death” processes of macroevolution. When speciation rates begin to outstrip extinction rates, the number of species will increase likewise, the number of species will decrease when extinction rates begin to overtake speciation rates. Throughout Earth’s history, these two processes have fluctuated—sometimes leading to dramatic changes in the number of species on Earth as reflected in the fossil record ((Figure)).

    Figure 4. Percent extinction occurrences as reflected in the fossil record have fluctuated throughout Earth’s history. Sudden and dramatic losses of biodiversity, called mass extinctions, have occurred five times.

    Paleontologists have identified five strata in the fossil record that appear to show sudden and dramatic (greater than half of all extant species disappearing from the fossil record) losses in biodiversity. These are called mass extinctions. There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research. An argument can be made that the five mass extinctions are only the five most extreme events in a continuous series of large extinction events throughout the Phanerozoic (since 542 million years ago). In most cases, the hypothesized causes are still controversial however, the most recent mass extinction event seems clear.

    The Five Mass Extinctions

    The fossil record of the mass extinctions was the basis for defining periods of geological history, so they typically occur at the transition point between geological periods. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species. These transitions can be seen in the rock strata. (Figure) provides data on the five mass extinctions.

    This table shows the names and dates for the five mass extinctions in Earth’s history.
    Mass Extinctions
    Geological Period Mass Extinction Name Time (millions of years ago)
    Ordovician–Silurian end-Ordovician O–S 450–440
    Late Devonian end-Devonian 375–360
    Permian–Triassic end-Permian 251
    Triassic–Jurassic end-Triassic 205
    Cretaceous–Paleogene end-Cretaceous K–Pg (K–T) 65.5

    The Ordovician-Silurian extinction event is the first recorded mass extinction and the second largest. During this period, about 85 percent of marine species (few species lived outside the oceans) became extinct. The main hypothesis for its cause is a period of glaciation and then warming. The extinction event actually consists of two extinction events separated by about 1 million years. The first event was caused by cooling, and the second event was due to the subsequent warming. The climate changes affected temperatures and sea levels. Some researchers have suggested that a gamma-ray burst, caused by a nearby supernova, was a possible cause of the Ordovician-Silurian extinction. The gamma-ray burst would have stripped away the Earth’s protective ozone layer, allowing intense ultraviolet radiation from the sun to reach the surface of the earth—and may account for climate changes observed at the time. The hypothesis is very speculative, and extraterrestrial influences on Earth’s history are an active line of research. Recovery of biodiversity after the mass extinction took from 5 to 20 million years, depending on the location.

    The late Devonian extinction may have occurred over a relatively long period of time. It appears to have mostly affected marine species and not so much the plants or animals inhabiting terrestrial habitats. The causes of this extinction are poorly understood.

    The end-Permian extinction was the largest in the history of life. Indeed, an argument could be made that Earth became nearly devoid of life during this extinction event. Estimates are that 96 percent of all marine species and 70 percent of all terrestrial species were lost. It was at this time, for example, that the trilobites, a group that survived the Ordovician–Silurian extinction, became extinct. The causes for this mass extinction are not clear, but the leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event. The oceans became largely anoxic, suffocating marine life. Terrestrial tetrapod diversity took 30 million years to recover after the end-Permian extinction. The Permian extinction dramatically altered Earth’s biodiversity makeup and the course of evolution.

    The causes of the Triassic–Jurassic extinction event are not clear, and researchers argue hypotheses including climate change, asteroid impact, and volcanic eruptions. The extinction event occurred just before the breakup of the supercontinent Pangaea, although recent scholarship suggests that the extinctions may have occurred more gradually throughout the Triassic.

    The causes of the end-Cretaceous extinction event are the ones that are best understood. It was during this extinction event about 65 million years ago that the majority of the dinosaurs, the dominant vertebrate group for millions of years, disappeared from the planet (with the exception of a theropod clade that gave rise to birds).

    The cause of this extinction is now understood to be the result of a cataclysmic impact of a large meteorite, or asteroid, off the coast of what is now the Yucatán Peninsula. This hypothesis, proposed first in 1980, was a radical explanation based on a sharp spike in the levels of iridium (which enters our atmosphere from meteors at a fairly constant rate but is otherwise absent on Earth’s surface) in the rock stratum that marks the boundary between the Cretaceous and Paleogene periods ((Figure)). This boundary marked the disappearance of the dinosaurs in fossils as well as many other taxa. The researchers who discovered the iridium spike interpreted it as a rapid influx of iridium from space to the atmosphere (in the form of a large asteroid) rather than a slowing in the deposition of sediments during that period. It was a radical explanation, but the report of an appropriately aged and sized impact crater in 1991 made the hypothesis more believable. Now an abundance of geological evidence supports the theory. Recovery times for biodiversity after the end-Cretaceous extinction are shorter, in geological time, than for the end-Permian extinction, on the order of 10 million years.

    Another possibility, perhaps coincidental with the impact of the Yucatan asteroid, was extensive volcanism that began forming about 66 million years ago, about the same time as the Yucatan asteroid impact, at the end of the Cretaceous. The lava flows covered over 50 percent of what is now India. The release of volcanic gases, particularly sulphur dioxide, during the formation of the traps contributed to climate change, which may have induced the mass extinction.

    Art Connection

    Figure 5. In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michels discovered, across the world, a spike in the concentration of iridium within the sedimentary layer at the K–Pg boundary. These researchers hypothesized that this iridium spike was caused by an asteroid impact that resulted in the K–Pg mass extinction. In the photo, the iridium layer is the light band. (credit: USGS)

    Scientists measured the relative abundance of fern spores above and below the K–Pg boundary in this rock sample. Which of the following statements most likely represents their findings?


    What’s Normal: How Scientists Calculate Background Extinction Rate

    You may be aware of the ominous term “The Sixth Extinction,” used widely by biologists and popularized in the eponymous bestselling book by Elizabeth Kolbert. Essentially, we’re in the midst of a catastrophic loss of biodiversity. Scientists agree that the species die-offs we’re seeing are comparable only to 5 other major events in Earth’s history, including the famously nasty one that killed the dinosaurs. But how do we know that this isn’t just business as usual? In order to compare our current rate of extinction against the past, we use something called the background extinction rate.

    Background extinction rate, or normal extinction rate, refers to the number of species that would be expected to go extinct over a period of time, based on non-anthropogenic (non-human) factors. The background extinction rate is often measured for a specific classification and over a particular period of time. For example, a high estimate is that 1 species of bird would be expected to go extinct every 400 years. Sometimes it’s given using the unit “millions of species years (MSY)” which refers to the number of extinctions expected per 10,000 species per 100 years.

    Scientists calculate background extinction using the fossil record to first count how many distinct species existed in a given time and place, and then to identify which ones went extinct. When using this method, they usually focus on the periods of calm in Earth’s geologic history—that is, the times in between the previous five mass extinctions.

    Another way to look at it is based on average species lifespans. Extinction is a natural part of the evolutionary process, allowing for species turnover on Earth. Sometimes when new species are formed through natural selection, old ones go extinct due to competition or habitat changes. Basically, the species dies of old age. Scientists can estimate how long, on average, a species lasts from its origination to its extinction– again, through the fossil record. For example, mammals have an average species lifespan of 1 million years, although some mammal species have existed for over 10 million. Given these numbers, we’d expect one mammal to go extinct due to natural causes every 200 years on average—so 1 per 200 years is the background extinction rate for mammals, using this method of calculation. Instead, in just the past 400 years we’ve seen 89 mammalian extinctions. This is why scientists suspect these species are not dying of natural causes—humans have engaged in “foul play.”

    Number of years that would have been required for the observed vertebrate species extinctions in the last 114 years to occur under a background rate of 2 E/MSY.

    At our current rate of extinction, we’ve seen significant losses over the past century. A recent study looked closely at observed vertebrate extinction data over the past 114 years. They then considered how long it would have taken for that many species to go extinct at the background rate. (A conservative estimate of background extinction rate for all vertebrate animals is 2 E/MSY, or 2 extinctions per 10,000 species per 100 years.) As you can see from the graph above, under normal conditions, it would have taken anywhere from 2,000 to 10,000 years for us to see the level of species loss observed in just the last 114 years.

    This is why it’s so alarming—we are clearly not operating under normal conditions. To explore this and go deeper into the math behind extinction rates in a high school classroom, try our lesson The Sixth Extinction, part of our Biodiversity unit. And stay tuned for an additional post about calculating modern extinction rates.

    Image credit: Coelurosaur, National Geographic Extinction rate graph, Berkeley Species loss graph, “Accelerated modern human–induced species losses: Entering the sixth mass extinction”


    Extinction

    Extinction is the dying out of a species. Extinction plays an important role in the evolution of life because it opens up opportunities for new species to emerge.

    Biology, Ecology, Earth Science, Geology, Geography, Physical Geography

    Dinogorgon Skull

    Many species have gone extinct throughout history and all that marks their presence on Earth are fossils, such as this one of a dinogorgon.

    Photograph by Jonathan Blair

    When a species disappears, biologists say that the species has become extinct. By making room for new species, extinction helps drive the evolution of life. Over long periods of time, the number of species becoming extinct can remain fairly constant, meaning that an average number of species go extinct each year, century, or millennium. However, during the history of life on Earth, there have been periods of mass extinction, when large percentages of the planet&rsquos species became extinct in a relatively short amount of time. These extinctions have had widely different causes.

    About 541 million years ago, a great expansion occurred in the diversity of multicellular organisms. Paleobiologists, scientists who study the fossils of plants and animals to learn how life evolved, call this event the Cambrian Explosion. Since the Cambrian Explosion, there have been five mass extinctions, each of which is named for the geological period in which it occurred, or for the periods that immediately preceded and followed it.

    The first mass extinction is called the Ordovician-Silurian Extinction. It occurred about 440 million years ago, at the end of the period that paleontologists and geologists call the Ordovician, and followed by the start of the Silurian period. In this extinction event, many small organisms of the sea became extinct. The next mass extinction is called Devonian extinction, occurring 365 million years ago during the Devonian period. This extinction also saw the end of numerous sea organisms.

    The largest extinction took place around 250 million years ago. Known as the Permian-Triassic extinction, or the Great Dying, this event saw the end of more than 90 percent of the Earth&rsquos species. Although life on Earth was nearly wiped out, the Great Dying made room for new organisms, including the first dinosaurs. About 210 million years ago, between the Triassic and Jurassic periods, came another mass extinction. By eliminating many large animals, this extinction event cleared the way for dinosaurs to flourish. Finally, about 65.5 million years ago, at the end of the Cretaceous period came the fifth mass extinction. This is the famous extinction event that brought the age of the dinosaurs to an end.

    In each of these cases, the mass extinction created niches or openings in the Earth&rsquos ecosystems. Those niches allowed for new groups of organisms to thrive and diversify, which produced a range of new species. In the case of the Cretaceous extinction, the demise of the dinosaurs allowed mammals to thrive and grow larger.

    Scientists refer to the current time as the Anthropocene period, meaning the period of humanity. They warn that, because of human activities such as pollution, overfishing, and the cutting down of forests, the Earth might be on the verge of&mdashor already in&mdasha sixth mass extinction. If that is true, what new life would rise up to fill the niche that we currently occupy?

    Many species have gone extinct throughout history and all that marks their presence on Earth are fossils, such as this one of a dinogorgon.


    Watch the video: Stop Extinction (February 2023).