Megafauna

Source: Wikipedia, the free encyclopedia.
This article is about living or extinct large animals. For other uses, see Megafauna (disambiguation).
The African bush elephant (foreground), Earth's largest extant land mammal, and the Masai ostrich (background), one of Earth's largest extant birds

In

Quaternary extinction event
, many species of megafauna went extinct as part of a slowly progressing extinction wave that effected ecosystems worldwide.

In practice, the most common usage encountered in academic and popular writing describes land mammals roughly larger than a human that are not (solely) domesticated. The term is especially associated with the

became extinct within the last forty thousand years
.

History

One of the earliest occurrences of the term "megafauna" is Alfred Russel Wallace's 1876 work The geographical distribution of animals. He described the animals as "the hugest, and fiercest, and strangest forms". In the later 20th and 21st centuries, the term usually refers to large animals. There are variations in thresholds used to define megafauna as a whole or certain groups of megafauna. Many scientific literature adopt Martin's threshold of (<45 kg) to classify animals under this group. However, for freshwater species, 30 kg is the preferred threshold. Some scientists define herbivorous terrestrial megafauna as having a weight exceeding 100 kg, and terrestrial carnivorous megafauna as more than 15 kg. Additionally, Owen-Smith coined the term megaherbivore to describe herbivores that weighed over a tonne.[1]

Among living animals, the term megafauna is most commonly used for the largest

elephants), megacarnivores (e.g., lions), and, more rarely, megaomnivores (e.g., bears).[2][3]

Ecological strategy

Megafauna animals – in the sense of the largest mammals and birds – are generally

K-strategists, with high longevity, slow population growth rates, low mortality rates, and (at least for the largest) few or no natural predators capable of killing adults.[4] These characteristics, although not exclusive to such megafauna, make them vulnerable to human overexploitation, in part because of their slow population recovery rates.[5][6]

Evolution of large body size

One observation that has been made about the evolution of larger body size is that rapid rates of increase that are often seen over relatively short time intervals are not sustainable over much longer time periods. In an examination of mammal body mass changes over time, the maximum increase possible in a given time interval was found to scale with the interval length raised to the 0.25 power.[7] This is thought to reflect the emergence, during a trend of increasing maximum body size, of a series of anatomical, physiological, environmental, genetic and other constraints that must be overcome by evolutionary innovations before further size increases are possible. A strikingly faster rate of change was found for large decreases in body mass, such as may be associated with the phenomenon of insular dwarfism. When normalized to generation length, the maximum rate of body mass decrease was found to be over 30 times greater than the maximum rate of body mass increase for a ten-fold change.[7]

In terrestrial mammals

Large terrestrial mammals compared in size to one of the largest sauropod dinosaurs, Patagotitan

Subsequent to the

Indricotherium 30 Ma ago. (Since generation time scales with body mass0.259, increasing generation times with increasing size cause the log mass vs. time plot to curve downward from a linear fit.)[7]

Megaherbivores eventually attained a body mass of over 10,000 kg. The largest of these,

arboreal habits, had the lowest rate (0.39) among the mammalian groups studied.[7]

Terrestrial mammalian carnivores from several

hyaenodontid Simbakubwa may have been somewhat larger). The largest known metatherian carnivore, Proborhyaena gigantea, apparently reached 600 kg, also close to this limit.[10] A similar theoretical maximum size for mammalian carnivores has been predicted based on the metabolic rate of mammals, the energetic cost of obtaining prey, and the maximum estimated rate coefficient of prey intake.[11] It has also been suggested that maximum size for mammalian carnivores is constrained by the stress the humerus can withstand at top running speed.[10]

Analysis of the variation of maximum body size over the last 40 Ma suggests that decreasing temperature and increasing continental land area are associated with increasing maximum body size. The former correlation would be consistent with Bergmann's rule,[12] and might be related to the thermoregulatory advantage of large body mass in cool climates,[8] better ability of larger organisms to cope with seasonality in food supply,[12] or other factors;[12] the latter correlation could be explained in terms of range and resource limitations.[8] However, the two parameters are interrelated (due to sea level drops accompanying increased glaciation), making the driver of the trends in maximum size more difficult to identify.[8]

In marine mammals

Baleen whale comparative sizes

Since tetrapods (first

pakicetids, no larger than dogs, of about 53 million years (Ma) ago.[15] By 40 Ma ago, cetaceans had attained a length of 20 m or more in Basilosaurus, an elongated, serpentine whale that differed from modern whales in many respects and was not ancestral to them. Following this, the evolution of large body size in cetaceans appears to have come to a temporary halt, and then to have backtracked, although the available fossil records are limited. However, in the period from 31 Ma ago (in the Oligocene) to the present, cetaceans underwent a significantly more rapid sustained increase in body mass (a rate of increase in body mass0.259 of a factor of 3.2 per million years) than achieved by any group of terrestrial mammals.[7] This trend led to the largest animal of all time, the modern blue whale. Several reasons for the more rapid evolution of large body size in cetaceans are possible. Fewer biomechanical constraints on increases in body size may be associated with suspension in water as opposed to standing against the force of gravity, and with swimming movements as opposed to terrestrial locomotion. Also, the greater heat capacity and thermal conductivity of water compared to air may increase the thermoregulatory advantage of large body size in marine endotherms, although diminishing returns apply.[7]

Among toothed whales, maximum body size appears to be limited by food availability. Larger size, as in

balaenid whales; the latter technique is used with less dense and patchy plankton.[16] The cooling trend in Earth's recent history may have generated more localities of high plankton abundance via wind-driven upwellings, facilitating the evolution of gigantic whales.[16]

Cetaceans are not the only marine mammals to reach tremendous sizes.[17] The largest carnivorans of all time are marine pinnipeds, the largest of which is the southern elephant seal, which can reach more than 6 m (20 ft) in length and weigh up to 5,000 kg (11,000 lb). Other large pinnipeds include the northern elephant seal at 4,000 kg (8,800 lb), walrus at 2,000 kg (4,400 lb), and Steller sea lion at 1,135 kg (2,502 lb).[18][19] The sirenians are another group of marine mammals which adapted to fully aquatic life around the same time as the cetaceans did. Sirenians are closely related to elephants. The largest sirenian was the Steller's sea cow, which reached up to 10 m (33 ft) in length and weighed 8,000 to 10,000 kg (18,000 to 22,000 lb), and was hunted to extinction in the 18th century. The semi-aquatic hippopotamus, which is the terrestrial mammal most closely related to cetaceans.[20]

In flightless birds

Dinornis robustus

Because of the small initial size of all mammals following the extinction of the non-avian dinosaurs, nonmammalian vertebrates had a roughly ten-million-year-long window of opportunity (during the Paleocene) for evolution of gigantism without much competition.

paleognaths evolved to large size on Gondwanan land masses and Europe. Gastornithids and at least one lineage of flightless paleognath birds originated in Europe, both lineages dominating niches for large herbivores while mammals remained below 45 kg (in contrast with other landmasses like North America and Asia, which saw the earlier evolution of larger mammals) and were the largest European tetrapods in the Paleocene.[22]

Flightless paleognaths, termed

Neotropic tinamous. However, recent genetic studies have found that tinamous nest well within the ratite tree, and are the sister group of the extinct moa of New Zealand.[21][23][24] Similarly, the small kiwi of New Zealand have been found to be the sister group of the extinct elephant birds of Madagascar.[21] These findings indicate that flightlessness and gigantism arose independently multiple times among ratites via parallel evolution.[25]

Predatory megafaunal flightless birds were often able to compete with mammals in the early

phorusrhacids shared the dominant predatory niches with metatherian sparassodonts during most of the Cenozoic but declined and ultimately went extinct after eutherian predators arrived from North America (as part of the Great American Interchange) during the Pliocene. In contrast, large herbivorous flightless ratites have survived to the present.[25]

However, none of the flightless birds of the Cenozoic, including the predatory

Dromornis stirtoni[25] or herbivorous Aepyornis, ever grew to masses much above 500 kg, and thus never attained the size of the largest mammalian carnivores, let alone that of the largest mammalian herbivores. It has been suggested that the increasing thickness of avian eggshells in proportion to egg mass with increasing egg size places an upper limit on the size of birds.[26][note 1] The largest species of Dromornis, D. stirtoni, may have gone extinct after it attained the maximum avian body mass and was then outcompeted by marsupial diprotodonts that evolved to sizes several times larger.[29]

In giant turtles

Megalochelys atlas, an animal that probably weighed about 1,000 kg (2,200 lb).[32]

Some earlier aquatic Testudines, e.g. the marine Archelon of the Cretaceous[33] and freshwater Stupendemys of the Miocene, were considerably larger, weighing more than 2,000 kg (4,400 lb).[34]

Megafaunal mass extinctions

Timing and possible causes

Correlations between times of first appearance of humans and unique megafaunal extinction pulses on different land masses
climate change
over the last 450,000 years (based on Antarctic temperatures and global ice volume), showing that there were no unique climatic events that would account for any of the megafaunal extinction pulses

The

extraterrestrial impact, competition from other animals or other causes. However, this extinction near the end of the Pleistocene was just one of a series of megafaunal extinction pulses that have occurred during the last 50,000 years over much of the Earth's surface, with Africa and southern Asia (where the local megafauna had a chance to evolve alongside modern humans) being comparatively less affected. The latter areas did suffer gradual attrition of megafauna, particularly of the slower-moving species (a class of vulnerable megafauna epitomized by giant tortoises), over the last several million years.[36][37]

Outside the mainland of

extinct fauna were smaller, but still displayed island gigantism.[38][39]

An analysis of the timing of

interstadials, periods of abrupt warming, but only when humans were also present. Humans may have impeded processes of migration and recolonization that would otherwise have allowed the megafaunal species to adapt to the climate shift.[64] In at least some areas, interstadials were periods of expanding human populations.[65]

An analysis of

Shasta ground sloth dung left in over half a dozen caves in the American Southwest.[74][75]

Continuing human hunting and environmental disturbance has led to additional

serious danger of further extinctions in the near future (see examples below). Direct killing by humans, primarily for meat or other body parts, is the most significant factor in contemporary megafaunal decline.[76][77]

A number of other mass extinctions occurred earlier in Earth's geologic history, in which some or all of the megafauna of the time also died out. Famously, in the Cretaceous–Paleogene extinction event the non-avian dinosaurs and most other giant reptiles were eliminated. However, the earlier mass extinctions were more global and not so selective for megafauna; i.e., many species of other types, including plants, marine invertebrates[78] and plankton, went extinct as well. Thus, the earlier events must have been caused by more generalized types of disturbances to the biosphere.[79]

Consequences of depletion of megafauna

Effect on nutrient transport

Megafauna play a significant role in the lateral transport of mineral nutrients in an ecosystem, tending to translocate them from areas of high to those of lower abundance. They do so by their movement between the time they consume the nutrient and the time they release it through elimination (or, to a much lesser extent, through decomposition after death).

Amazon Basin, it is estimated that such lateral diffusion was reduced over 98% following the megafaunal extinctions that occurred roughly 12,500 years ago.[81][82] Given that phosphorus availability is thought to limit productivity in much of the region, the decrease in its transport from the western part of the basin and from floodplains (both of which derive their supply from the uplift of the Andes) to other areas is thought to have significantly impacted the region's ecology, and the effects may not yet have reached their limits.[82] In the sea, cetaceans and pinnipeds that feed at depth are thought to translocate nitrogen from deep to shallow water, enhancing ocean productivity, and counteracting the activity of zooplankton, which tend to do the opposite.[83]

Effect on methane emissions

Large populations of megaherbivores have the potential to contribute greatly to the atmospheric concentration of

sauropods could have emitted 520 million tons of methane to the atmosphere annually,[84] contributing to the warmer climate of the time (up to 10 °C warmer than at present).[84][85] This large emission follows from the enormous estimated biomass of sauropods, and because methane production of individual herbivores is believed to be almost proportional to their mass.[84]

Recent studies have indicated that the extinction of megafaunal herbivores may have caused a reduction in

Pleistocene epoch after the extinction of megafauna in the Americas. After early humans migrated to the Americas about 13,000 BP, their hunting and other associated ecological impacts led to the extinction of many megafaunal species there. Calculations suggest that this extinction decreased methane production by about 9.6 million tons per year. This suggests that the absence of megafaunal methane emissions may have contributed to the abrupt climatic cooling at the onset of the Younger Dryas.[86] The decrease in atmospheric methane that occurred at that time, as recorded in ice cores, was 2–4 times more rapid than any other decrease in the last half million years, suggesting that an unusual mechanism was at work.[86]

Gallery

Pleistocene extinct megafauna

Other extinct Cenozoic megafauna

Extant

See also

Notes

  1. ^ Nonavian dinosaur size was not similarly constrained because they had a different relationship between body mass and egg size than birds. The 400 kg Aepyornis had larger eggs than nearly all dinosaurs.[27][28]
  2. ^ Analysis indicates that 35 genera of North American mammals went extinct more or less simultaneously in this event.[51]

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External links

Retrieved from "https://en.wikipedia.org/w/index.php?title=Megafauna&oldid=1217739987"