In zoology, megafauna (from Greek 'large' and Neo-Latin 'animal life') are large animals. The precise definition of the term varies widely, though a common threshold is approximately , this lower end being centered on humans, with other thresholds being more relative to the sizes of animals in an ecosystem. The spectrum of lower-end thresholds ranges from to , the latter largely restricted to megaherbivores. Large body size is generally associated with other traits, such as having a slow rate of reproduction and, in large herbivores, reduced or negligible adult mortality from predation.
Megafauna species have considerable effects on their local environment, including the suppression of the growth of woody vegetation and a consequent reduction in wildfire frequency. Megafauna also play a role in regulating and stabilizing the abundance of smaller animals.
During the Pleistocene, megafauna were diverse across the globe, with most continental ecosystems exhibiting similar or greater species richness in megafauna as compared to ecosystems in Africa today. During the Late Pleistocene, particularly from around 50,000 years ago onwards, most large mammal species became extinct, including 80% of all mammals greater than , while small animals were largely unaffected. This pronouncedly size-biased extinction is otherwise unprecedented in the geological record. Humans and climatic change have been implicated by most authors as the likely causes, though the relative importance of either factor has been the subject of significant controversy.
Terminology 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 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 Paul S. Martin's proposed threshold of to classify animals as megafauna. However, for freshwater species, is the preferred threshold. Some scientists define herbivorous terrestrial megafauna as having a weight exceeding , and terrestrial carnivorous megafauna as more than . Additionally, Owen-Smith coined the term megaherbivore to describe herbivores that weighed over , which has seen some use by other researchers.
Among living animals, the term megafauna is most commonly used for the largest extant terrestrial mammals, which include (but are not limited to) elephants, giraffes, hippopotamuses, rhinoceroses, and larger bovines. Of these five categories of large herbivores, only bovines are presently found outside of Africa and Asia, but all the others were formerly more wide-ranging, with their ranges and populations continually shrinking and decreasing over time. Wild equines are another example of megafauna, but their current ranges are largely restricted to the Old World, specifically in Africa and Asia. Megafaunal species may be categorized according to their dietary type: megaherbivores (e.g., elephants), megacarnivores (e.g., lions), and megaomnivores (e.g., bears).
Definition
Since the term "megafauna" was first proposed, the scientific community has not settled on a single, succinct definition. However, recent scholarship has been in support of ecological definitions. A 2020 study in Proceedings of the Royal Society B: Biological Sciences analyzed 276 pieces of literature to categorize the different uses of the term. The study determined two typical uses: "keystone megafauna" and "functional megafauna," with a subcategory of "apex megafauna." Subsequently, these categories reflected how the term has evolved amongst the academic scientific community. Despite each of these attempts at categorization, there remains no set standard, as definitions vary based on specific habitats, groups being studied and context.
Keystone megafauna First defined by Robert T. Paine in 1969, a keystone species is a population whose activity meets a disproportional amount of needs that other species in a habitat depend on. Their function is further illustrated by the effects of their absence. Losing these species can cause natural ecological processes to unravel, leading to further environmental decline. Keystone megafauna's larger mass and range is significant to such roles they play in their ecosystems. Distinguishing a keystone megafauna from the other categories depends on the individual species' size and their implications for the function of the habitat. Although small species can also function as keystones, they do not meet the sizing criteria of megafauna.
Functional megafauna Functional megafauna refers to the species that are currently seen as the largest in their taxonomical group and that tend to have identifiable advantages in their habitat due to their size. The term "functional" in this case refers to the traits that contribute to the ability allowing these species to use their size. The actual mass minimum with this definition will vary between the individuals SPECIFY classification, like how herbivores typically reach around 1000 kg to be considered whereas carnivores need only be around 13–16 kg. Apex megafauna is an additional term to be used for these species within this category.
Apex megafauna "Apex" is a term usually used to describe apex predators. Apex predators are carnivorous species who dominate the habitat they reside in, being at the top of the food chain and having large hunting ranges. The difference for these megafauna species, humans for example, lies in their unlikeliness to become prey which increases significantly into adulthood, whether it is because of predation status or their size. Large animals that are not predators can fit into this apex ideation because, similarly, their range is more influenced by the capacity of the habitat rather than the typical phenomenon of being hunted for population control.
Ecological strategy Megafauna tend to display the ecological role of 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. These characteristics, although not exclusive to such megafauna, make them vulnerable to human overexploitation, in part because of their slow population recovery rates.
Megafauna are considerable contributors to the environments they populate in. The lasting effects of megafauna have been studied, such as Enquist's paper that considered the results from notable megafauna extinction events. During the Anthropocene, many larger species declined in populations (flora and fauna both), which was found to have profound downward effects on the total biosphere activity. This can be attributed to losing these species that assist in many natural processes, like creating new soil, cycling carbon, and population control of the other species. According to simulations and studies done based on current global conditions, removing megafauna from a habitat would leave niches to be filled by smaller plants and animals, upsetting the stable chain of systems that is currently functioning.
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. 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.
In terrestrial mammals
Subsequent to the Cretaceous–Paleogene extinction event that eliminated the non-avian dinosaurs about Ma (million years) ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant. Starting from just a few kg before the event, maximum size had reached ~ a few million years later, and ~ by the end of the Paleocene. This trend of increasing body mass appears to level off about 40 Ma ago (in the late Eocene), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude. However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of 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.)
Megaherbivores eventually attained a body mass of over . The largest of these, indricotheres and proboscids, have been hindgut fermenters, which are believed to have an advantage over foregut fermenters in terms of being able to accelerate gastrointestinal transit in order to accommodate very large food intakes. A similar trend emerges when rates of increase of maximum body mass per generation for different mammalian clades are compared (using rates averaged over macroevolutionary time scales). Among terrestrial mammals, the fastest rates of increase of body mass0.259 vs. time (in Ma) occurred in perissodactyls (a slope of 2.1), followed by rodents (1.2) and proboscids (1.1), all of which are hindgut fermenters. The rate of increase for artiodactyls (0.74) was about a third of the perissodactyls. The rate for carnivorans (0.65) was slightly lower yet, while primates, perhaps constrained by their arboreal habits, had the lowest rate (0.39) among the mammalian groups studied.
Terrestrial mammalian carnivores from several eutherian groups (the artiodactyl Andrewsarchus – formerly considered a mesonychid, the oxyaenid Sarkastodon, and the carnivorans Amphicyon and Arctodus) all reached a maximum size of about (the carnivoran Arctotherium and the hyaenodontid Simbakubwa may have been somewhat larger). The largest known metatherian carnivore, Proborhyaena gigantea, apparently reached , also close to this limit. 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. It has also been suggested that maximum size for mammalian carnivores is constrained by the stress the humerus can withstand at top running speed.
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, and might be related to the thermoregulatory advantage of large body mass in cool climates, better ability of larger organisms to cope with seasonality in food supply, or other factors; the latter correlation could be explained in terms of range and resource limitations. 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.
In marine mammals