Scavenging is the process in which animals consume dead prey or plant material that have died from causes other than predation. As both a carnivorous and herbivorous feeding behavior, scavenging aids in overcoming fluctuations in food resources in the environment. Scavengers play an important role in the ecosystem by consuming the dead animal and plant material. Decomposers and detritivores complete this process, by consuming the remains left by scavengers. The process and rate of scavenging is affected by both abiotic factors such as habitat, temperature and season and biotic factors such as carcass size and location
Scavenger is an alteration of scavager, from Middle English skawager meaning "customs collector", from skawage meaning "customs", from Old North French escauwage meaning "inspection", from schauwer meaning "to inspect", of Germanic origin; akin to Old English scēawian and German schauen meaning "to look at", and modern English "show" (with semantic drift).
Obligate scavenging is rare among vertebrates, due to the difficulty of finding enough carrion without expending too much energy. In vertebrates, only vultures and possibly some pterosaurs are obligate scavengers, as terrestrial soaring flyers are the only animals able to find enough carrion.
Most scavenging animals are facultative scavengers that gain most of their food through other methods, especially predation. Many large carnivores that hunt regularly, such as hyenas and jackals, but also animals rarely thought of as scavengers, such as African lions, leopards, and wolves will scavenge if given the chance. They may also use their size and ferocity to intimidate the original hunters (the cheetah is a notable exception). Almost all scavengers above insect size are predators and will hunt if not enough carrion is available, as few ecosystems provide enough dead animals year-round to keep its scavengers fed on that alone. Scavenging wild dogs and crows frequently exploit roadkill.
Scavengers of dead plant material include termites that build nests in grasslands and then collect dead plant material for consumption within the nest. The interaction between scavenging animals and humans is seen today most commonly in suburban settings with animals such as opossums, polecats and raccoons. In some African towns and villages, scavenging from hyenas is also common.
In the prehistoric eras, the species Tyrannosaurus rex may have been an apex predator, preying upon hadrosaurs, ceratopsians, and possibly juvenile sauropods, although some experts have suggested the dinosaur was primarily a scavenger. The debate about whether Tyrannosaurus was an apex predator or scavenger was among the longest ongoing feud in paleontology; however, most scientists now agree that Tyrannosaurus was an opportunistic carnivore, acting mostly as a predator but scavenging when it could. Recent research also shows that while an adult Tyrannosaurus rex would energetically gain little though scavenging, smaller theropods of approximately 500 kg (1,100 lb) may have potentially gained levels similar to that of hyenas, though not enough for them to rely on scavenging.
Animals which consume feces, such as dung beetles, are referred to as coprovores. Animals that collect small particles of dead organic material of both animal and plant origin are referred to as detritivores.
Ecological importance of scavengingEdit
Scavengers play a fundamental role in the environment through the removal of carrion, or dead animal flesh, serving as an environmental sanitation surface. Carcass removal by vertebrate scavengers results in a larger diffusion of nutrients into the overall food web compared to removal of carcasses by microorganisms or invertebrate scavengers.
Scavenging unites animals which normally would not come into contact, and results in the formation of highly structured and complex communities which engage in non random interactions. Scavenging communities function in the redistribution of energy obtained from carcasses and reducing decomposition associated diseases. Oftentimes vertebrate scavenger communities differ in consistency due to carcass size and carcass types, as well as by seasonal effects as consequence of differing invertebrate and microbial activity.
Competition amongst scavengers for carrion may occur resulting in the inclusion and exclusion of certain scavengers from access to carrion, shaping the scavenger community. Seasonal changes may result in the alteration of competition amongst scavengers. During cooler seasons, it has been observed that competition amongst scavengers decreases as carrion decomposes at a slower rate and therefore is available for a longer period of time. Such longer persistence time also results in the increase of scavenger species present to feed from the carrion.
Alterations in scavenging communities may result in drastic changes to the scavenging community in general, reduce ecosystem services and have detrimental effects on animal and humans. The reintroduction of gray wolves (Canis lupus) into Yellowstone National Park in the United States caused drastic changes to the prevalent scavenging community, resulting in the provision of carrion to many mammalian and avian species. Likewise, the reduction of vulture species in India lead to the increase of opportunistic species such as feral dogs and rats. The presence of both species at carcasses resulted in the increase of diseases such as rabies and bubonic plague in wildlife and livestock, as feral dogs and rats are transmitters of such diseases. Furthermore, the decline of vulture populations in India has been linked to the increased rates of anthrax in humans due to the handling and ingestion of infected livestock carcasses. An increase of disease transmission has been observed in mammalian scavengers in Kenya due to the decrease in vulture populations in the area, as the decrease in vulture populations resulted in an increase of the number of mammalian scavengers at a given carcass along with the time spent at a carcass.
Transmission of diseaseEdit
Scavenging may provide a direct and indirect method for disease transmission of animals. More specifically, scavengers of infected carcasses may become hosts for certain pathogens and consequently vectors of disease themselves. An example of this phenomenon is the increased transmission of tuberculosis observed when scavengers engage in eating infected carcasses. Likewise, the ingestion of bat carcasses infected with rabies virus by striped skunks (Mephitis mephitis) resulted in increased infection of these organisms with rabies virus.
A major vector of transmission of diseases are various bird species, with outbreak being influenced by such carrier birds and their environment. An increase of transmission of avian influenza virus to chickens by domestic ducks from Indonesian farms permitted to scavenge surrounding areas was observed in 2007. The scavenging of ducks in rice paddy fields in particular resulted in increased contact with other bird species feeding on leftover rice, which may have contributed to increased infection and transmission of the avian influenza virus. The domestic ducks may not have demonstrated symptoms of infection themselves, though were observed to excrete high concentrations of the avian influenza virus. Similarly, an avian cholera outbreak during December 2006 to March 2007 off the coast Newfoundland, Canada resulted in the mortality of many marine bird species. Interestingly, the transmission, perpetuation and spread of the outbreak was mainly restricted to gull species who scavenge for food in the area.
Transmission of diseases may occur indirectly through animal by-products.
Threats to scavengersEdit
Many species that scavenge face persecution globally. Vultures in particular have faced incredible persecution and threats by humans.
Poisoning of the food sources of these animals is a common form of persecution, and as vulture feed communally many organisms may be affected at a given feeding period. Poisoning of the vulture food source such as various carnivorous species with the drug diclofenac has resulted in a 95% decline of Gyps vultures in Asia, and as a result diclofenac was banned by regional governments in 2006. Poisoning of carnivorous species that are predators of game species is still a common practice in Europe and consequently contributes to the poisoning of vultures.
Habitat loss has also contributed to the decline of vulture species, specifically in West Africa due to the growing human population.
Food shortage has also affected various vulture populations in West Africa, mainly as a result of overhunting of vulture food sources as well as changes in livestock husbandry. Food shortage is a major threat to vulture populations as it has been correlated with lower breeding success and high mortality rates in juvenile vultures.
In the 1970s Lewis Binford suggested that early humans primarily obtained meat via scavenging, not through hunting. In 2010, Dennis Bramble and Daniel Lieberman proposed that early carnivorous human ancestors subsequently developed long-distance running behaviors which improved the ability to scavenge and hunt: they could reach scavenging sites more quickly and also pursue a single animal until it could be safely killed at close range due to exhaustion and hyperthermia.
In modern humans, necrophagy (eating of dead/decaying flesh) occurs rarely in most societies. This is likely an adaptation to the risk of disease, due to humans having lower levels of protective acids in the digestive tract, compared to species that are dedicated scavengers. Many instances have occurred in history, especially in times of war, where necrophagy and cannibalism can emerge as a survival behavior.
"Scavenger" appears as an occupation in the 1911 Census of England and Wales. This job title applied to someone who cleans the streets and removes refuse, generally a workman (a modern-day garbage collector, janitor, or street cleaner) employed by the local public health authority.
In India, the term "manual scavenging" is used to describe the removal of raw (fresh and untreated) human excreta from buckets or other containers that are used as toilets or from the pits of pit latrines. The workers pile the excreta into baskets and may carry these on their heads to locations sometimes several kilometers from the latrines. India has officially prohibited the employment of manual scavengers since 1993, but the practice continues as of 2014[update].
The term "scavenger" originated as "scavager" or "scaveger", an official concerned with the receipt of custom duties and the inspection (scavage) of imported goods. The "scavagers" are found[by whom?] with such officials of the City of London as an aleconner or beadle. These officials seem to have been charged also with the cleaning of the streets, and the name superseded the older rakyer for those who performed this duty. These professions are essential to urban settings operating at the highest capacity. The garbage-collection jobs and scavenging professions allow urban populations to continue unhindered by outbreaks of disease most commonly caused by the build-up of physical waste. These jobs had great importance before the times of functional sewer systems and of indoor plumbing.
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