Ecological communities can be envisioned as collections of species that are organized into food chains and webs in which each species is a consumer of resources and is itself a resource for other consumers. Ecologists call these consumptive interactions trophic interactions. And so, species engaging in a particular kind of trophic interaction are said to belong to a distinct trophic group. Ecologists routinely idealize food chains and webs as being comprised of four trophic groups. Species that consume mineralized nutrients and CO2 in order to photosynthesize carbohydrates belong to the plant trophic group, species that consume living plant tissue belong to the herbivore trophic group, species that prey on herbivores belong to the carnivore trophic group, and species that recycle dead organic material back into the nutrient pool belong to the decomposer trophic group.
Such classic idealization of ecological systems typically ignores another trophic group, scavengers. After all, mobs of bloody headed vultures vying for their share of a carcass or insect larvae crawling in stinking, rotting meat do not engender the same sense of awe and natural wonder as do grazing antelope and lions coexisting on the Serengeti plains of Africa. Scavengers get short shrift in ecological thinking because their role is typically viewed as being a repulsive behavioral curiosity or the ecological equivalent of garbage men that sustain themselves on nature's offals. Scavengers are sometimes viewed merely as parasites that steal food - called kleptoparasitism - from the more noble carnivores. These are, however, unfortunate and inaccurate characterizations of scavenging. As we will show below, scavenging serves an important role to the welfare of many species and it can be an important determinant of the structure and functioning of ecological communities. Moreover, scavenging involves many more species than those few specialists that are routinely highlighted as serving this role.
Scavenging, like carnivory, involves the act of consuming the flesh of dead animals - carrion. Technically, however, scavenging differs from carnivory in that it does not actively involve killing animals. Scavengers also differ from decomposers. Decomposers like bacteria and fungi break down the protein of dead animals into its constituent carbon-, nitrogen-, and hydrogen-based elements (Figure 1). Those elements are then broken down further into mineralized form to be taken up later by plants. Scavengers, on the other hand, consume organismal protein and convert it into their own body tissue (Figure 1).
Research has shown that many organisms die from sources other than predation. Although the exact value varies among species and sizes of prey, predation accounts for between 2% and 75% of organism losses annually, thus leaving 25-98% to be scavenged. In the Serengeti alone, the annual amount available to scavengers is estimated to be on the order of 26 million kg. Clearly, neither the Serengeti plains, nor any other location globally, is littered with dead animal carcasses, testimony to the magnitude of this trophic interaction. Depending on the size and species of carrion, a carcass can be despatched within hours to days. Research has also shown that scavenging efficiency, defined as the proportion of a carcass that was consumed within this time frame, averages 75%, a value that rivals the efficiencies of carnivores consuming their hunted prey.
Scavenging can be temperature dependent because of interplay between microbial decomposition and chemical detection of carcasses. This interplay leads to intermediate, optimal temperatures for scavenging, especially within temperature regions of the globe. Decomposers alone are rarely able to utilize entire carcasses. So, to avoid competition with scavengers, decomposers have evolved capacities to produce noxious and odorous chemicals that can make the entire carcass distasteful or even toxic. At moderate temperatures (e.g., 10-15 °C) microbial decomposition is at a level that produces modest concentration of chemicals leading to putrid odors that signal the location of edible carrion to scavengers. Indeed, experimental studies have demonstrated that under such conditions scavengers can find and begin to remove carrion within minutes to hours after becoming available. Higher temperatures and associated higher rates of decomposition lead to higher concentrations of toxic amines and sulfur compounds that signal to scavengers that the item is inedible. Lower temperatures are less favorable for microbial activity and accordingly there is little or no production of chemical odors. Scavenging may thus be limited at low temperatures because, without the chemical cues, scavengers may have difficulty finding a potentially edible carcass.
Carrion tends to be an unreliable resource in any one location or point in time. This has hindered the evolution of strict or obligate scavenging behavior, except in a few notable species like vultures or certain flying insects. Vultures in particular have several traits that facilitate specialization as scavengers. First, they have large broad wings that enable them to expend minimal energy by soaring over vast areas to locate carrion. They have sharp eyesight and a keen sense of smell. They also are able to consume carcasses very rapidly once they have been discovered. Even so, they represent a very small faction of the range of species that scavenge.
Most scavenging is facultative; in essence a dietary supplement. It is undertaken by a broad range of species. Most notably carnivores of all stripes, including the majestic birds of prey such as eagles, hawks and falcons but also other birds such as ravens and magpies; canid, felid, ursid, and hyenid mammal predators; snakes, lizards, and spiders all consume fresh carrion when it is found. After all, it does not make evolutionary sense (in terms of improving individual survival and reproduction) to pass up a free meal, one that effectively differs little from a hunted prey item, whenever it is encountered. The proportion of the diet that comes from carrion, however, can vary widely among carnivore species, making some species like hyenas and ravens seem close to being obligate in their scavenging.
Many seemingly unlikely species such as herbivores also scavenge. For example, on islands in Lake Michigan, white-tailed deer consume large quantities of dead alewives, herring-like freshwater fish that undergo annual mass die offs and wash onto shore in spring. It has been estimated that alewives comprise 30-54% of the daily diet of individual deer during the spring period. Such purposeful scavenging appears to provide the deer with an important dietary supplement during a period when terrestrial food resources are in critically low supply after lengthy winter browsing. Alewives have higher protein, fat, energy, and mineral (especially salt) contents and are more easily digested than the heavily browsed plants on the islands. Other herbivores including grasshoppers and hippopotamus also readily engage in scavenging.
Scavengers do not actively hunt and kill prey. Instead, they must seek out carrion across broad distances on landscapes. But carrion is highly ephemeral in space and time and so it can be quite difficult to find it unless one can search wide distances quickly and efficiently. Most scavengers do not have this searching ability. So they beat the odds against finding carcasses by associating themselves with species that actively hunt and kill prey.
A classic example of such association is between wolves and ravens. Ravens are typically present at wolf-killed carcasses and in some locations such as on Isle Royale in Lake Superior they are omnipresent. There are even cases in which ravens are rarely found on the landscape except in close association with wolves. Ravens can derive a very good livelihood from scavenging carcasses. An individual can ingest and hoard between 0.5 and 2 kg of wolf-killed prey per day. Thus, wolves may routinely lose between 2 and 20 kg of food per day to flocks of ravens. There are notable cases in which flocks of ravens devour up to half of the moose carcasses.
Such a high level of scavenging imposes strong competitive pressure on wolves to the extent that it may alter wolf grouping dynamics. A classically held belief is that groups of wolves comprise related kin in which altruistic behavior of the kin contribute toward overall family welfare (survival and reproduction). But recent research shows that wolf packs contain unrelated individuals. Moreover, pack sizes are often larger than one would expect if individual wolves were attempting to maximize their foraging returns. Such behavior is not expected to be favored by natural selection. This counterintuitive behavior can, however, be reconciled if we add in the costs of food loss to scavengers. In the absence of scavenging, wolves maximize their foraging returns by associating in groups of two or three individuals because one individual alone is inefficient at killing prey and beyond two or three individuals competition for access to a carcass increases with group size leading to diminishing per individual foraging return rate. Loss of food to scavengers may change this structure because it forces wolves to hunt more frequently. Larger packs tend to be more efficient at killing prey frequently. Also, individual foraging return varies little with group size under conditions of scavenging and frequent hunting. Thus, the foraging cost of living in large groups may be offset by the benefit of frequent prey capture in wolves and perhaps in other social carnivores like lions that also face competition with scavengers.
Because scavengers associate closely with predators, their vocalizations and movement behavior may signal imminent predation risk to prey. Moose in boreal forest ecosystems that face high predation risk have been shown to respond dramatically to this signal. In boreal forests, ravens associate closely with wolves, especially during wolf hunting forays. In these regions, the probability of survival, especially of young individuals, can often be as low as 30%. Research using playback calls of ravens has shown that moose in such high risk areas decrease their foraging rates and become increasingly vigilant by being watchful of imminent danger. This contrasts sharply with a lack of a behavioral response to playback of raven calls in geographic locations where wolves and other predators of moose have long been extirpated. In such areas, survival probability is at least three times higher than in the high risk areas. Differences in foraging rates between high and low risk areas are known to have differential effects on ecosystems because they lead to differences in the abundance of plant species that comprise the herbivore's forage.
Because carnivores hunt year-round, they often provide a steady supply of carrion. The exact supply rate of such a resource is known to change the seasonal behavior of scavenger species as well as be an important determinant of the spatial composition of scavenging species within landscapes.
Grizzly bears are important scavengers throughout most of their geographic range. In most cases, however, they hibernate during the winter months as a means to survive periods of chronic food shortages. However, Grizzly bears are known to forego hibernation in conditions when the supply of carrion is high. This may often happen in winters with high snow depth because moose and elk species that comprise the prey base for wolves are encumbered by deep snow and thus are especially vulnerable to being captured. Under such conditions, wolves frequently abandon partially eaten carcasses in favor of capturing new prey, leaving a continuous and plentiful supply of left-over meat, bone and hide to be scavenged.
Scavenging is undertaken by many generalist species that opportunistically use carrion when it is available while sustaining themselves on other resources when carrion is unavailable. These species do not live in isolation of one another on landscapes. So the availability of carrion within the landscape can lead to strong interactions among species as they vie for their share of the resource. Moreover, the nature of carrion supply in space and time can have an important bearing on the kinds of scavenger species found within a location.
If the amount of carrion provided by carnivores is small and much localized, then this resource will attract scavenger species with small foraging radii - those species that forage largely within a small local area. This highly limited resource will be most likely consumed by species that are competitively dominant. These are typically the more fearsome species like coyotes or hyenas that are able to usurp the food by scaring away other species. If the local supply of carrion is large, then it will saturate the ability of the local, competitively dominant scavengers to consume the carrion in its entirety. In such cases, wandering species - those with large foraging radii - will also be attracted to the resource leading to a high diversity of scavenger species at a carcass. The plentiful supply of the resource also diminishes the intensity of competitive interactions among the scavenger species. Because many of these scavengers are also generalist carnivores, such a high, local resource supply represents an important survival subsidy that maintains the multiple trophic level structure of ecological food chains and webs. Predator species that temporarily resorted to scavenging can resume their normal carnivore role once the pulse of carrion supply subsides.
In addition, carnivores, by adding to natural mortality of prey, can add to the spatial and temporal supply of carrion. In the absence of carnivores, herbivore species often die in high numbers during parts of the year in which resources are in short supply or poor in quality such as drought periods in savanna grasslands or late winter in northern temperate regions. Scavengers take advantage of these short pulses of resources to sustain their populations. Nevertheless, their population dynamics are influenced by the vagaries of this carrion supply because it can fluctuate widely with weather conditions from year to year. Large hunting carnivores can change the temporal dynamics of carrion supply from a short seasonal pulse to one that is more even and protracted throughout the year. This subsidy in turn can help to stabilize the long-term population dynamics of carnivore species that scavenge opportunistically, leading to a higher diversity of species on the landscape.
Energetic subsidies in the form of carrion can also undergird food chain structure in locations where long food chains are unlikely to be sustained by local levels of resource production. Arid oceanic island ecosystems off Baja Mexico normally provide an inhospitable environment: they are covered with Opuntia cactus and myriad species of flying insects and their web-building spider predators. Curiously, however, the islands support extraordinarily high densities of spider predators. This occurs because a considerable abundance of nutrient-rich resources in the form of drowned animal carcasses washes up onto the shore from oceanic drift. This resource input sustains insect species that scavenge the decomposing carcasses, thereby creating a highly abundant resource for carnivore species, especially on islands where there is little plant production and hence limited production of herbivore prey. The marine-island food energy conduit thus bolsters the structure of the island food web. In turn, the abnormally high abundance of spiders led to an unusually high capacity to control the abundance of the island's herbivorous insects, thereby lessening the insect damage to plants. Thus, the effects of the subsidy, mediated by scavenging, reverberate through the whole island system. Shut off the supply of carrion and the island ecosystem could collapse to a comparatively barren desert.
The view that scavengers are repugnant, behavioral oddities is an unfortunate and inaccurate representation of an ecological role. Scavenging provides an important means to bolster the structure and functioning of ecological systems because it mediates the ebb and flow of a major resource in space and time. It seems evident that in its absence, many ecological systems would have considerably lower species diversity because of reduced productivity and longevity of the myriad species that avail themselves of carrion when it is in supply. In addition, scavenging may serve an important and integral role in the functioning of ecosystems in that it provides a source of energy to top carnivores that that can rival or exceed in magnitude levels of energy supply provided by the supply chain from plants through herbivores to carnivores.
See also: Competition and Competition Models; Predation; Trophic Structure
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