Carnivore Scavenger Interactions

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.

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