Carrion

Every animal in a forest is destined to die. An obvious point perhaps, but why then are there not heaps of lifeless bodies scattered among the trees? Animal carcasses, or carrion, contain a high concentration of energy and nutrients that need to be recycled back into the ecosystem. While it may seem unpleasant to us, to the scavengers that eat it, carrion is a valuable (and appetising!) source of food. It doesn't always have the same appeal as many other features of the forest, but carrion and its scavengers are a vital and fascinating part of the forest ecosystem.

Carrion becomes available for many reasons, including death by disease, predation or starvation. Whatever the cause, when an animal dies a highly nutritious meal is there for those with the stomach for it. Scavenging can be an effective strategy as it saves on the energy that would be used in chasing and killing prey, and avoids the potential dangers of doing so; living prey can put up a fight after all! However, while it can be easier than hunting, scavenging isn't necessarily easy. When a carcass becomes available (at the moment of death) a fierce competition ensues for this precious source of energy. A wide range of animals and other organisms are quick to take advantage of a carcass; let us take a look at some of the main players.

Carrion scavengers

Bacteria set to work immediately, starting to decompose the animal's tissues, and certain insects such as burying beetles and blowflies are carrion specialists. One of the first blowflies on the scene will be the beautiful, metallic looking greenbottle (Lucilia caesar). Many carnivores and omnivores don't specialise in carrion, but are opportunists and will scavenge carcasses when they find them. Foxes (Vulpes vulpes), badgers (Meles meles), wild boar (Sus scrofa) and brown bears (Ursus arctos) are among the mammals that eat carrion and there are plenty of birds that will also feed on dead animals.

Members of the crow family such as ravens (Corvus corax) and hooded crows (Corvus corone cornix) are highly intelligent and adaptable, and can often be seen on a carcass (they seem to find eyeballs a particular delicacy!). For both of our native eagles ? the golden eagle (Aquila chysaetos) and the white-tailed eagle (Haliaetus albicilla) ? as well as the red kite (Milvus milvus), carrion is a very important source of food. All of these organisms are potentially competing for the same food source, so it is interesting to look at some of the different strategies used to ensure that scavengers can detect the carrion, and get their share.

Detecting carrion

Finding food, living or dead, can pose a challenge, especially among denser woodland. Fortunately for carrion-eaters, decomposing flesh can be very strong smelling. From the moment of death the body's own chemicals, as well as bacteria, begin breaking down the tissues, releasing gases in the process that are responsible for the odour. Not surprisingly then, most scavengers follow their noses (or other olfactory organs!) to a carcass.

In addition, the noise and commotion caused by a group of crows on carrion can alert other animals that there's something worth squawking about in the forest! In this way bears or foxes for example may also use their ears to locate carrion. Bears in particular would be able to scavenge a fairly fresh carcass by muscling in on the efforts of a smaller predator (including other bears).

Sight plays a much more important role for birds of prey, such as golden eagles, which scavenge and hunt in more open forest and moorland; it is still not clear if, and to what extent, their sense of smell is involved, although it is thought by some that it could be used to some degree.

Scavenging strategies

Scavenging a carcass requires a tough stomach. Anyone who has been unfortunate enough to suffer from food poisoning will have experienced the effects of powerful toxins produced by bacteria. There is a theory that these toxins are actually a protection mechanism on the part of the bacteria which are effectively preventing their food source from being eaten by large animals. In any case, animals that eat them need robust digestion and immune systems to be able to deal with such a potentially harmful source of food.

This being the case, some scavengers prefer their carrion at a particular stage of decay, and many mammals choose fresher corpses over really putrid ones. In contrast, most flesh flies (family Sarcophagidae) lay their eggs in flesh that is in an advanced state of decay to give their maggots the best start in life.

While the organs and flesh of a carcass are the easiest parts to tackle, even the tougher bits don't go to waste. Dermestid beetles can digest the soft tissue around bones, and are even used in museums to clean skeletons for exhibition! Some Tineid moths (the family of moths that includes clothes moths) consume keratin in hair and feathers in later stages of decomposition. Minerals such as calcium can be difficult to come by, and bones (as well as discarded deer antlers) are a valuable part of the ecosystem. Take a close look at old bones on the forest floor, and you may see the tiny teeth marks of mice or voles that have used this important food source.

Among the most intriguing and important carrion eaters are the burying or sexton beetles (Nicrophorus spp.). These undertakers of the forest can detect a dead bird or mouse, for example, from a over a mile away, within an hour or two of death. They do this by using special olfactory (ie odour detecting) organs on their antennae. The beetle can bury a carcass many times its own size, excavating the ground beneath the body, and then laying its eggs in the soil near the carrion. Burying the carcass helps reduce the risk of it being scavenged by other animals.

The burying beetles are very unusual insects in that they show a surprising amount of care for their offspring. Carrion is a valuable food source and competition is high, so with both parents caring for the young, they are able to fend off competitors more easily. Another fascinating part of their life is the mutualistic relationships with the mites (Poecilochirus necrophori) they carry on their bodies. In return for a free ride to new hunting grounds, the mites prey on the blowfly larvae in the carrion that would otherwise be competing with the burying beetles.

While competition is a major feature in the ecology of carrion, it is only part of a very complex picture, as the relationship between burying beetles and their mites clearly illustrates. In addition, the fact that some organisms have evolved to specialise on certain parts of a carcass is a good example of what ecologists call 'resource partitioning': through natural selection, certain species (Tineid moths are a good example) find their own specialist niche, and so avoid too much competition.

There are other surprising examples of cooperation and mutual benefit in the quest for carrion. Before the wolf (Canis lupus) was eradicated in Scotland, its kills would have been a source of food for various forest scavengers. Wolves tend to eat their fill and leave the rest, in contrast to the lynx (Lynx lynx) which makes more effort to hide its kill for later. As the wolves leave to sleep off their meal, scavengers have an opportunity to move in and feed.

Overall, wolf kills provide a fairly steady supply of food to other wildlife throughout the lean months of winter. Scientists studying the re-introduced wolves in Yellowstone National Park in North America (an ecosystem with many parallels to the Caledonian Forest) have found that climate change is having an adverse effect on scavengers, as milder winters mean that more elk (Cervus elaphus) survive, and so less carrion is available. Wolves were re-introduced to Yellowstone in 1995, and the resulting increase in carrion has benefited many other species: even in mild winters, wolves ensure that elk carcasses are available throughout the season.

The raven (Corvus corax) is known in other parts of its range to associate with wolves as a reliable provider of food. There are reports of ravens congregating and calling where potential prey animals, such as deer, are present. Wolves follow these calls, and the ravens benefit from the spoils if the wolves make a successful kill. Wolves also open up the tough hide, giving the ravens easier access to the interior of the carcass.

Salmon carrion plays an important role in some forest ecosystems. In the temperate rainforests of western Canada, bears often leave parts of salmon (Onchorynchus spp.) carcasses in the forest, which are then available to other terrestrial scavengers. In this way nitrogen is brought into the forest, enriching the soil. This is a very important nutrient exchange, as the tendency is for nutrients to move from land to sea, so a transfer in the opposite direction is a very valuable one. It is possible that a similar process took place on a significant scale in Scotland's ancient forests (with Atlantic salmon Salmo salar), forming an important link between between ocean (where salmon spend much of their lives) and the forest ecosystems.

It is important to remember that, as well as the food carrion provides for scavengers, there are also predators that eat the scavengers. Blowflies, beetles, foxes and crows - in fact most scavengers - are potential prey for species further up the food chain, illustrating how carrion can really enrich the web of life.

The influence of humans

Sadly, carrion, laced with poison, is sometimes used as bait for scavengers such as crows and birds of prey. This illegal practice is used by a minority of gamekeepers. As well as being inhumane, it is also devastating for wildlife that may not be directly targeted. The corpses of the unfortunate bird or animal killed by the bait then kill other scavengers that feed upon them. The fact that carrion is used as bait shows what an important source of food it is.

When deer are culled on the hills of Scotland there is a process known as 'gralloching' that involves leaving behind deer entrails before the carcass is removed. It would at first seem that this process might simulate the carrion that was previously left by wolves. In fact it doesn't, because gralloch is often very concentrated in time and space, rather than being more widely scattered through the forest and throughout the year. While the practice of gralloching can provide food for eagles, there can be problems when high concentrations of gralloch artificially boosts numbers of crows and foxes, which can then prey upon rare ground nesting birds. Because of this, in sensitive areas 'gralloch pits' are sometimes dug to discourage unnaturally high numbers of scavengers.

The loss of large predators in the Caledonian Forest presents something of a conundrum. Deer control is essential for forest regeneration, and selling the venison can provide further revenue for woodland restoration. However, while the gralloch is left, removing carcasses (and livestock) from the hill has resulted in a long-term depletion of nutrients from the forest ecosystem, leading to a loss of minerals, as well as food for the many animals that can benefit from carrion.

The broader picture

In a balanced forest ecosystem, carrion provides food for some of the most microscopic to some of the largest creatures in the forest, and forms a unique 'microclimate', offering shelter and protection to the myriad organisms that inhabit it. It is one of many ecological features in the forest that contributes to the complexity and health of the overall ecosystem. The scavengers that thrive on carrion play a crucial role in the food chain, ensuring that the nutrients locked up in animals' bodies are released back into the soil. These can then be used again by all the other life forms in the forest. In this way, carrion and its scavengers form a vital link between the death of an animal, and newly emerging life.

Dan Puplett

 

Sources & further reading

  • Begon M., Harper J.L., and Townsend C.R, (1996). Ecology (3rd ed.) Blackwell Science: Oxford.
  • Burkepile, D.E., Parker, J.D., Woodson, B.C., Mills, H.J., Kubanek, J, Patricia A. Sobecky, P.A., and Hay, M.E. (2006)
  • Chemically mediated competition between microbes and animals: microbes as consumers in food webs. Ecology 87 (11) pp. 2821?2831.
  • Chinery, M. (2005) Complete British Insects. Collins: London.
  • McGrady, M.J., Petty, S.J. and McLeod, D.R.A. (2004). Potential impacts of new native woodland expansion on golden eagles in Scotland. Scottish Natural Heritage Commissioned Report No.018 (ROAME No. F99LD01).
  • Melis, C., Teurlings, I., Linnell, J., Andersen, R., Bordoni, A., (2004) Influence of a deer carcass on Coleopteran diversity in a Scandinavian boreal forest: a preliminary study
  • European Journal of Wildlife Research 50 (3), pp. 146-149
  • Reimchen, T.E., Mathewson, D., Hocking, M.D., Moran, J. and Harris, D. (2002)
  • Isotopic Evidence for Enrichment of Salmon-Derived Nutrients in Vegetation, Soil, and Insects in Riparian Zones in Coastal British Columbia
  • American Fisheries Society Symposium XX. http://web.uvic.ca/~reimlab/n15clayoquot.pdf
  • Steinhart, P. (1995) The Company of Wolves. Vintage Books: New York.
  • Wilmers, C & Getz, W (2005) Gray Wolves Help Scavengers Ride Out Climate Change. PLoS Biology 3(4)

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