The ecology of dung is often overlooked. Admittedly, it doesn't make the best topic for dinner conversation or at parties, but the fact remains that every living thing produces waste - excretion happens, regardless of our sensibilities. Squeamishness aside, it is fascinating to realise how efficiently nature can break down and recycle the nutrients contained within waste products. This process is crucial for healthy ecosystems.
When an animal eats, there is inevitably a certain amount of waste that must be expelled from its body as urine and faeces. Undigested material passes through the gut and emerges as excrement, containing vast amounts of bacteria and often a coating of mucus. The distinct smell is hydrogen sulphide and methane, produced by bacteria.
However, the journey doesn't end there: dung usually passes through the digestive tracts of many creatures before being thoroughly broken down into nutrients such as nitrogen and phosphorus, as well as various gases. There is a rich community of animals that both depends on dung, and plays an essential part in its decomposition.
All shapes and sizes
Perhaps unsurprisingly, the make-up and shape of dung varies from species to species. Even with the same individual the composition of its droppings is affected by various factors, such as diet and state of health. It can however fall into several distinct types.
Carnivore dung is easily distinguishable from that of herbivores. In appearance, it is often longer and thinner, and may contain telltale feathers, fur and bones. Fox (Vulpes vulpes) scat is a classic example: narrow and elongated, with a distinctive tapered twist at one end. Carnivores have more efficient digestive systems than herbivores, so there is relatively little energy remaining in their faeces. It is also a scarce resource (as there are normally fewer carnivores than herbivores in an ecosystem), which accounts for the absence of specialist animals feeding upon it. As a result, it is decomposed primarily by bacteria and fungi.
The droppings of insectivores such as hedgehogs (Erinaceus europaeus) are generally harder to find, but usually contains obvious insect remains, such as beetle carapaces. As insect exoskeletons are composed of a tough substance called chitin, parts of the insect pass fairly unscathed through the digestive tract.
The elimination habits of birds (carnivorous or otherwise) deserve some special attention. Most people are familiar with the white droppings of many bird species. Birds do not urinate as such their urine and faeces are eliminated from the same opening, known as a cloaca. The whiteness in their droppings is uric acid. Many raptors (birds of prey) and scavengers regurgitate pellets, as well as defecating. While they are not strictly dung these pellets consist of some of the more indigestible parts of their diet. In the case of owls, there are often lots of bones present in the pellet, as their digestive juices are somewhat weaker than those of other birds of prey; also owls tend to swallow their prey whole, rather than picking off pieces of flesh. Therefore raptor droppings are usually very white and paint-like, containing little solid matter.
The dung of herbivorous mammals is very variable, but tends to be rounded or pellet-shaped. It often contains visible, fine plant remains. Deer droppings for example are generally rounded (resembling chocolate raisins!), while those of grouse species are relatively long and cylindrical. In contrast to that of carnivores, herbivore dung can contain up to around fifty per cent of the energy value of the original food the animal ate. While dung is harmful to animals not adapted to consume it (this is the reason humans have evolved the capacity to be repelled by the smell), there are many for which dung is a welcome feast.
Guests at the banquet
Nothing in nature is wasted and dung is no exception. The act of eating dung is called coprophagy. From the moment it falls, there are plenty of hungry mouths ready to join the feast. In fact, some herbivores - including mountain hares (Lepus timidus), a number of rodents and even millipedes - eat their own droppings to digest more of the nutrients; this behaviour is known as refection. The first set of droppings produced by hares is soft and greenish, and are rarely seen as they are eaten quite promptly. It is usually the second round of more well-digested pellets that are found. Unpleasant as this may sound, it is an adaptation that makes sense in nutritional and survival terms.
While herbivore dung is more abundant than that of carnivores, it is still a relatively scattered resource and provides islands of food and habitat which are mostly used by opportunists. Cow dung, for instance, is consumed by many organisms such as earthworms, as well as being broken down by bacteria and fungi. There are a number of fungi, such as the dung roundhead fungus (Stropharia semiglobata), that are found primarily on deer and other droppings. Some of the more charismatic dung specialists are the various dung beetles. Among those commonly found in our native woodlands is the dor beetle (Geotrupes stercorarius). This robust and industrious looking insect has an incredible sense of smell, as do many dungfeeding insects. It catches the scent of dung from a considerable distance and is then able to follow the smell upwind to its source. Having located its prize and mated, it dig burrows beneath the dung, and drag pieces of it down into the tunnels as food for its larvae. This helps with both nutrient cycling and aeration of the soil.
The role of dung decomposers is highlighted when human-induced imbalances occur. For example, in parts of England the use of insecticides and parasite treatments in cattle has reduced the amount of dung beetles and other insects in their dung. This inhibits the breakdown of dung and also has knock-on effects for bats and other wildlife dependent on these insects.
As its name suggests, the yellow dung fly (Scatophaga stercoraria) - along with its other close relatives - is very keen on faeces. The adults feed on flies, nectar, and the occasional swig of liquid dung. It is the larvae however that are entirely dependent on dung. The adults congregate around fresh dung, mate, and then lay their eggs within it. The larvae hatch and then burrow down into the dung, emerging as a new generation of adults in 3-4 weeks. There are numerous other organisms that feed on dung, such as the European black slug (Arion ater), which is commonly seen on wet days in our native pinewoods.
With this wealth of life attracted to dung, there are knock-on effects throughout the food chain. For example, the myriad insects associated with faeces provide food for a variety of birds. It is quite common to see cowpats riddled with holes made by the beaks of starlings (Sturnus vulgaris) searching for insect larvae. Aerial hunting birds such as swallows (Hirundo rustica) often frequent areas where cattle are present, feasting on the flies associated with dung. In prehistoric times, birds may have demonstrated similar habits in areas where herds of wild cattle or aurochs (Bos primigenius) were present.
Dung and its decomposers are crucial to nutrient cycling. Wolves (Canis lupus) and other predators that ingest bone deposit phosphates in their droppings. Evidence from Canada suggests that bears (Ursus arctos) and other fish eating predators play a significant role in transporting nitrogen and other minerals. By eating salmon (Oncorhynchus spp. ), and then both depositing carcasses, and defecating in the woods (as bears are wont to do), they return carbon and nitrogen to the forest. It is could be speculated that bears had a similar influence in Scottish forests before their extinction here.
A clear example of the positive effects of dung are the faeces or casts of earthworms. While many parts of the Caledonian Forest are too acidic for earthworms, there can be pockets of brown earth where they are present. As any gardener will testify, the casts demonstrate the crucial role worms can play in breaking down larger organic matter into fine, rich soil.
Many plants have evolved to take advantage of animals and birds need to defecate. For example, trees such as rowan (Sorbus aucuparia) have tempting red berries, which are an important food for several species of bird as well as mammals such as the pine marten (Martes martes). Birds in particular can often travel a considerable distance from the source plant before excreting the seed, complete with a package of fertiliser!
It is not uncommon to find small rowan trees in otherwise treeless areas. These are often on rocky mounds which serve as perches for birds. The vegetation can be relatively lush as a result of repeated fertilising with bird droppings. Rowans are frequently found growing beneath Scots pines (Pinus sylvestris) in which redwings (Turdus iliacus) and their close relatives have perched and passed out the seeds. Similarly, birds such as mistle thrushes (Turdus viscivorus) help to disperse holly (Ilex aquifolium).
Local environmental changes
There are many other examples of how frequent deposition of droppings can result in local vegetation changes. For example otters (Lutra lutra) repeatedly deposit their droppings (called spraints) on the same prominent feature such as a rock protruding from the river. Over time, the steady supply of nutrients creates a flush of algae, making the rock a distinct green colour, allowing it to be spotted from some distance. They often have spraint sites beside these streams and again these can be spotted from a distance because of the rich mound of grass that develops.
Badgers (Meles meles) are very fastidious in their defecation habits, and have dedicated latrine areas, usually at a respectable distance from their setts (as their burrow systems are called). In some woodland types, areas that have been occupied by badgers over long periods often have considerable amounts of nettle (Urtica dioica) and elder (Sambucus nigra), which thrive on the elevated levels of nitrogen in the soil. Badgers also eat elderberries so again their dung plays a role in seed dispersal.
The influence of agriculture and former human settlements can often be seen for many years after an area has been abandoned by humans. It is common to see abundant growth of nettles around such areas as many years of animal manure being deposited again increases the nitrogen content of the soil.
Many animals use dung and urine as a means of marking their territory. These deposits are usually scented with secretions from special glands. Bear in mind that for a lot of mammals smell is the dominant sense, so scent-based messages provide a wealth of information for other animals in the area. Otter spraints are a good example. These are olfactory notices, informing others of the same species that the territory is occupied. For some species these markers indicate whether an animal is ready to mate. Clear marking of territories is vital as it prevents excessive competition for the same resources. This helps the species to stay healthy and ensures that its food source is not depleted, thereby helping to keep the wider ecosystem in balance and health.
Surveys and detective work
As many of our native animals are so elusive (mammals are often nocturnal and are also able to detect our presence long before we see them!), droppings are among the main clues that naturalists and biologists use to study certain wildlife.
As well as indicating the presence of a particular species, the amount of dung can also give an indication of its abundance. Biologists have actually calculated the average number of times that red deer (Cervus elaphus) and roe deer (Capreolus capreolus) defecate in a day. Dung counts can be made along a transect line, and then a formula can be used to calculate the number of deer in the area!
The contents of faeces can be examined (of course using appropriate hygiene precautions!), giving an insight into an animals diet, and sometimes an indication of other creatures in the area. For example, in spring, fox (Vulpes vulpes) droppings are often quite white, because in areas with large vole populations, the high levels of calcium and phosphorous ingested when eating whole voles colours their droppings. These kinds of seasonal changes are quite common, and are particularly noticeable in the droppings of pine martens. During the summer it is not uncommon to find the wing cases of beetles in their scat, while in early autumn they are often filled with rowan and other berries.
Some faeces can be seen from a considerable distance revealing the whereabouts of certain wildlife. The droppings of peregrine falcons (Falco peregrinus), can stain parts of the cliffs and crags where they like to nest, while bats can sometimes be located by the dark stain of urine that appears below the entrance to their roosting places in tree crevices.
Vast amounts of dung are produced in the Caledonian Forest, and without the animals, fungi and bacteria that break it down there would soon be an alarming build-up, smothering vegetation and withholding essential nutrients from the ecosystem. Fortunately, with the assistance of numerous organisms adapted to the task, the nutrients contained in dung are eventually returned to the soil and so the cycle continues.
Sources and further reading
Bang, P. and Dahlstrom, P. (2001).Animal Tracks and Signs. Oxford University Press: Oxford.
Begon, M., Harper, J.L. and Townsend, C.R, (1996). Ecology (3rd ed.) Blackwell Science: Oxford.
Ben-David, M., Hanley, T.A. and Schell D. M. (1998). Fertilization of terrestrial vegetation by spawning Pacific salmon: the role of flooding and predator activity Oikos 83 (1), 47-55.
Chinery, M. (2005). Complete British Insects. Collins: London.
Jones. R. (2008). How is dung recycled? BBC Wildlife 26 (3), 66-67.
Wester Ross Fisheries Trust www.wrft.org.uk/files/soilecosystem.pdf