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 discover how efficient nature can be. There are countless living things that help recycle the nutrients contained within animal waste. These processes are crucial for healthy ecosystems.
When an animal eats, there is a certain amount of waste that must leave its body as urine and faeces. Undigested material passes through the gut and emerges as excrement. Animal poo contains vast amounts of bacteria and often a coating of mucus. The distinct smell is hydrogen sulphide and other gases, produced by bacteria.
However, the journey doesn’t end there. Dung usually passes through the digestive tracts of many creatures before it is completely broken down. It will eventually end up as 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
The make-up and shape of dung varies from species to species. Even in the same individual the composition of its droppings can vary day to day. Things like diet and state of health all have an effect. It can however fall into several distinct types.
Carnivore dung is often easy to tell apart from that of herbivores. In appearance, it is often longer and thinner, and may contain telltale feathers, fur and bones. Fox 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 isn’t much energy left in their faeces. It is also a scarce resource (as there are normally fewer carnivores than herbivores in an ecosystem). This is why there are few specialists that feed on carnivore dung. As a result, it is mainly decomposed by bacteria and fungi.
The droppings of insectivores such as hedgehogs usually contain obvious insect remains, such as beetle carapaces. Insect exoskeletons are made of a tough substance called chitin, so parts of the insect pass the digestive tract intact.
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 weaker than those of other predatory birds. Owls also tend to swallow their prey whole, rather than picking off pieces of flesh. Thus 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, and this applies to bird droppings as well. Deer droppings for example are generally rounded or cylindrical, often with a pointed tip. Those of grouse species are relatively long and also cylindrical, usually with a white cap. 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.
Guests at the banquet
Dung is harmful to animals not adapted to consume it. This is the reason humans have evolved to be repelled by the smell). However, there are many for which dung is a welcome feast.
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, and even millipedes – eat their own droppings to digest more of the nutrients. 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. Droppings provide valuable ‘islands’ of food and habitat. 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 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. This robust and industrious looking insect has an incredible sense of smell, as do many dung–feeding 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 burrows beneath the dung, and drags 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. In many places the use of insecticides and parasite treatments in cattle has reduced the amount of dung beetles and other insects. 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 – along with its other close relatives – is partial to faeces. The adults feed on flies, nectar, and the occasional swig of liquid dung. It is the larvae however that are entirely dependent on the brown stuff. 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 many other creatures that feed on dung, such as the European black slug.
With this wealth of life attracted to dung, there are knock-on effects throughout the food chain. The many 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 searching for insect larvae. Swallows often frequent cattle fields, feasting on the flies attracted to cow pats. In prehistoric times, our ancestors may have seen a similar sight when herds of wild cattle or aurochs were present.
Dung and its decomposers are crucial to nutrient cycling. Wolves and other predators that ingest bone deposit phosphates in their droppings. Evidence from Canada shows that bears play a significant role in transporting nitrogen and other minerals. By eating salmon and then both depositing carcasses, and defecating in the woods (as bears are wont to do), they return carbon and nitrogen to the forest. Bears may have 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. As any gardener will testify, these casts show the role worms 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. Trees such as rowan have tempting red berries, which are an important food for several species of bird as well as mammals such as the pine marten. Birds in particular can often travel a long way 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 lush as a result of repeated fertilising with bird droppings. Rowans are frequently found growing beneath Scots pines in which redwings and their close relatives have perched. Similarly, birds such as mistle thrushes help to disperse holly.
Local environmental changes
There are other examples of how regular dung deposits can change the vegetation. Otters repeatedly deposit their droppings (called spraints) on a prominent feature such as a rock protruding from the river. Over time, the supply of nutrients creates a flush of algae, making the rock a distinct green colour. Otters around the Scottish coast need a source of freshwater to keep their coats in good condition. They often have spraint sites beside streams entering the sea which you can spot from a distance because of the rich mound of grass that develops.
Badgers are very fastidious in their defecation habits, and have dedicated latrine areas. In some woodland types, areas where badgers have lived for a long time often have a lot of nettle and elder, which thrive on the nitrogen rich soil. Badgers also eat elderberries so again their dung plays a role in seed dispersal.
Many animals use dung and urine to mark their territories. These deposits are usually scented with secretions from special glands. 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. These markers provide other information such as whether an animal is ready to mate.
Surveys and detective work
Many of our native animals are nocturnal and are wary of humans. Because they are so elusive, droppings are among the main clues that naturalists and biologists use to study certain wildlife.
As well as giving away the presence of a particular species, the amount of dung can also give us an idea of its abundance. Biologists have actually calculated the average number of times that red deer and roe deer 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 precautions!), giving an insight into an animal’s diet. By identifying the prey in an animal’s diet we can shed light on other species in the area. For example, in spring, fox droppings are often quite white. This is because foxes eat a lot of voles and the high levels of calcium and phosphorus ingested then colours their droppings. These kinds of seasonal changes are quite common, and are particularly noticeable in the droppings of pine martens. During the summer beetle wing cases often show up 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. Peregrine falcons droppings stain parts of the cliffs and crags where they like to nest. You can sometimes locate bats by locating the dark stain of urine that appears below the entrance to their roosting places in tree crevices.
Vast amounts of dung are produced in many ecosystems including the Caledonian pinewoods. Without the animals, fungi and bacteria that break it down there would soon be an alarming build-up. Vegetation would be smothered and essential nutrients withheld from the ecosystem. Thanks to those adapted to the task, the nutrients contained in dung are eventually returned to the soil – and so the cycle continues.
Sources and further reading
- Baker, N. (2013). The Nature Tracker’s Handbook. Bloomsbury: London.
- 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.
- Rhyder, J. (2021). Track & Sign. The History Press: Cheltenham.
- Wester Ross Fisheries Trust www.wrft.org.uk/files/soilecosystem.pdf (Accessed September 2021)