Food, in whatever form, is essential for the survival of living things. Millions of years of evolution have resulted in a myriad of strategies for obtaining food, and these different interactions are part of the glue that binds ecosystems together.

Some feeding strategies are more familiar: carnivores are animals (and plants) that eat other animals, while herbivores feed on plants. Then there are the different kinds of symbiotic relationship which involve more close and complex interactions. Mutualistic relationships are a partnership: each of the life-forms involved benefits the other in a ‘win-win’ situation. Bees pollinating flowers are an example. (Slightly confusingly, we commonly use the word ‘symbiotic’ to mean just ‘mutualistic’). Commensalism is when one organism ‘hitches a ride’ on another, or uses it as a home, but causes no obvious harm. An example would be mosses living on the bark of a tree. Now enter the bizarre world of parasitism.

A parasite is a living thing that gets food from another organism, harming, but not killing it. Its feeding habits are known as parasitism. More than half of the Earth’s species are parasites while virtually every species may play the host.

Parasites come in many shapes and sizes, and include viruses and bacteria, various worms, plants, fungi, insects, and even birds and mammals. They have a huge effect on living things and their populations, and are a major cause of disease in humans and other organisms. As we will see below, taking essential nutrients from another organism can cause it to weaken and affect its fertility, but the wider consequences of this are sometimes unexpected.

The Caledonian Forest is literally crawling with parasites – some of which are invisible, while others may be easily seen – or felt! There are some fascinating examples of more well-studied species that can help us glimpse their role in our native forest ecosystems.


Fungi are virtually everywhere, and many species are parasitic. Some of the more obvious ones are those found in trees, and form a variety of ‘bracket’ fungal bodies. A good example is the aspen bracket fungus. Unlike most parasites, a lot of the bracket fungi kill their host tree. But these fungi indirectly benefit other species by creating the dead wood habitat required by rare invertebrates such as the aspen hoverfly.

Parasitic Plants

Even some plants have a darker, parasitic side. Common cow-wheat (Melampyrum pratense) is semi-parasitic on the roots of a number of plants including blaeberry (Vaccinium myrtillus), while lousewort (Pedicularis sylvatica) and yellow rattle (Rhinanthus minor) are found on grasses. Parasitic plants can increase the diversity of plant species in an area by keeping more dominant species in check.


Galls on a plant are a sign that a parasite has been at work. These abnormal growths are a response to certain parasites invading the plant’s tissues. There are many gall-inducing parasites: fungi and gall wasps are among the more common ones. A wasp, for instance, causes a plant to produce a gall in order to create a source of food and shelter in which its larvae can develop. The plant suffers wasted energy and nutrients that go into the extra growth.

Some galls, such as oak apples (caused by the wasp Biorhiza pallida), can support communities of other insects which in turn can be food for birds. Look into the canopy of a birch tree (Betula spp.) and you may see dense bunches of twigs which look a lot like birds’ nests. These are the result of infection by the parasitic witches’ broom fungus (Taphrina betulina).


Blood is a rich source of nutrients, and there are a number of bloodsucking, or haematophagous, invertebrates. Certain species of ticks (Ixodes ricinus) and midges (Culicoides impunctatus) are among the most feared parasites in Scotland! The females of these species use their piercing mouthparts to feed on the blood of warm-blooded animals. They do this to get the protein they need to nourish the eggs they are carrying.

It may be a small comfort when being bitten by midges to know that the larvae of certain species of red mite are themselves parasites on adult midges. This illustrates clearly that being a parasite doesn’t make you immune from parasitism: most parasites have their own parasites to deal with!

Invaders, usurpers and thieves

Parasitism doesn’t always involve feeding directly from a host: some species take advantage of a host’s food or resources. The cuckoo (Cuculus canorus) is well-known for its habit of laying its eggs in the nests of smaller birds, a strategy known as brood parasitism. Meadow pipits (Anthus pratensis) and dunnocks (Prunella modularis) are among the most regular victims of this ruse.

Remarkably, individual strains of cuckoo specialise in parasitising a particular species of bird. They lay an egg of a colour that matches the eggs of their host, thus disguising it. The young cuckoo then grows much larger than its unfortunate hosts. It also pushes the young of the host out of the nest.

One of the wood ant species, the slave-making ant (Formica sanguinea) is a social parasite. After entering the nest of smaller ant species such as Formica fusca, the queen slave-maker kills the host queen and ‘enslaves’ the hatching workers to care for her own offspring.

Food theft, or kleptoparasitism, is when an animal gets its food by stealing prey killed by another animal. Food theft has the advantage that an animal can eat prey it may not have been able to kill otherwise. It can also save the energy and potential danger involved in tackling the prey. Members of the crow family often use this strategy, as do birds of prey. Among the mammals that were once in the Caledonian Forest, the brown bear (Ursus arctos) and the wolf (Canis lupus) would also have stolen kills. Wolves can drive lynx (Lynx lynx) or raptors off a kill, and bears are able to see off any other predator!

Parasitoids are parasites that eventually kill their host. They are somewhere between predator and parasite. There are many parasitoid wasps, such as the ruby-tailed, or cuckoo wasp (Chrysura hirsuta) which lays its eggs within the brood cells in the nests of the mason bee (Osmia uncinata). The wasp larvae then feed on the bee larvae.

More than meets the eye

One animal may be a host to a wide range of parasites. As is also the case with mutualistic relationships, when we see an individual animal, we are actually looking at a whole community of life-forms. For example, red deer (Cervus elaphus) parasites have been fairly well-studied, and the deer serves as a good illustration of the range of parasites that can inhabit one animal.

Among the internal or endoparasites are liver flukes (Fasciola hepatica), a variety of worms, and microscopic gut parasites. External or ectoparasites include ticks, deer keds (Lipoptena cervi) and the larvae of the nasal botfly (Cephenemyia auribarbis).


From viruses to ticks, parasites have a wide range of strategies for spreading from host to host. Lice, for example, usually transfer when two hosts make contact. Fleas crawl and leap for relatively long distances while ticks crawl to the top of a piece of vegetation, such as a bracken stem, and wait for a passing host to brush by. Rusts and mildews are transmitted by spores, and some parasitic fungi spread when plants roots make contact.

Midges locate their hosts by detecting chemical signals, particularly carbon dioxide, in their hosts’ breath. Borrelia burgdorferi, the bacterium responsible for Lyme disease, lives in the gut of some ticks, and can be passed into another organism when the tick bites a host for a meal of blood. This is an example of how a parasite can use another parasite as a vector: a means of travelling between, and infecting, new hosts.

Defences against parasites

A host will usually try to repel or remove parasites where possible, and there are a multitude of ways in which this can be achieved. Mammals groom, and birds preen, activities which, among other functions, remove unwelcome guests such as ticks. Red deer use mud wallows to help get rid of ectoparasites such as deer keds, and ticks. A number of birds, including the capercaillie (Tetrao urogallus) rid themselves of ectoparasites by ‘anting’. This is when a bird allows ants to crawl through its feathers. The ants spray their defensive formic acid, which helps to kill the bird’s parasites.

Another defence is through avoidance. Fleas parasitise a wide range of warm-blooded animals. Certain species of flea infest the nests of crested tits (Parus cristatus). This forces the birds to change nesting sites every year, increasing the demand for suitable sites.

The starling (Sturnus vulgaris) has an intriguing way of coping with the parasites that prey on nestlings. The male starling collects aromatic plants to weave into the nest. The aromatic oils in the plants boost the young birds’ resistance to parasites and increase their chances of survival.

At the microscopic end of the scale, some of the most dangerous parasites – viruses and bacteria – are fought directly by the host’s immune system.

The influence of parasites

Parasites have a powerful and complex influence on the populations of living things in the forest ecosystem. They play a key role in regulating extreme swings in populations. By reducing fertility they also prevent certain species from becoming to plentiful, thus giving other organisms a chance to thrive.

The tendency for ecosystems to naturally evolve towards diversity is illustrated by the fact that monocultures created by humans (such as fields of wheat, or conifer plantations) are often prone to parasitic disease, and it takes a great deal of input to keep parasites at bay. Diverse ecosystems are much less vulnerable to devastating epidemics. The presence of parasites, over time, promotes biodiversity.

Breeding chaffinches (Fringilla coelebs) illustrate the effect parasites have on natural selection. The fewer parasites a male has, the brighter the pigments in his feathers. Why does this matter? Brighter coloured males are more attractive to females, so his genes for fitness and immunity are passed on to the next generation. This shows that parasites can actually improve the long-term health of a population.

Parasitism also influences the movements of animals. Midges, in particular, force red deer onto higher ground (as well as keeping tourists at bay!), and this has an effect on grazing patterns.

Parasites rarely have the same public appeal as more glamorous species. But while by definition parasitism harms individual organisms, in a larger context these fascinating interactions play an indispensable role in promoting health and diversity in the forest.

Sources and further reading
  • Parasites and Parasitism
  • Begon, M., Harper, J.L. and Townsend, C.R, 1996. Ecology (3rd ed.) Blackwell Science: Oxford.
  • Engel, C. 2002. Wild Health: How Animals Keep Themselves Well and What We Can Learn From Them. Houghton Mifflin: New York.
  • Windsor, D.A. 1995/6. Endangered Interrelationships – The Ecological Cost of Parasites Lost. Wild Earth 78-83.

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