You scratch my back... the beneficial (and not so beneficial) relationships between organisms

To call someone a “parasite” is an insult. But the word has rather a different meaning in biology.

Etymologically speaking, the earliest known record of the word parasite in the English language was in 1539, when it was defined as “a hanger-on, a toady, a person who lives on others”. The word itself was derived from the Greek parasitos, meaning “a person who eats at the table of another”.

The social use appears to precede the scientific use, which was first recorded in 1646 as “an animal or plant that lives on others”.

Parasite might trigger distant memories of school lessons about fleas and tapeworms. But is this view accurate? As with most things in life, the answer is not as straightforward as it first appears.

It’s complicated

Parasites are a group of often unrelated organisms that share a way of life. Parasitism is only one example in the spectrum of ways organisms relate to each other.

Today, ecologists use “symbiosis” to refer to any relationship between two organisms. Anton de Bary, the pioneering mycologist (a fungi specialist), defined symbiosis when he wrote in his 1879 monograph Die Erscheinung der Symbiose that “any two organisms living in close association, commonly one living in or on the body of the other, are symbiotic, as contrasted with free living”.

Symbiosis can be subdivided into four broad categories, with clear examples in each, but the boundaries between them are sometimes blurred.

Parasitism

Parasitism is a relationship in which one partner (the parasite) benefits at the expense of the other (the host). Parasites hurt their hosts in many ways, ranging from general or specialised pathology and impairment of sexual characteristics, to the modification of host behaviour. Parasites increase their own fitness by exploiting hosts for food, habitat and/or dispersal.

Less obvious but familiar examples include the cuckoo, which is a brood parasite, laying its eggs in the nests of other bird species. This relieves the parasitic parent from the investment of rearing young or building nests, enabling them to spend more time foraging and producing more offspring.

The risk of losing an egg to raiders such as small mammals is reduced by distributing the eggs among different nests – literally not putting all their eggs in one basket.

Another interesting example, a parasite for life and not just at Christmas, is mistletoe. This plant grows on a wide range of host trees and commonly stunts their growth, but can kill them with heavy infestation.

Mistletoe is not completely dependent on its host and has its own leaves that do some photosynthesis. It uses the host mainly for water and mineral nutrients.

A lion eating a wildebeest or zebra is certainly benefiting from the other organism’s loss, but lions are predators, not parasites. Well-adapted parasites have typically evolved not to kill their hosts.

What about mosquitoes, which drink human blood? Parasites usually live in a very intimate relationship with their host, depending on it for more than nutritional requirements. The host is a source of food and at the same time provides a more-or-less permanent habitat. So, a mosquito is more properly a tiny predator.

But mosquitoes also transmit disease-causing micro-organisms such as the malaria protozoan or dengue virus. These are true parasites.

Mutualism

Mutualism is a relationship in which both partners benefit from the interaction.

The classic example of mutualism is lichen, a long-term association between a fungus and a green alga (or blue-green cyanobacterium). It is this that the German mycologist Heinrich Anton de Bary described as “the living together of unlike organisms”.

The fungus benefits from the relationship because algae or cyanobacteria produce food by photosynthesis. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungus, which also gather moisture and nutrients from the environment and (usually) provide an anchor to it.

A further example may be observed in a tropical aquarium. Well known to fans of the Disney film Finding Nemo, the clownfish is protected by a sea anemone, which stings the fish’s predators; in turn, the clownfish removes ectoparasites from the anemone.

Commensalism

Commensalism is a similar concept, but only one partner benefits, while the other is unaffected. The cattle egret is a classic example of a commensal.

This bird forages in fields among cows and horses, feeding on insects stirred up when the animals graze. The egret benefits from this relationship because the livestock inadvertently help it find a meal, while they are seemingly unaffected by its presence.

Another, more recently appreciated example is the colonisation of the human gut by so-called “good bacteria”, also known as probiotics, which multiply in the mammalian gut and apparently aid digestion. Whether this relationship is in fact commensal or mutual may depend on the species of bacteria involved.

Some biologists argue that any close relationship between two organisms is unlikely to be completely neutral for either party, and that relationships identified as commensal are more likely mutualistic or even parasitic in a subtle way that has not been identified.

Competition

Competition is an interaction between organisms in which the fitness of one, or potentially both, is lowered by the presence of the other. In some cases, both partners may be harmed by the relationship.

The behaviour of male red deer during the rutting season is an example of competition within a species, while trees of different species compete for light in a rainforest.

So, the next time you are tempted to call someone a parasite, think again. Your relationship with them may in fact be an example of competition, commensalism or perhaps even mutualism: you scratch my back and I’ll scratch yours.