Symbiosis is a close and often long term interaction between two different species. Some argue that the term symbiosis should only apply to persistent mutualism. Others follow the simplest definition, “the living together of unlike organisms”, and apply it to all interspecies relationships. Since we cover both mutually beneficial relationships and less balanced relationships in this lesson, we will fall in with the second group. In a symbiotic relationship, the species that hosts the other is called the host. The organism inside or on the host is called the symbiont, and is typically much smaller than the host. Depending on the specific example, each host might have a single symbiont, several, or even millions.
Mutualism is a relationship between two different species in which both individuals benefit from the other. The most common, and perhaps most often overlooked, example of mutualism is the relationship between animals and their gut flora. The bacteria in the intestines of animals facilitate digestion and overall health for the host animal. In return, the bacteria are provided with a steady diet and a more or less safe environment in which to thrive. This is also a good example of a case where a single host could have millions or even billions of symbionts.
Another example of a large colony of symbionts living on and in their host is the acacia tree and it's ants. The stinging ants hollow out the tree's thorns and live inside them. They protect the trees both from pathogens and from the invasion of other insects that would eat the tree. In return, the tree provides a very protective shelter for the ants and the nectar that the ants eat.
The oxpecker is a species of bird that shares a mutalistic relationship with a variety of large mammals and serves as a great example of mutualism with a very small number of symbionts per host. These birds, either alone or in small groups, perch on the backs of large mammals and eat various bugs that feed or just annoy the larger animals. The birds are granted a relatively safe perch and a meal. In return, they clear the mammals of potentially dangerous parasites.
As you can see from the examples above, the larger the symbiont, the fewer there tend to be on or in each host. Also apparent from the above examples, some mutualistic relationships are crucial to the survival of both symbiont and host. In others, such as is the case with the oxpeckers, both species benefit from the relationship, but could likely survive without it.
On the other end of the symbiotic specturm, parasitism is a non-mutual relationship between two species. In parasitic relationships, the symbiont is called a parasite. Parasites benefit from the relationship while the host typically suffers. Traditionally, the term parasite is reserved for organisms visible to the naked eye. However, by the simplest definition of the term, even harmful bacteria, viruses, and other harmful microorganisms are parasites. Just as with mutualism, these symbionts can be either relatively close in size to the host or millions of times tinier, and each example is different from the next.
Unlike predators, parasites do not typically kill their hosts. However, in some cases, as is with many viral and bacterial infections, they can directly kill the host. Others that don't directly kill the host, like ticks or intestinal worms, can cause the death of the host indirectly by spreading disease or causing malnutrition. Most parasites, however, do not kill or cause the death of their host under normal infestation circumstances.
Fleas are a well known example of animal parasites. Fleas gain the benefit of a home and food source from their hosts, while the host animals gain no benefits at all. In return for providing room and board for the fleas, host animals get skin irritation and potential infection by any disease the flea may be carrying. In this example, the parasite is annoying, but generally not extremely harmful.
Tapeworms live in the intestines of larger animals, and survive by absorbing the nutrients that would otherwise go to the host. This nutritional theft might not always harm, or even annoy the host. However, it does have the potential to cause malnutrition. If a host is malnourished due to an intestinal worm, it will suffer a variety of health issues, possibly leading to death.
About half way between mutualism and parasitism is commensalism. Commensalism is a relationship in which the symbiont benefits from the relationship, but the host is neither helped nor harmed.
Clown fish live among the tentacles of anemones. The anemones are neither harmed nor helped by the presence of the small fish, but the clown fish are protected from larger predators by the poisonous tentacles of the anemone.
Many plants, such as orchids, grow on larger trees and receive the benefit of habitat from the trees. The trees themselves are neither helped nor harmed. While this is a good specific example of commensalism, there are other plants that do harm their host trees. These, of course, would actually be parasites.
Barnacles attach themselves to whales and mollusks. In doing so, they gain a mode of transportation and a source of food, as the barnacles eat the same plankton as the host. Some do argue that barnacles are actually parasites. Those who argue this typically say that the barnacles take food away from the host or that they slow the host's movement. While any plankton eaten by a barnacle is, in truth, plankton the host can not eat, the amount eaten is so small that it doesn't pose any risk of harm to the host. There is also not a significant difference, if any at all, in movement speed between mollusks or whales with barnacles and those without.
Amensalism is quite different from the other examples of symbiosis, in that there isn't really a symbiont and host relationship. Amensalism is an association between individuals of two different species in which one is inhibited or destroyed outright while the other is unaffected. There are two basic modes of amensalism. In competition, the larger or stronger organism excludes the smaller or weaker one from the living space or deprives it of food. In antibiosis, one organism is unaffected and the other is killed by the unaffected organism's chemical secretion.
The bread mold Penicillium secretes penicillin, which kills off many types of bacteria. Many plants, like the black walnut tree, secrete chemicals that inhibit the growth of or kill out right other plants in the area.