When two species—that is, at least two individuals representing two different species—live and interact closely in such a way that either or both species benefit, it is symbiosis. It is also possible for a symbiotic relationship to exist between two organisms of the same species. Organisms engaging in symbiotic relationships are called symbionts.
There are three basic types of symbiosis, differentiated as to how the benefits (and the detriments, if any) are distributed. These are commensalism, parasitism, and mutualism. In the first two varieties, only one of the two creatures benefits from the symbiotic relationship, and in both instances the creature who does not benefit—who provides a benefit to the other creature—is called the host. In commensalism the organism known as the commensal benefits from the host without the host's suffering any detriment. By contrast, in parasitism the parasite benefits at the expense of the host.
Mutualism is distinguished from the other two types of symbiosis, because in this variety both creatures benefit. Thus, there is no host, and theoretically the partners are equal, though in practice one usually holds dominance over the other. An example of this inequality is the relationship between humans and dogs. In this relationship, both human and dog clearly benefit: the dog by receiving food, shelter, and care and the human by receiving protection and loving companionship—the last two being benefits the dog also receives from the human. Additionally, some dogs perform specific tasks, such as fetching slippers, assisting blind or disabled persons, or tracking prey for hunting or crime-solving purposes.
For all this exchange of benefits, one of the two animals, the human, clearly holds the upper hand. There might be exceptions in a few unusual circumstances, such as dog lovers who are so obsessive that they would buy food for their dogs before feeding themselves. Such exceptions, however, are rare indeed, and it can be said that in almost all cases the human is dominant.
Most forms of mutualism are facultative, meaning that the partners can live apart successfully. Some relationships of mutualism are so close that the interacting species are unable to live without each other. A symbiotic relationship in which the partners, if separated, would be unable to continue living is known as an obligate relationship. In commensalism or parasitism, the relationship is usually obligate for the commensal or the parasite, since by definition they depend on the host. At the same time, and also by definition, the host is in a facultative relationship, since it does not need the commensal or parasite—indeed, in the case of the parasite, would be much better off without it. It is possible, however, for an organism to become so adjusted to the parasite attached to its body that the sudden removal of the parasite could cause at least a short-term shock to the system.
A special variety of commensalism is inquilinism, in which the commensal species makes use of the host's nest or habitat, without causing any inconvenience or detriment to the host. Inquilinism (the beneficiary is known as an inquiline) often occurs in an aquatic environment, though not always. In your own yard, which is your habitat or nest, there may be a bird nesting in a tree. Supposing you benefit from the bird, through the aesthetic enjoyment of its song or the pretty colors of its feathers—in this case the relationship could be said to be a mutualism. In any case, the bird still benefits more, inasmuch as it uses your habitat as a place of shelter.
The bird example is an extremely nonintrusive case of inquilinism; more often than not, however, a creature actually uses the literal nest of another species, which would be analogous to a bird nesting in your attic or even the inside of your house. This is where the analogy breaks down, of course, because such an arrangement would no longer be one of commensalism, since you would be suffering a number of deleterious effects, not the least of which would be bird droppings on the carpet.
Inquilinism sometimes is referred to as a cross between commensalism and parasitism and might be regarded as existing on a continuum between the two. Certainly, there are cases of a creature making use of another's habitat in a parasitic way. Such is the case with the North American cowbird and the European cuckoo, both of which leave their offspring in the nests of other birds to be raised by them. (See Instinct and Learning for a discussion of how these species exploit other birds' instinctive tendency to care for their young.)