There are two fundamental types of positive interactions: mutualism and commensalism.

Mutualism is a mutually beneficial interaction between individuals of two or more species (+/+ relationship) (see Figure 12.3). Commensalism is an interaction in which individuals of one species benefit, while those of the other species do not benefit and are not harmed (+/0 relationship) (see Figure 12.3).

In some cases, the species involved in a positive interaction form a symbiosis, a relationship in which individuals of the two species live in close physical and/or physiological contact with each other. Examples include the relationships between pea aphids and their bacterial symbionts (see Concept 13.2) and between humans and bacteria (we have a diverse set of bacteria living in our guts, many of which are beneficial). However, parasites also form symbiotic associations with their hosts (see Figure 13.3). Thus, symbiotic relationships can range from parasitism (+/—) to commensalism (+/0) to mutualism (+/+).

In mutualism and commensalism, the growth, reproduction, or survival of individuals of one or multiple species is increased by their interaction with other species (and no species is harmed). The benefits can take a variety of forms. A species may provide its partner with food, shelter, or a substrate to grow on; it may transport its partner's pollen or seeds; it may reduce heat or water stress; or it may decrease the negative effects of competitors, herbivores, predators, or parasites. In a mutualism, there can be costs to an organism that provide a benefit to its partner, as when supplying food to its partner reduces its own opportunity for growth. Nevertheless, the net effect of the interaction is positive because the benefits are greater than the costs for each of the partners.

In the remainder of this section, we will discuss some general observations that apply to both mutualism and commensalism; in Concept 15.2, we'll examine some characteristics that are specific to mutualism.