Organisms from very different groups sometimes establish complex relationships with each other, in some cases of total interdependence. The forms of cooperation, at least apparent, are among the most fascinating results of the evolution of life.
In the natural world, almost all beings live in a complex web of relationships that are established between representatives of different taxonomic groups. Predator-prey, herbivore-plant and host-parasite relationships are an example of this. In all of these cases, there is a common characteristic: between the partners of these relations there is a common selective pressure, but in opposite directions. That is, to the pressure imposed by predators, herbivores and parasites, on prey, plants and hosts, the latter respond with a counter adaptation to avoid pressure.
On the contrary, in the case of mutualistic relations, both partners benefit from selective reciprocal pressures, through a process that is called co-evolution. Which in practice means that they help each other. Within mutualism there are many different modalities, but the common factor that led different organisms to evolve in this direction remains to be explained. In fact, it turns out that many mutualistic relationships are more reciprocal explorations, than cooperative efforts between individuals.
In any case, this type of relationship particularly attracts the attention of humans, who easily find an analogy here with feelings such as friendship or altruism. Even so, if we look more deeply into the parallel, the conclusions about why friendship could also be somewhat disconcerting: is friendship motivated by issues of altruism or selfishness? After all, maybe even the relationships between people can be arranged, at least metaphorically, within the categories: predation, parasitism and mutualism …
Mutualism may or may not be symbiotic and may be optional or mandatory. Symbiotic mutualism, or simply symbiosis, is one in which both organisms live together in a very close physical association and in which at least one cannot live independently of the other. Therefore, symbiosis is always a case of obligatory mutualism.
In fact, although symbiosis relationships exist a little everywhere, most of them are not at all obvious, and this designation (symbiosis) is often used in an abusive way for other types of mutualism. The relationships between symbiont organisms are sometimes so deep that it is difficult to distinguish between them. An example of this is an association of algae and fungi that make up lichens. Also in coral, Celenterates associate with algae that provide them with more than 80% of the energy they need, in exchange for retaining essential nutrients that come from their ability to capture zooplankton in suspension in the Ocean. In the roots of many plants, which live in poor soils, symbiotic relationships with fungi are established, which in exchange for the photosynthetic energy provided by the plants provide mineral nutrients that they capture from the soil.
Mandatory non-symbiotic mutualism is a type of mutualism more frequent than the previous one. In this case, the actors depend on each other to survive but do not physically live that close. The pollination of flowers and the dispersion of seeds is in some cases absolutely dependent on an agent, which may be an insect, a bird, or another animal, which depends on this resource, nectar, pollen or fruit, to survive. Also, some species of ants live inside the trunks of trees, which in addition to shelter, provide them with food through sugary substances that secrete, in exchange for protection against defoliating insects. The termites of the African savannah also create fungi inside their nests, which find the essential characteristics there to develop and which partially degrade their food.