Facilitation

In a broad sense, biological coevolution is "the change of a biological object triggered by the change of a related object". Coevolution can occur at multiple levels of biology: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as covarying traits between different species in an environment. Each party in a coevolutionary relationship exerts selective pressures on the other, thereby affecting each others' evolution. Species-level coevolution includes the evolution of a host species and its parasites (host-parasite coevolution), and examples of mutualism evolving through time. Evolution in response to abiotic factors, such as climate change, is not coevolution (since climate is not alive and does not undergo biological evolution). Evolution in a one-on-one interaction, such as that between predator and prey, host-symbiont or host-parasitic pair, is coevolution. But many cases are less clearcut: a species may evolve in response to a number of other species, each of which is also evolving in response to a set of species. This situation has been referred to as "diffuse coevolution".

Mutualism is the way two organisms biologically interact where each individual derives a fitness benefit (i.e. increased survivorship). Similar interactions within a species are known as co-operation. It can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the expense of the other. Mutualism and symbiosis are sometimes used as if they are synonymous, but this is strictly incorrect: symbiosis is a broad category, defined to include relationships which are mutualistic, parasitic or commensal. Mutualism is only one type.

Facilitation describes species interactions that benefit at least one of the participants and cause harm to neither. Facilitations can be categorized as mutualisms, in which both species benefit, or commensalisms, in which one species benefits and the other is unaffected.

"Species coextinction is a manifestation of the interconnectedness of organisms in complex ecosystems. The loss of species through coextinction represents the loss of irreplaceable evolutionary and coevolutionary history. In view of the global extinction crisis, it is imperative that coextinction be the focus of future research to understand the intricate processes of species extinctions. While coextinction may not be the most important cause of species extinctions, it is certainly an insidious one."

Facilitation is a significant ecological process that produces community-level effects through individual interactions. By improving dispersal, increasing access to resources, and providing protection from stress, predation, and competition, facilitation can impact community structure, diversity, and invasibility. Incorporation of facilitation into such classic theories as natural selection and niche separation should be a goal of current and future ecologists. This will require further research into the mechanisms of facilitation at the level of individuals, and the impacts of facilitation at the level of population, community, and ecosystem. Continued study of positive species interactions will serve to improve our understanding of processes and application of theories.