Table 1 Factors affecting the likelihood or strength of facilitation between symbionts and parallel examples for free-living organisms

Developmental stage

Interactions between microparasites in rodents can be facilitative when infections are new, but competitive when chronic (or vice versa)⁴⁰.

The facilitative interaction between a nurse plant and a beneficiary becomes competitive as the beneficiary ages⁹¹.

Genetic identity

 Facilitated/facilitators

Facilitation only occurs between certain genotypes of spider mites on tomato plants⁹⁷.

Only some genotypes of dinoflagellate prey are able to facilitate others by producing toxins that kill a predatory dinoflagellate⁹².

 Resource/host

Facilitation between two strains of powdery mildew only occurs in more susceptible genotypes of ribwort plantain hosts⁷⁷.

The genotype of a host plant can change the intensity of facilitative interactions occurring between beneficiaries⁹⁸.

Functional overlap

In aphids, co-occuring bacterial endosymbionts often display complementary (protective and/or nutritional) functions for their hosts⁴¹, which increases facilitation via enhanced host fitness.

Character displacement reducing overlap in resource use between interacting free-living decomposer bacteria leads to the emergence of facilitation, as some species evolve to use the waste products of other species⁴⁸.

Genetic diversity

Infection success of trematode eye-fluke parasites in rainbow trout is higher when the inoculum contains greater symbiont diversity⁹⁹.

Species diversity of aquatic arthropods increases resource consumption compared to monospecies cultures⁶³.

Phylogenetic distance

Facilitation occurs between both closely (e.g. two rodent malaria parasites¹⁸) and distantly related species (e.g. microparasites such as viruses, bacteria, fungi or protozoa, and macroparasites such as helminths²²).

Nurse and beneficiary plants are often phylogenetically distant⁷.

Environmental factors

Demography

 Prevalence of each player

The likelihood of facilitation is affected by the density of the facilitator (e.g. low density of a rodent malaria parasite facilitates another in mice¹⁸, and a high density of HIV facilitates a human malaria parasite⁴²).

The strength of facilitation by the co-occurring facilitators, ribbed mussels and fiddler crabs, is positively correlated with their respective density⁹⁸.

 Order of arrival (priority effects or sequential infection)

Facilitation occurs only if the facilitator is the first to infect the host (e.g. rodent malaria parasites in mice¹⁸, and trematode eye-fluke genotypes in rainbow trout⁹⁹).

Recruitment of a new grassland plant species establishing in an environment depends on the plant species that are already present¹⁰⁰.

Environmental stress

The strength of facilitation between two strains of powdery mildew can be reduced in more resistant ribwort plantain hosts⁷⁷.

Facilitation occurs under more stressful conditions³⁴, but might disappear at the harshest end of the stress gradient (the stress-gradient hypothesis)³⁴.

Site/localisation

The site of infection within a host determines whether facilitation occurs (e.g. scabies mites facilitate opportunistic pathogens at the wound site only²³).

In sessile organisms, such as plants, the condition of the micro-site (soil, topography, etc) affects the intensity of the interaction among individuals⁹¹.