Is domatia production in Coprosma rotundifolia (Rubiaceae) induced by mites or foliar pathogens?

Plant–invertebrate mutualisms involve the production of food and/or shelter by plants to co-opt invertebrate groups in order to either prevent herbivore or pathogen damage or facilitate seed dispersal. Plant structures and the provision of food are relatively expensive, and a reactive plant response to attack may reduce those costs provided the fitness benefit of the mutualism is maintained. We investigated whether foliar domatia in the New Zealand shrub Coprosma rotundifolia (Rubiaceae) were an induced mutualism, whose density is dependent on the abundance of foliar mites and/or foliar fungi. Alternatively, domatia may be a defence that is always present irrespective of local conditions, i.e. a constitutive mutualism. Beneficial mites inhabit the domatia (mite houses), feeding on leaf fungi and small herbivorous arthropods from the leaf surface. We attempted to manipulate mite and fungal densities to test (1) whether the density of foliar mites on shrubs stimulated increased domatia production and the domatia opening size of new leaves, and (2) whether the density of foliar fungi on old leaves influenced domatia production in new leaves. Under experimental treatments (with or without miticide and different levels of foliar fungal detritus) C. rotundifolia shrubs showed no significant fungi, or a defence that is always present irrespective of local conditions, i.e. a constitutive mutualism. Beneficial mites inhabit differences in the mean relative change in domatia production in new season’s leaves compared with old leaves. We propose that domatia on C. rotundifolia are potentially part of a constitutive mutualism, as plants produce many domatia, apparently in excess of requirements. A constitutive mutualism suggests that plants have a consistent fitness advantage by maintaining these structures every year, presumably because of constant pressure from foliar invertebrate herbivores and pathogens.