Plants form the first trophic level in terrestrial ecosystems and provide energy and nutrients to higher trophic levels. Herbivores, frugivores or fungal endophytes use plants directly, while predators consume plants indirectly by consuming herbivores. However, species are often simultaneously interacting with antagonistic and mutualistic partners at various trophic levels. For this reason, the outcomes of species interactions can indirectly affect other species in the community. The aim of my thesis was to study tritrophic interactions between plants, their antagonists, such as insect or avian herbivores, and mutualists such as insectivorous or frugivorous birds and symbiotic endophyte fungus. 

In Chapters I-III I concentrated on the interactions among plants, herbivores and protective plant mutualists. In the first two Chapters, I investigated whether birds use volatile organic compounds or changes in visual properties of leaves from herbivore-damaged trees as foraging cues. I found that trees respond to herbivore damage both locally and systemically, but the olfactory foraging cue hypothesis was not supported. Instead, herbivory affected visual properties of leaves viewed by birds, although these changes may be in the limit of detection to them. In addition, my results indicate that cryptically coloured herbivores may have slightly better camouflage when on herbivore-damaged trees, although the herbivores are discriminable to birds against the leaves of the host plant regardless of the treatment (Chapter II). In Chapter III I studied the relationship between plants and protective fungal symbiont by testing whether systemic endophyte fungi can protect grasses against wild avian grazers. In this study I used two grass species, red fescues and tall fescues, which differ in texture. Both species have naturally both endophytic and non-endophytic individuals. I found that softer red fescue was preferred over coarse tall fescue, regardless of the endophyte status. 

In Chapter IV I studied the interaction between plants, herbivores and seed-dispersing mutualists. I tested whether insect herbivory causes allocation cost to fleshy fruiting plants by affecting ripening or chemical composition of berries. I also investigated potential ecological cost of herbivory measured as probability for ripe berries to be removed by frugivorous birds. I found that berries in undamaged ramets neighbouring herbivore-damaged conspecifics had lower probability to be removed by frugivores, although herbivory did not affect ripening or chemical composition of berries. This indicates that in clonal plants, herbivore damage may cause priming effect on neighbouring ramets, which can affect plant mutualists. 

The results of this thesis extend current knowledge about plant responses to herbivory, and also how these responses affect plant mutualists. In addition, my thesis provides information about the foraging behaviour of herbivores and plant mutualists. This kind of knowledge is essential for biological control and agricultural procedures, as well as on the planning of urban grass areas.