Trophic, Indirect, and Evolutionary Interactions in a Plant–Herbivore–Parasitoid System

Abstract: The aim of this thesis project was to elucidate patterns and processes associated with the biotic interactions in a natural plant–herbivore–parasitoid food web characterized by spatial and temporal heterogeneity with regard to species composition.The system examined is based on island populations of the perennial herb Meadowsweet (Filipendula ulmaria, Rosaceae), located in the Skeppsvik Archipelago. The area is subject to isostatic rebound, amounting to 0.85 cm per year; this makes it possible to calculate the age of the rising islands. Meadowsweet colonizes new islands when they are about 100 years old.Meadowsweet is consumed by two major herbivores in the study area: Galerucella tenella and Altica engstroemi (Coleoptera: Chrysomelidae). Both herbivores overwinter in the topsoil and successful colonization occurs when the islands reach a height that prevents the beetles from being removed or killed as a result of wave wash during the winter. I found that both herbivores significantly reduced individual plant fitness and population growth rate. A “cafeteria experiment” with Galerucella showed that this beetle discriminated between plants from different islands, avoiding plants from old islands which contained high concentrations of putative defence compounds, while readily accepting plants from younger islands which contained lower concentrations of these chemicals. Further, the plant species exhibited a trade-off between growth and production of the putative defence compounds. Taken together, these results were interpreted as providing evidence of herbivore-driven evolution of resistance in Meadowsweet. Further, laboratory studies suggested that Galerucella gradually includes a less preferred host plant (Rubus arcticus, Rosaceae) in its diet as Meadowsweet resistance increases. This implies that Galerucella drives its own host-breadth enlargement by selectively inducing a ‘rent rise’ in the original host, Meadowsweet.In a number of field studies I showed that the oligophagous parasitoid Asecodes mento (Hymenoptera: Eulophidae) has a strong positive effect on Meadowsweet seed set by removing large numbers of G. tenella larvae. This top-down effect is, however, altered by the presence of a close relative of G. tenella, namely G. calmariensis, which is monophagous on Purple loosestrife (Lythrum salicaria, Lythraceae). G. tenella experiences associational susceptibility when coexisting with G. calmariensis since the latter supports a higher and more fit pool of shared parasitoids and because Meadowsweet attracts a higher proportion of the shared parasitoid females than Purple loosestrife. This implies that G. tenella densities are very low in coexisting populations and that Meadowsweet experiences associational resistance and produces more seeds when co-occurring with Purple loosestrife. Thus, selection for increased resistance in Meadowsweet is likely to be relaxed in populations mixed with Purple loosestrife.I conclude that the evolution of plant resistance is likely to depend on the length of time and intensity of selection. When Meadowsweet colonizes new islands it experiences a period of enemy-free space; followed by a midlife and ageing with selection by herbivores. The intensity of this selection does, however, depend on the presence of additional plant and herbivore species.