Host plant population size determines cascading effects in a plant–herbivore–parasitoid system

Cascading effects from a third level consumer to the basal plant level have been documented several times in terrestrial systems. However, less is known about how these effects are influenced by characteristics of the host plant population. To examine if properties of the spatial context can affect tritrophic interactions we studied 85 populations of a perennial plant, Actaea spicata, the associated specialist moth seed predator, Eupithecia immundata and a guild of parasitoids. Based on the Fretwell–Oksanen model we developed a hypothesis for how the level of herbivory is related to the spatial aggregation of plant populations. In agreement with predictions, incidences of both higher trophic levels were correlated to plant population size. However, there was no effect of population connectivity or environmental parameters. Seed predation was unimodally related to population size with the highest levels in populations of intermediate size. In small populations, usually either a high proportion of seeds was preyed upon due to seed predator presence and parasitoid absence or there was no seed predation when the seed predator was absent. Intermediately sized plant populations suffered from intense seed predation since the seed predator was present but parasitoids were often not. In all large plant populations parasitoids reduced the level of seed predation, thereby enhancing plant fitness. The effects of predation on seed output was correlated with effects on plant population structure, in terms of the proportion of seedlings. Our results demonstrate that host plant population size is important for the distribution of herbivores and their natural enemies, as well as for the outcome of the tritrophic interaction. Moreover, natural enemies of a specialist herbivore might have long-term cascading effects in terms of changes in plant population structure. In terrestrischen Systemen wurden verschiedentlich Kaskadeneffekte von einem Konsumenten der dritten Ebene auf die basale Pflanzenebene dokumentiert. Es ist jedoch weniger bekannt, wie diese Effekte von den Eigenschaften der Wirtspflanzenpopulation beeinflusst werden. Um zu bestimmen, ob die Eigenschaften des räumlichen Kontextes tritrophische Interaktionen beeinflussen kann, untersuchten wir 85 Populationen einer perennierenden Pflanzenart, Actaea spicata, den spezialisierten assoziierten Samenprädator, die Motte Eupithecia immundata, und eine Gilde von Parasitoiden. Basierend auf dem Fretwell–Oksanen-Modell ent