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There is growing evidence that genetic and ecological factors interact in determining population persistence. The demographic effects of inbreeding depression can largely depend on the ecological milieu. We used demographic data of the perennial herb Succisa pratensis from six populations in grazed and ungrazed sites with different soil moisture. We built an individual-based model assessing the demographic consequences of inbreeding depression in populations with different management and habitat. Today this plant has to cope with severe landscape fragmentation, deteriorating habitat conditions in terms of decreasing grazing intensity, and the effects of inbreeding depression. For each population we performed simulations testing two inbreeding depression hypotheses (partial dominance and overdominance) and three epistatic functions among loci. The results indicated stronger inbreeding depression effects for populations in unfavourable sites without grazing or in xeric habitats compared to populations in favourable mesic sites with grazing. Overall, we found stronger effects with overdominance, a result that emphasizes the importance of understanding the genetic mechanisms of inbreeding depression. Hence, management practices can interact with the genetic consequences of inbreeding depression in population dynamics, which may have important implications for plant population ecology and evolutionary dynamics of inbreeding depression, [GER] Es gibt zunehmend Hinweise darauf, dass genetische und okologische Faktoren bei der Bestimmung der Persistenz von Populationen zusammenwirken. Die demographischen Effekte von Inzuchtdepressionen hangen in großem Ausmaß von der okologischen Umwelt ab. Wir nutzten die demographischen Daten des perennierenden Krautes Succisa pratensis aus sechs Populationen, die entweder beweidet oder unbeweidet waren und unterschiedliche Bodenfeuchten zeigten. Wir erstellten ein individuenbasiertes Modell um die demographischen Konsequenzen der Inzuchtdepression in verschiedenen Populationen bei verschiedenen Bewirtschaftungen und Habitaten abzuschatzen. Heute muss diese Pflanze mit ernster Landschaftsfragmentierung, sich verschlechternden Habitatbedingungen in Form von abnehmender Beweidungsintensitat und den Auswirkungen der Inzuchtdepression zurechtkommen. Fur jede Population fuhrten wir Simulationen durch, um zwei Inzuchtdepressions-Hypothesen (teilweise und Uberdominanz) und drei epistatische Funktionen uber die Loci zu testen. Die Ergebnisse wiesen auf starkere Auswirkungen der Inzuchtdepression bei Populationen in ungunstigen Habitaten ohne Beweidung oder in xerischen Habitaten im Vergleich zu Populationen in den bevorzugten mesischen Habitaten mit Beweidung hin. Insgesamt fanden wir starkere Effekte bei Uberdominanz, ein Ergebnis, dass die Wichtigkeit betont, die genetischen Mechanismen der Inzuchtdepression zu verstehen. Die Bewirtschaftungsweise kann also mit den genetischen Konsequenzen der Inzuchtdepression in der Populationsdynamik interagieren. Dies kann wichtige Implikationen fur die Pflanzenpopu- lationsokologie und die evolutionare Dynamik der Inzuchtdepression haben

Summary 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.