The distribution and abundance of plant species is determined by the ability to disperse, recruit and persist in a suitable habitat. Plant-animal interactions influence recruitment by reducing seeds and seedlings through predation and herbivory. This thesis examines and quantifies the impact of pre-dispersal seed predation by Eupithecia immundata (Geometridae) on population dynamics of the perennial herb Actaea spicata (Ranunculaceae). The seed predator E. immundata consumed between 21 and 49% of A. spicata seeds produced in the decuduous forest, and between 24 and 80% in the mixed coniferous forest. Analysis of demographic information from seven years and two populations by transition matrix models resulted in population growth rates ranging from 0.84 to 1.14 in the deciduous forest, and from 0.78 to 1.08 in the mixed coniferous forest. Elasticity analysis indicated that survivorship of reproductive individuals contributed most to population growth rate, as is often found in long-lived perennials. Experimental seed addition intended to compensate for seed predation, increased seedling emergence significantly in the deciduous forest. Population growth rate shifted from negative to positive values in two more transition years when seed predation was excluded from matrices.

Understorey forest perennials were subjected to treatments in seed sowing experiments in the field to examine effects of post-dispersal seed predation, seedling herbivory, density, litter and seed size on seedling recruitment. Reduced rodent seed predation increased seedling emergence in A. spicata, but seed density did not affect predation rates, although there was a density-dependent seed and seedling mortality in the high density. Seedling herbivory by slugs did not affect seedling emergence of e.g. A. spicata, Convallaria majalis, Paris quadrifolia or Ranunculus acris. Seedling emergence was significantly higher in species with large seeds (>3mg) than in those with small seeds (<2mg), irrespective of predation. Litter removal improved seedling emergence occasionally.

Inventory data on 45 understorey species in a deciduous forest were used to investigate life history features, colonization and extinction. Local colonization was positively related to seed size and seed dispersal features, whereas local extinction rate was negatively related to seed size. In conclusion, several biotic interactions, seed predation in particular, causes significant reductions in recruitment. Although the effect on population dynamics is generally small, it is likely to translate to significant effects if viewed over a long time period.