Summary1 Habitat fragmentation and the resulting decline in the local abundance of plant species can affect biological interactions. We examined the effects of abundance on plant-pollinator interactions by observing the pollinator service and subsequent reproductive output of a mostly outbreeding, but self-compatible, plant, Lychnis viscaria , in experimental populations of different sizes (number of individuals) and densities (distance between individuals). 2 Bumblebees, the main pollinators of L. viscaria , preferred larger populations, but visitation rates were higher in sparser populations. Pollinators were attracted to the larger inflorescences in sparse populations, which were also more visible due to their larger area for a given size. 3 Bumblebees probed more flowers within plants in sparse populations, probably due to the larger inflorescences and longer flight distances between individuals. 4 Subsequent reproductive success (capsule production) was higher in sparse populations, due to differences in pollination success and resource competition, and their interaction. In self-compatible species, such as L. viscaria , reproductive success may be determined more by resource availability, whereas self-incompatible plants may be more sensitive to changes in pollinator abundance. 5 We conclude that plant-pollinator interactions are sensitive to changes in both the size and spatial arrangement of plant populations, which can affect their demography and genetics. In this study, species density had a greater effect than size and the unexpectedly beneficial effects of low density may be due to greater resource availability.

Genetic diversity is expected to decrease in small and isolated populations as a consequence of bottlenecks, founder effects, inbreeding, and genetic drift. The genetics and ecology of the rare perennial plant Lychnis viscaria (Caryophyllaceae) were studied in both peripheral and central populations within its distribution area. We aimed to investigate the overall level of genetic diversity, its spatial distribution, and possible differences between peripheral and central populations by examining several populations with electrophoresis. Our results showed that the level of genetic diversity varied substantially among populations (  Hexp = 0.000–0.116) and that the total level of genetic diversity (mean Hexp = 0.056) was low compared to that of other species with similar life‐history attributes. The peripheral populations of L. viscaria had less genetic variation (mean Hexp = 0.034) than the central ones (0.114). Analysis of genetic structure suggested limited gene flow (mean FST = 0.430) and high differentiation among populations, emphasizing the role of genetic drift (  Nem = 0.33). Isolation was even higher than expected based on the physical distance among populations. We also focused on the association between population size and genetic diversity and possible effects on fitness of these factors. Population size was positively correlated with genetic diversity. Population size and genetic diversity, however, were not associated with fitness components such as germination rate, seedling mass, or seed yield. There were no differences in the measured fitness components between peripheral and central populations. Even though small and peripheral populations had lower levels of genetic variation, they were as viable as larger populations, which emphasizes their potential value for conservation.

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We studied inbreeding depression in a perennial plant, Lychnis viscaria, in three populations differing in their inbreeding history and population size by measuring several traits at two nutrient levels over the plant's life cycle. The observed levels of inbreeding depression (cumulative inbreeding depression, from -0.057 to 0.629) were high for a plant with a mixed mating system. As expected, the population with a low level of isozyme variation expressed the least inbreeding depression for seed germination. Highest inbreeding depression for germination was found in the largest and genetically most variable population. No clear differences between populations in expression of inbreeding depression in the later life stages were found. The population level inbreeding depression varied with the nutrient conditions and among populations and life stages, but we found no evidence that inbreeding depression increased with lower nutrient availability. These results emphasize the importance of measuring inbreeding depression under several environmental conditions and over life stages.