The effect of plant population size on the interactions between the rare plant Gentiana cruciata and its specialized herbivore Maculinea rebeli

1. Many rare plant species are restricted to small isolated populations in which fitness may be reduced because of inbreeding, environmental and demographic stochasticity, and reduced pollination. However, specialist herbivores are less likely to be present in such populations because of higher probabilities of herbivore extinction and lower rates of colonization, and may therefore affect fitness only in larger plant populations. 2. We studied the relationships between the size of populations of the endangered grassland plant Gentiana cruciata and the probability of occurrence and population size of its specialist herbivore, the endangered butterfly Maculinea rebeli, and their effects on plant size, fruit herbivory and seed production. 3. The 29 G. cruciata populations studied ranged in size from 1 to 337 genets and 18 of them supported a M. rebeli population. M. rebeli populations were both more likely and larger in larger G. cruciata populations. Estimated adult herbivore populations were small, ranging from 1 to 42 individuals, with a median of 11. We conclude that the conservation of M. rebeli requires the conservation of large G. cruciata populations. 4. Although large populations of G. cruciata produced more flowers, a greater proportion of their fruits were attacked by herbivores. Fruit herbivory, which considerably decreased the number of seeds per fruit, appears to have been caused largely by Maculinea. The number of seeds both per fruit and per genet significantly decreased with the number of M. rebeli eggs per genet. The overall independence of G. cruciata seed production from population size may result from the opposing effects on fruit production and herbivory. 5. Our study suggests that complex interactions between different trophic levels may determine the population dynamics of rare species. Furthermore, small population size may have both negative and positive effects on the fitness of endangered species.