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

Kery, M Matthies, D Fischer, M

M. Kery et al., The effect of plant population size on the interactions between the rare plant Gentiana cruciata and its specialized herbivore Maculinea rebeli, J ECOLOGY, 89(3), 2001, pp. 418-427

47

INGLESE

Article

Environment/Ecology

JOURNAL OF ECOLOGY

0022-0477 → ACNP

89

3

2001

418 - 427

ISI

0022-0477(200106)89:3<418:TEOPPS>2.0.ZU;2-5

1 Many rare plant species are restricted to small isolated populations in w hich fitness may be reduced because of inbreeding, environmental and demogr aphic stochasticity, and reduced pollination. However, specialist herbivore s are less likely to be present in such populations because of higher proba bilities of herbivore extinction and lower rates of colonization, and may t herefore affect fitness only in larger plant populations. 2 We studied the relationships between the size of populations of the endan gered grassland plant Gentiana cruciata and the probability of occurrence a nd population size of its specialist herbivore, the endangered butterfly Ma culinea rebeli, and their effects on plant size, fruit herbivory and seed p roduction. 3 The 29 G. cruciata populations studied ranged in size from 1 to 337 genet s and Is of them supported a M. rebeli population. M. rebeli populations we re 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 nl. rebeli requir es the conservation of large G. cruciata populations. 4 Although large populations of G. cruciata produced more flowers, a greate r 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 p er genet significantly decreased with the number of M. rebeli eggs per gene t. 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 le vels may determine the population dynamics of rare species. Furthermore, sm all population size may have both negative and positive effects on the fitn ess of endangered species.