Seed predators and the evolutionarily stable flowering strategy in the invasive plant, Carduus nutans

In plants where reproduction is fatal, seed-feeding insects may have a major impact on the evolutionarily stable reproductive strategy by altering fecundity schedules in a size-dependent manner. We explored this in Carduus nutans, a facultative biennial native to Europe, using two years of data from the South of France. An integral projection model based on detailed statistical models of the demography of Carduus nutans and characteristics of herbivore attack showed that seed predators select for smaller flowering size. An elasticity analysis showed that changes in the slope relating seed herbivore attack rates to plant total receptacle area had a large effect on lifetime reproductive success relative to most other plant demographic rates. Together, these two results indicate that in the absence of seed predators, as is the case in the exotic range of this invasive species, flowering size could evolve to be larger. Further analysis also showed that subsequent introduction of different species of seed-feeders as biocontrol agents could lead to different evolutionary outcomes dependent on the ecological characteristics of the seed-feeders, allowing the direction and magnitude of evolutionary change in flowering size to be predicted based on what seed predators have been introduced where and when. Such data would allow us to distinguish between the effect of seed predators and other hypotheses for size increase in the invasive habitat. © Springer Science+Business Media B.V. 2008.