Population dynamics of a monocarpic thistle: simulated effects of reproductive timing and grazing of flowering plants

In monocarpic plants, which die after flowering once, the timing of reproduction plays an important role. The optimal time for reproduction is when reproductive output and survival are maximized. This optimum may be altered by herbivores that consume reproductive plants of different sizes disproportionally. I examined plant survival, flowering probability, reproductive output and the probability of becoming grazed in relation to plant size in two populations of the short-lived monocarpic herb Cirsium palustre. Moreover, I simulated the consequences of changes in reproductive timing and grazing preference for population dynamics. Plant survival, flowering probability and reproductive output tended to increase with plant size, whereas the probability of becoming grazed was unaffected by plant size. According to the stochastic simulations, intense grazing would have been required to significantly reduce the stochastic population growth rate (logλs) and therefore, the observed levels of grazing had no impact on logλs in the study populations. Stochastic simulations conducted with selective grazing focusing on either early or late flowering plants and with different reproductive timings revealed that the grazing of early flowering plants had a constant effect on logλs despite the proportions of early and late flowering plants in the population, suggesting that there is no optimal time for reproduction. The grazing of late flowering plants reduced logλs with delayed reproduction, favouring reproduction early in the life cycle.