Context-dependent pollinator limitation in stochastic environments: can increased seed set overpower the cost of reproduction in an understorey herb?

1. In the understorey herb Lathyrus vernus seed production is pollen limited, but increased reproduction results in a lower probability of remaining reproductive. Putting these two results together, previous research reported that population growth rate lambda was negatively impacted by high pollination. 2. Thus, costs and benefits have to be translated into the common currency of their respective effects on population dynamics to determine whether populations are truly pollen limited or whether they are already at an optimal level of pollination. 3. Also, when pollinators and demography vary from year to year we require a framework that examines reproductive benefits and demographic costs in the context of a variable environment. Whether or not additional pollination will increase the stochastic population growth rate lambda(S) depends upon the balance of stochastic elasticities of the costs and benefits. 4. In constant environment models, where seed survival, germination and seedling survival were increased, we found that the high cost of reproduction could be offset by improvements in seed survival and germination, but not by improvements of seedling survival. 5. In variable environment models, where changes in the sequence and frequencies of high- and low-pollination years mixed with occasional high-germination years were modelled, we found that increasing the frequency of high-germination conditions could offset the cost of reproduction, and the offset was even greater if high-germination years occurred after a high-pollination year or if high pollination was accompanied by high-germination conditions in the same year. 6. Both deterministic lambda and stochastic lambda(S) were less sensitive to perturbation of reproduction than to perturbation of the probability for flowering plants to remain reproductive. In other words, a small change in the parameter which is related to the 'cost' of reproduction had a bigger effect than a small change in the parameter which is related to the 'benefit' of increased pollination for Lathyrus. 7.Synthesis. Stochastic environment-specific elasticities for reproduction and stasis of flowering plants differ in their response to environmental context. The cost-benefit relationships, the ultimate fitness consequences of supplemental pollen, are influenced by the frequency and sequence of years differing in pollen availability and recruitment conditions.