Modelling the quantitative effects of pre- and post-dispersal seed predation in Pinus sylvestris L.

1 Despite three decades of intensive research on seed predation, rather little is known about how important a role it plays in plant demography. Here a balance sheet of seed predation is drawn up for Pinus sylvestris, based on mortality caused by its main seed predators. This species is of special interest because trees carry seeds throughout the year and can therefore support specialist as well as generalist seed predators. Also, its regeneration in Scotland is weak and some of its main seed predators (red squirrels, crossbills, wood ants) are themselves threatened. 2 Field data from two P. sylvestris forests were used to estimate rates of predation on seeds due to pre- and post-dispersal predators, including both specialists and generalists. The rates of predation changed systematically over time and were fitted to time-dependent functions. The resulting functions were incorporated into a model, which was solved numerically to determine the fate of a cohort of seeds. 3 On average, approximately 20% of seeds in the cohort were consumed by pre-dispersal seed predators, approximately 25% of seeds by post-dispersal seed predators, and approximately 55% of seeds were still alive at the end of 2 years. This reduces the seed pool for regeneration of P. sylvestris, but is unlikely to be a serious problem for regeneration. 4 Seed survival was most sensitive to the rate at which seeds became invulnerable to further seed predation, and to the rate of seed dispersal from cones on trees. Seed survival was less sensitive to seed predation. However, among seed predators, carabids feeding on loose seeds and birds eating seeds in open cones were the most important. 5 Cones open as they dry out, and the timing of seed dispersal is therefore weather-dependent. Late opening of cones had a deleterious effect on seed survival because it led to an increase in pre-dispersal seed predation. Thus, in a period of climate change, patterns of seed predation would be expected also to change, altering the success of regeneration.