Patterns of variation in Ilex aquifolium fruit traits related to fruit consumption by birds and seed predation by rodents

Abstract

The drupes of Ilex aquifolium contain up to four pyrenes, but brood size reduction by both pyrene number reduction and/or seed abortion producing blank pyrenes affected 2/3 of fruit production. The effect of both types of brood size reduction on fruit traits related to seed dispersal and seed predation were examined. Within-fruit allocation, the hierarchical distribution of fruit variation (localities, trees, branches), and fruit allometry were also documented. The production of blank pyrenes increases fruit profitability in terms of pulp mass to burden mass ratio, which might have important consequences for frugivorous birds and seed dissemination. However, the production of blank pyrenes did not affect fruit consumption by the main dispersers of holly pyrenes (Turdus iliacus). The pyrenes were heavily predated by rodents that discard the fibrous coat and consume the seed. The examination of pyrene remains after predation by rodents showed that they opened most of the seeded pyrenes and left most of the blank pyrenes intact. However, the rodents opened some blank pyrenes, which may increase the handling time and might be interpreted as an escape from predation. Larger fruits invest proportionately less in pulp and more in pyrenes, especially in pyrene fibrous coat. In the same way, heavier seeds had proportionately heavier fibrous coats; hence, larger fruits invested proportionately more in defence than in dispersal. There is a significant degree of phenotypic variability in fruit traits that might affect both dispersal and predation; however, frugivorous birds probably do not exert selective pressures on fruit traits because they consume the entire crop.