Seed dispersal and fitness determinants in wild rose: Combined effects of hawthorn, birds, mice, and browsing ungulates.

Factors not directly related to either the plants or their avian seed-dispersal agents are ultimately responsible for the sign and magnitude of the average contribution of seed vectors to the fitness of Rosa canina plants in southern Spain. Coexistence with simultaneously fruiting Crataegus monogyna, reproductive depression caused by browsing ungulates, and seed predation by mice, are some of these factors. Disperser behaviour may either enhance, depress, or be neutral to Rosa fitness depending on the relative importance of the reproductive depression caused by browsing ungulates (pre-dispersal) and mice (post-dispersal). The contribution of seed vectors to Rosa fitness is largely independent of their dispersal-related traits and out of the control of the parent plants. Environmental constraints external to the plant-seed disperser interaction seem therefore to impose very restrictive, limits on the maximum degree of adaptedness attainable by dispersal-related plant traits, thus operating against coevolution.