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

Abstract

Summary.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.pressures exerted by dispersers on dispersal-related plant traits could hardly result in observable adaptations if they are offset (or reversed) by stronger pressures from nondisperser organisms influencing plant fitness and/or if dispersal-related determinants of plant fitness are largely nonheritable (Herrera 1984c). These assumptions have not been critically tested to date, but some evidence tends to suggest that they may often be unjustified (Janzen 1982(Janzen , 1983Herrera 1982a Herrera , 1984aManasse and Howe 1983; Manzur and Courtney in press).Seed dispersal of wild rose by frugivorous birds, and its complex relation to browsing mammals, granivorous rodents, and coexisting hawthorn, are investigated in this paper. The objectives are: (1)to compare the relative importance of disperser and non-disperser organisms in determining dispersers' contribution to wild rose fitness; and (2) to evaluate the extent to which observed seed dispersal patterns should be attributed to selection of dispersers on heritable wild rose traits.

Fruit selection by birds in relation to fruit abundance and appearance in the naturalised shrub Berberis darwinii

“…This degree of fruit removal by birds is at the higher end of the range recorded for most other plant species (e.g., Krtisi & Debussche 1988;Herrera 1984;Piper 1986), including some (e.g., Crataegus monogyna: Courtney & Manzur 1985) that are naturalised in New Zealand.…”

“…Alteration of the characteristics of a fruit that impart attractiveness should reduce the chance of that fruit being taken by the disperser (Herrera 1982). Manzur & Courtney (1984) demonstrated such a relationship in hawthorn (Crataegus monogyna*), where fruits damaged by insects were rejected by birds.…”

Recruitment of fleshy‐fruited species under different shrub species: Control by under‐canopy environment

“…At the German site the average density of seedlings and the number of fleshy-fruited species under about 200 shrubs was highest beneath Crataegus, intermediate in Rosa and Cornus and low in Prunus (see also Kollmann 1994). Markedly higher densities of birddispersed seeds and subsequent recruitment under Crataegus monogyna compared with Rosa canina were also observed by Herrera (1984) in Spain where Crataegus shrubs were much more attractive for frugivorous birds than Rosa.…”

Post-dispersal seed predation and the establishment of vertebrate dispersed plants in Mediterranean scrublands

“…The demographic implications of post-dispersal seed predation by rodents must be made with reference to the seed shadows of the plant species concerned. Thrushes (Turdus merula and T. viscivorus) disperse the majority of seed of all three plant species and deposit seeds heterogeneously in the study area, favouring shrub microhabitats while avoiding open areas (Herrera and Jordano 1981;Herrera 1984;Schupp 1993;P. Hulme, unpublished work).…”