Ingestion and dispersal:: direct and indirect effects of frugivores on seed viability and germination of Corema album (Empetraceae)

The effect of culls, blackbirds and rabbits on the viability and germination of Corema album seeds are compared. Frugivores can affect seed viability and germination (1) directly, through the effect of ingestion and (2) indirectly, dispersing seeds to different sites with different conditions. These two major factors in the quality of a seed disperser are not necessarily concordant in direction and magnitude. Gulls and blackbirds have similar direct effects, being much better than those of rabbits, due to the low probability of germination of seeds within rabbit pellets. Seed germination occurs mainly in the open ground, particularly in the sparse scrub, and is very low under vegetation cover. This pattern becomes crucial determining the indirect effects of seed dispersers that will depend on their capacity to carry seeds to the most suitable sites for germination. Gulls and rabbits disperse most of seeds to open ground, exerting a positive indirect effect on germination, whereas blackbirds disperse seeds mainly tinder shrubs, thus exerting a negative indirect effect. Direct and indirect effects on seed germination are concordant for gulls but discordant for blackbirds and rabbits. Gulls were the best dispersers; the overall probability of germination for a seed dispersed by gulls was 17.59%. The quality of blackbirds and rabbits was relatively low (3.49% and 1.17%, respectively). Frugivores seem to be essential for germination of C. album seeds, not as much for their direct effects but for their ability to carry seeds to suitable sites. (C) 2004 Elsevier SAS. All rights reserved.