CONICET | Buscador de Institutos y Recursos Humanos

The spatial context in which seed predation occurs may modify spatial structure of recruitment generated by seed dispersal. The Janzen-Connell (J-C) model predicts that granivores will exert greater pressure near the parent plant or at those sites where density of dispersed seeds is higher. We investigated how probability of post-dispersal survival of Juglans australis varies with nut density across a hierarchy of spatial scales. We experimentally evaluated the survival of 3120 nuts at three spatial scales: meso-scale (¡Ü 1.5 ha), as forest sites with two densities of fruiting J. australis individuals; intermediate scale (< 0.2 ha), as individual trees with two experimental crop sizes; and small scale (< 0.1 m2), as microsites with two factors (number of nuts and distance from source). Nut removal coincided with seed predation, a condition that allows us to test density-dependent seed predation hypothesis. Probability of nut survival was greater at forest sites with higher J. australis density. Nut survival was not affected by nut density in the seed shadow of individual specimens. At sites where J. australis density was greater, the proportion of surviving nuts did not differ between microsites located at different distances from the parent plant, but was greater at microsites with greater initial nut density. Nut survival depended on the scale at which rodents responded to nut density, being negatively density-dependent at meso-scale and spatially random at intermediate and small scales. At meso-scale, excess of nut supply increased the probability of nut survival, in agreement with a model of granivore satiation near the seed source. Rodent satiation at meso-scale might favour maintainance of sites with high J. australis density, where individual trees might have greater probabilities of passing their genes on to the next stage of the dispersal cycle.

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