Questions: Is post-dispersal seed predation a factor contributing to a positive feedback mechanism for dominance of tall-tussock grasslands? Do seeds dispersed into neighbouring microhabitats of contrasting dominance differ in their probabilities of being predated? Does predation rate vary among predator groups? Do seed eaters selectively forage among the available seeds?
Location: The southern and flooding sub-regions of the Argentinian Pampas.
Methods: We examined seed predation by vertebrate and invertebrate predators within two microhabitats of grassland mosaics (highly dominated tall-grass patches vs. scarcely dominated short-grass matrix) for different seed species in semi-natural grasslands. Proportion of seeds eaten by different predator groups was estimated through exclusion experiments and analysed using ANOVA for split-split-plot designs. Experiments were performed during the autumn of two consecutive years in both Pampa sub-regions.
Results: Removal of seeds after a five day trial was two to four times higher in the tall-grass patches than in the short-grass matrix. During the same period, vertebrate predation was six times higher than invertebrate predation in the tall-grass phase of the Southern Pampa, but it did not differ in the short-grass matrix. Relative predation among phases showed the same pattern in the Flooding Pampa, where preferences by seed species also varied according to phase.
Conclusions: The highest predation intensity shown by vertebrates in the tall-grass patches indicates that they are the main seed predators in these systems, possibly because this microhabitat grants them refuge against carnivorous predators. This could significantly reduce the available seeds for recruitment of subordinate species after different disturbance events (fires, trampling by large herbivores, burrowing), representing an active filter to the floristic composition of the patch and contributing with other mechanisms to the dominance of tall-grass species.