Can seed-eating birds exert top-down effects on grasses of the Monte desert?

Abstract

Granivorous animals can exert major effects on the abundance and diversity of plants when they are selective as well as efficient consumers. However, even under such conditions granivore impact will ultimately depend on whether environmental stress obscures plant-animal interactions. We studied diet and seed selection patterns of seed-eating birds to corroborate whether they are selective consumers in the central Monte desert of Argentina. Overall, 83% of seeds in bird stomachs were grass seeds, whereas only 30% of available seeds were from grass species. Therefore, we conclude that avian granivory is highly selective. We developed a set of a priori expectations to test whether birds are efficient consumers (i.e. whether they reduce seed reserves significantly), through short-term mechanism-explicit enclosure experiments. Birds decreased the number of selected grass seeds by >50%, and also reduced the amount of non-selected grasses and selected forbs when selected grass seeds were scarce in the habitat. Thus, consumption was context-dependent, varying with the composition of background seed reserves. The corroboration of foraging plasticity through mechanism-explicit trials seems to be crucial to correctly assign direct and indirect effects of seed predation in long-term enclosure experiments. The comparison of average grass seed reduction caused by bird predation with mean declines of grass seedlings caused by senescence (ca 95%) allowed us assess top-down (e.g. seed availability) vs bottom-up control (e.g. rainfall) on grass recruitment. Despite moderate to high seed predation, the number of grass seeds that remains in the soil in spring would not limit seed germination and seedling recruitment. By contrast, safe-site availability and drought may be important factors limiting grass recruitment, at least in the undisturbed habitats of the Biosphere Reserve of Ñacuñán.