Browsing by Author "Milesi, Fernando A."

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    Can seed-eating birds exert top-down effects on grasses of the Monte desert?
    (2008) Marone, Luis; de Casenave, Javier Lopez; Milesi, Fernando A.; Cueto, Victor R.
    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 Nacunan.
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    Influence of temporal fluctuations in seed abundance on the diet of harvester ants (Pogonomyrmex spp.) in the central Monte desert, Argentina
    (2009) Pirk, Gabriela I.; De Casenave, Javier Lopez; Pol, Rodrigo G.; Marone, Luis; Milesi, Fernando A.
    Harvester ants usually go through temporal fluctuations in environmental seed abundance and composition which could influence their behaviour and ecology. The aim of this study was to evaluate how these fluctuations influence the diet of Pogonomyrmex rastratus, P. pronotalis and P. inermis (Hymenoptera, Formicidae) in the central Monte desert during three consecutive growing seasons. Although seeds were the main item in the diet, these ants turned more generalist when seed abundance of the most consumed species (grasses Aristida spp., Trichloris crinita, Pappophorum spp., Digitaria californica and Stipa ichu) was low. Accordingly, diversity of items in the diet decreased with seed abundance in a logarithmical fashion, showing higher foraging efficiency for seeds at higher seed abundance. Seed diversity, however, was not related to seed abundance as ants always included several species in their diet, with alternating prevalence. The proportion of the most consumed species increased logarithmically in the diet of P. rastratus and P. pronotalis along with their abundance in the environment probably as a consequence of diet switching (from forb and shrub seeds to grass seeds) and by an increase in foraging efficiency at higher seed densities. In contrast, foraging activity of P. inermis was very low at low seed abundance and its diet included only the five grasses. Among the most consumed species, proportion in the diet was not associated with relative abundance in the environment. Aristida spp., Pappophorum spp. and D. californica were overall highly selected. However, the flexibility in the diet of P. pronotalis and P. rastratus and the low foraging activity of P. inermis during periods of low resource abundance could attenuate potential top-down effects in the central Monte desert. This study shows that bottom-up effects are important in ant-seed interactions and should be considered when predicting and evaluating ants' effects on seed resources.
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    Litter and seed burying alter food availability and foraging efficiency of granivorous birds in the Monte desert
    (2013) Cueto, Victor R.; Milesi, Fernando A.; Marone, Luis
    We assessed experimentally if the main granivorous bird species that feed on the ground in the central Monte desert are able to detect and consume seeds buried in the soil or trapped within litter. Understanding seed vulnerability to birds allows 1) a better understanding of how seed abundance translates into seed availability, a necessary step to assess seed limitation scenarios, and 2) whether birds alter the distribution of soil seeds through their consumption. Rufous-collared sparrows found and consumed high proportions of buried seeds, though less seeds were eaten at increasing depths. In contrast, many-colored chaco-finches, common diuca-finches and cinnamon warbling-finches did not find buried seeds. All bird species fed on every substrate offered but, as a whole, birds reduced by 50% their seed consumption in Prosopis litter, and by 30% in Larrea litter, compared to consumption in bare soil. This effect was less notable for rufous-collared sparrows, whose double scratch' foraging method would contribute to its great diet breath and abundance in the Monte desert. As birds do not reach a fraction of seeds buried and trapped by litter, seeds readily available for them may be scarcer than previously estimated through soil seed bank studies. Furthermore, since the four bird species detect and consume seeds from littered microhabitats, seed consumption by them surely affects the seasonal dynamics of the soil seed bank in all microhabitat types of the Monte desert.