Browsing by Author "Pirk, Gabriela I."

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    Grass seed production in the central Monte desert during successive wet and dry years
    (2010) Pol, Rodrigo G.; Pirk, Gabriela I.; Marone, Luis
    In desert regions, water availability triggers primary production, which determines seed production, the composition and size of soil seed reserves and the abundance and behaviour of seed-eating animals. In the central Monte desert, large precipitation events (a parts per thousand yen10 mm) account for a high proportion of growing season's rainfall. Our first objective here was to assess whether and how timing and amount of seed production of C-3 and C-4 perennial grasses are linked to spring and summer precipitation pulses and to estimate the seasonal and year-to-year variability in seed production. Our second aim was to calculate grass seed production and compare it with seed requirements by granivorous animals to infer whether the animals can exert top-down effects on plant populations. Seed production of C-3 and C-4 species was triggered by significant spring and summer rainfall, respectively. Such distinct response may be associated with the effect of precipitation during flower development and seed set in both functional groups. In all species, seed production varied among years. Rainfall pulses in the summer triggered and positively affected the magnitude of seed production in most C-4 grasses. However, all perennial grasses were able to produce high amount of seeds even during a year subjected to extreme drought, suggesting that perenniality would allow these species to make large reproductive investment despite harsh environmental conditions. The comparative assessment of seed production and consumer demands suggests that it is unlikely that granivory exerts a top-down control on grasses in the Monte desert.
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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.