Browsing by Author "De Casenave, Javier Lopez"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Evaluation of three techniques for the study of harvester ant (Pogonomynnex spp.) diet
    (2007) Pirk, Gabriela Ines; De Casenave, Javier Lopez; Marone, Luis
    The estimation of an ant's diet is crucial in many ecological studies. Different techniques, which involve different assumptions and field procedures, have been used to estimate the composition of harvester ant diet. In this study, three techniques are compared for the estimation of the diet of Pogonomynnex rastratus (Mayr). Pogonomyrmex pronotalis (Santschi), and Pogonomyrmex inermis, (Forel) in the central Monte desert, Argentina: (1) hand collection of items brought back to the nest by foragers, (2) collection of items with a semiautomated device with pitfall traps, and (3) collection of the discarded material accumulated in middens. The hand collection technique and the collection of middens provided the lowest mid the highest number of items, respectively. Midden samples and pitfall traps contained a higher proportion of nonseed items, probably coming from sources other than ants, than hand-collected items. The three techniques provided similar estimations of species richness but a bias against small seeds was detected for P.pronotalis and P. inermis with the hand collection technique, possibly because of the difficulty of collecting small items by hand. The percentage of seed species if) the diet obtained with different techniques was positively correlated in the great majority of colonies. Overall, despite their intrinsic differences, the three techniques proved consistent, which constitutes a robustness test for the estimations obtained, In comparative ecological studies, the awareness that results depend on the techniques and their assumptions is particularly important.
  • No Thumbnail Available
    Item
    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.