Browsing by Author "Marone, Luis"

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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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    EFFECTS OF NUTRITIONAL AND ANTI-NUTRITIONAL PROPERTIES OF SEEDS ON THE FEEDING ECOLOGY OF SEED-EATING BIRDS OF THE MONTE DESERT, ARGENTINA
    (2012) Manuel Rios, Juan; Mangione, Antonio; Marone, Luis
    Food quality and physiological capacities and limitations in recognizing and processing food are among the factors influencing the choices of desert birds in feeding. Seed-eating birds of the central Monte desert generally select grass seeds rather than forb seeds. We studied some of the mechanisms underlying seed-selection patterns in seed-eating birds of the Monte desert, analyzing nutrients and secondary compounds of the 15 most abundant seeds in the soil and their relationship with the diet of six species of seed-eating birds. Grass seeds contained more starch and less total phenols than did forb seeds and were free from alkaloids. The diet of the most graminivorous birds was correlated with seeds' starch concentration, while generalists' diet was correlated with seed abundance. To assess the plausible mechanisms underlying birds' selection of seed, we experimented with three species differing in the breadth of their diet: a generalist, the Rufous-collared Sparrow (Zonotrichia capensis), and two graminivores, the Many-colored Chaco-Finch (Saltatricula multicolor) and Common Diuca-Finch (Diuca diuca). We postulated that the level of starch and the presence of phenolic compounds and alkaloids influence food preference. Results suggest that most graminivorous birds prefer high-starch diets and avoid diets with phenols and alkaloids. In contrast, the generalist foraged regardless of starch content, and its food intake was reduced only by some of the phenols and alkaloids tested. Seed chemistry may explain some important features of seed selection by birds in the Monte desert.
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    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.
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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.
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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.
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    Seasonal dynamics of guild structure in a bird assemblage of the central Monte desert
    (2008) Lopez de Casenave, Javier; Cueto, Victor R.; Marone, Luis
    Most studies of ecological guilds consider guild membership as a fixed attribute of species, and the guild structure as a fixed attribute of a community. Consequently, almost no study tackled the examination of temporal changes in the guild structure of a particular assemblage. We used data on foraging patterns of central Monte desert birds to assess seasonal differences in resource use by resident species, and to examine seasonal changes in the guild structure of the assemblage. From 1993 until 1999, we quantified the use of foraging substrates, manoeuvres, and height strata, and the utilisation of plants by 18 bird species. Resident species showed different kinds of seasonal dynamics, from no noticeable to drastic changes in their foraging patterns. Seasonal changes in guild structure were remarkable. Only two guilds were present all year round (ground foragers and foliage foragers), although they suffered important species turnover. Three other guilds had a seasonal occurrence: graminivores (during the non-breeding season), arboreal herbivores, and aerial feeders (during the breeding season). Two species changed from one guild to another between seasons. Both presence-absence of migrant species, and changes in the feeding ecology of resident species had an influence on the temporal dynamics of guild structure. These factors appeared to be related with changing resource availability, especially with the birds' ability for facing lean conditions through migration and with the opportunistic use of seasonally superabundant resources. Our results undermine the trust in studies in which guild structure was assessed irrespective of temporal variations. We show also that guild membership is not a fixed attribute of a species: we should rather evaluate the individuals' resource use in a given moment to assign them to a guild, instead of obtaining a single measure purporting to represent a species' fixed attribute. (C) 2007 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.
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    Tolerance to Dietary Phenolics and Diet Breadth in Three Seed-eating Birds: Implications for Graminivory
    (2012) Manuel Rios, Juan; Marcelo Mangione, Antonio; Marone, Luis
    The ability to cope with plant secondary compounds (PSCs) has profound implications for an animal's behavior. In the present study, we assessed the tolerance to dietary phenolics in three seed-eating birds: Zonotrichia capensis, Saltatricula multicolor, and Diuca diuca, which differ in their diet breadth. Seeds in their habitat have distinct chemical composition: grass seeds have less PSCs, specifically, less total phenolics than forb seeds. Based on the detoxification limitation hypothesis and using published data of the natural history of these birds in the central Monte desert, we postulate that predominantly and exclusively graminivorous birds such as D. diuca and S. multicolor, respectively, are less tolerant due, in part, to a lower detoxification capacity than those with greater diet breadth, Z. capensis. To achieve this goal, we measured the food intake of diets varying in their concentration of tannic acid (TA). Indicators of tolerance were body mass change, food, TA and water intake, and glucuronic acid output throughout the experiment. Zonotrichia capensis performed better along the tolerance experiment: it maintained body mass from 0 to 4% TA diet, food and TA intake were higher than the other two species at the end of the experiment, and glucuronic acid output by Z. capensis was greater than D. diuca and S. multicolor from 2% TA diet until the end of the experiment. Our results suggest that Z. capensis is the most tolerant species and this physiological trait may explain their greater diet breadth. J. Exp. Zool. 00A:1-9, 2012. (c) 2012 Wiley Periodicals, Inc.