Gabriela Inés Pirk

Diet and seed removal rates by the harvester ants Pogonomyrmex rastratus and Pogonomyrmex pronotalis in the central monte desert, Argentina

Abstract.Granivorous animals, through seed consumption, may have an important influence on plant abundance, distribution and species composition in desert ecosystems. The aims of this study are twofold: to quantify the diet of Pogonomyrmexrastratus (Mayr) and pogonomyrmex pronotalis (Santschi), and to estimate seed removal per colony of both species throughout their activity season (October-April) in the central Monte desert, Argentina. Both species rely heavily upon seeds, which account for 87–94 % of the items carried to the nests. Their diets are similar, consisting mainly of grass seeds, which represent more than 93 % of the seeds. Among them, three species predominate: Aristida spp., Trichloriscrinita and Pappophorum spp. Seasonal variations as well as seed species richness in the diet are also similar between species. However, their food-handling behaviour differs: most caryopses carried by P. pronotalis bear bracts whereas most caryopses carried by P. rastratus lack them. Seed removal per colony by P. rastratus (6 × 104 seeds/colony) and by P. pronotalis (5 × 104 seeds/colony) throughout the season is similar to the one reported for P. occidentalis in North America. However, seed removal per hectare, which could be estimated for P. rastratus (8.3 × 105 seeds/ha), is lower than removal rates reported for the North American species P. barbatus, P. desertorum, P. rugosus and P. californicus, probably because P. rastratus has lower activity levels and smaller colonies than the North American studied species.

Seed Preferences of Three Harvester Ants of the Genus Pogonomyrmex (Hymenoptera: Formicidae) in the Monte Desert: Are They Reflected in the Diet?

ABSTRACT Harvester ants play an important ecological role as seed consumers in arid areas. We performed choice experiments to study preferences of Pogonomyrmex rastratus (Mayr), Pogonomyrmex mendozanus (Cuezzo & Claver), and Pogonomyrmex inermis (Forel) (Hymenoptera: Formicidae) for seeds of six native species (three grasses, two forbs, and a shrub) in the central Monte desert, Argentina. We hypothesized that ant diet composition would reflect ant preferences. Thus, based on previous ant diet studies, we expected that 1) the three species would prefer grass to nongrass seeds, 2) P. inermis would have a lower preference for nongrass seeds than P. rastratus and P. mendozanus and 3) relative abundance of seeds in the diet would be positively associated with relative preference. In general, relative abundance of seeds in the diet was positively associated with relative preferences. Ants preferred grass seeds, but P. inermis did not have a lower preference for nongrass seeds. We also analyzed the relationship between preferences for seeds of the six species and their size and morphology, and we found higher preferences for seeds of intermediate size but no relationship with seed morphology. The overall match between seed preferences and diet composition could increase the chances of ants affecting the abundance and composition of some seed resources in the Monte desert, with important community implications.

Influence of seed size on feeding preferences and diet composition of three sympatric harvester ants in the central Monte Desert, Argentina

Selective seed consumption by harvester ants may affect seed abundance and composition and, ultimately, plant communities. We evaluated the influence of seed size on preferences and diet of Pogonomyrmex mendozanus, P. rastratus, and P. inermis in the central Monte Desert, Argentina. In choice experiments with Pappophorum spp. seeds of different sizes, P. mendozanus and P. rastratus preferred large seeds, maximizing energy reward. P. inermis showed a less-marked preference for large seeds, which was probably due to morphological constraints imposed by its small body size. Under natural conditions, none of the three species selected larger Pappophorum spp. seeds probably because of high travel and handling costs. Seeds of intermediate size predominated in the diet of the three species but a slight size match was detected as P. mendozanus carried larger seeds than P. rastratus and this than P. inermis, matching body-size differences. Thus, ants probably maximize energy reward but face morphological restrictions and higher costs when carrying and holding large seeds. While seeds of intermediate size are the most vulnerable ones to ant predation, small seeds are favored, as they are abundant in the soil seed bank and lowly predated.

Diet of two sympatric Pheidole spp. ants in the central Monte desert: implications for seed-granivore interactions.

Ants of the genus Pheidole are important seed consumers in several desert ecosystems. In South American deserts, although several Pheidole spp. have been characterized as seed harvesters, studies on their diet and ecological role are still missing. Pheidole spininodis (Mayr) and Pheidole bergi (Mayr) are capable of removing seeds in the central Monte desert. The aim of this study was to quantify and compare the diet of these species and to interpret the results in the context of seed–granivore interactions. Diet was estimated during mid-summer by collecting items brought back to the nest by foragers in ten colonies per species. While P. spininodis was mainly granivorous, P. bergi was mainly insectivorous. However, they both collected ~40% of other types of items. Among seeds, the diet of P. spininodis included mostly grass seeds, whereas the diet of P. bergi was mainly made up of shrub and tree seeds, usually retrieved cooperatively. This behavior allowed P. bergi to carry larger seeds, resulting in diet partitioning in terms of seed size. However, diet of P. spininodis is very similar to that of three sympatric Pogonomyrmex species. Thus, specialized harvester ants remove large quantities of grass seeds in the central Monte desert during the summer, potentially affecting their abundance in the soil seed bank. P. bergi directs its feeding pressure to shrub and tree seeds, and although seeds constitute ~10% of its diet, its high colony density and high activity levels, added to the lower proportion of large seeds in the soil seed bank, indicate that their importance as seed consumers cannot be ruled out.

Evaluation of Three Techniques for the Study of Harvester Ant (Pogonomyrmex spp.) Diet

Abstract 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 Pogonomyrmex 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 and 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 in 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.

Foliar herbivory and its effects on plant growth in native and exotic species in the Patagonian steppe

Studies of herbivory and its consequences on the growth of native and exotic plants could help elucidate some processes involved in plant invasions. Introduced species are likely to experience reduced herbivory in their new range due to the absence of specialist enemies and, thus, may have higher benefits if they reduce the investment in resistance and increase their compensatory capacity. In order to evaluate the role of herbivory in disturbed areas within the Patagonian steppe, we quantified and compared the leaf levels of herbivory of four native and five exotic species and recorded the associated insect fauna. We also performed greenhouse experiments in which we simulated herbivory in order to evaluate the compensatory capacity of native and exotic species under different herbivory levels that resembled naturally occurring damage. Natural herbivory levels in the field were similar between the studied exotic and native plants. Field observations confirmed that they both shared some herbivore insects, most of which are generalists. In the greenhouse experiments, both exotic and native plants fully compensated for herbivory. Our results suggest that the studied exotic plants are not released from herbivory in the Patagonian steppe but are able to fully compensate for it. The capacity to recover from herbivory coupled with other potential adaptations, such as a better performance under disturbance and greater competitive ability than that of the native species, may represent some of the mechanisms responsible for the success of plant invasion in the Patagonian steppe.