Influence of temporal fluctuations in seed abundance on the diet of harvester ants (Pogonomyrmex spp.) in the central Monte desert,Argentina

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.     

Influence of temporal fluctuations in seed abundance on the foraging behaviour of harvester ants (Pogonomyrmex spp.) in the central Monte desert,Argentina

The simultaneous study of the temporal dynamics of foraging behaviour, diet and seed abundance is essential to assess the way in which resources affect the behaviour and ecology of harvester ants. Here, we evaluate how fluctuations in grass seed abundance during three consecutive growing seasons influenced the foraging behaviour and diet of the harvester ants Pogonomyrmex rastratus, P. mendozanus and P. inermis in the central Monte desert, Argentina. Seed abundance of the most consumed grasses varied greatly through ant activity season, and ants altered their foraging behaviour in response to those changes. Foragers spent more time travelling and searching for food, and their foraging trips took longer during the low seed availability season. Foraging distance was very similar among species and, contrary to our expectations, did not vary between seasons. Foraging success of P. rastratus and P. inermis increased during the high availability season. This matched the seasonal pattern of foraging activity, suggesting that colonies may detect seed abundance and regulate their foraging effort with the rate of forager success. Although grass seeds were the main item in the diet of the three species, P. mendozanus, and to a lesser extent P. rastratus, turned more generalist when grass seeds were scarce. In contrast, P. inermis showed a very narrow diet breadth, only harvesting grass seeds in both seasons. Our results indicate the relevance of seed availability on foraging behaviour of harvester ants, which should be taken into account when predicting and evaluating the effect of ants on seed resources as well as numerical responses of harvester ant populations to the temporal and spatial variations in grass seed abundance.     

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

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 Pogonomyrmex rastratus (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., Trichloris crinita 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. Received 6 April 2005; revised 1 September 2005; accepted 2 September 2005.     

Effect of harvester ants of the genus Pogonomyrmex on the soil seed bank around their nests in the central Monte desert,Argentina

1. The abundance and composition of soil seed banks is a key determinant of plant community structure. Harvester ants can remove huge quantities of preferred seeds close to the nest affecting composition and spatial distribution of plants. 2. In the central Monte desert (Argentina) ants of the genus Pogonomyrmex have high seed removal rates, especially of the five main grasses. The aim of this study was to establish if their foraging activity affects spatial patterns of the soil seed bank around their nests. Our hypotheses were: (1) removal by ants decreases seed abundance of preferred species in the soil; and (2) the effect varies in time. 3. Soil seed abundance was assessed at different distances from Pogonomyrmex nests in the litter and in bare soil at the beginning, the middle, and the end of the season (late spring‐early autumn). 4. A lower seed abundance of preferred species was observed close to the nest in the litter at the end of the season. Non‐preferred species showed no distance gradient. 5. The lower foraging activity and seed consumption at the beginning of the season could explain the temporal variation of the spatial effect. This was only observed in the litter, probably because of the higher removal frequency in this substrate. 6. Colonies of Pogonomyrmex spp. could enhance the heterogeneity of soil seed banks in the central Monte desert from the summer to the beginning of the autumn. Implications for vegetation dynamics depend on the degree to which seed density limits perennial grasses recruitment after ant activity season.     

Worker size and seed size selection by harvester ants in a neotropical forest

The rules ants follow when selecting seeds are important both to theories of colony organization and to the shaping of their impact as harvesters. Two Costa Rican forests yielded the most diverse harvester ant assemblages yet studied (41 and 44±4 species). I assayed seed size preferences within and between species using milled barley. Seed size selection varied at a number of levels. First, species differed in seed size use: small species mainly carried off small seeds; large species retrieved a broad range of seed sizes. Within species, one-third of those tested yielded inter-colony differences in preferred seed sizes. Finally, workers of polymorphic species tested showed significant worker/seed size correlations. Species, colony, and worker level differences are common among harvester ants. Their significance to community organization and colony ergonomics however is hotly debated and requires a better understanding of the dynamics of food limitation. From the plant's perspective, small seeds will be harvested by a wider variety of ants than large seeds.     

Factors affecting nesting success of a bird assembly in the central Monte Desert, Argentina

We estimated nesting success of a breeding bird assemblage in the central Monte Desert in four breeding seasons (1995–1999). We analyzed the effect of year, period within the breeding season, nest type, and nest location on daily survival rates (DSR). Averaged nesting success of passeriforms in natural habitats was very low (<20% on average, n=12 species). DSR tended to be lower during the egg-laying stage in Furnariidae, Rhinocryptidae, Mimidae, and Emberizidae. Tyrannidae had the highest DSR for all nesting stages and Emberizidae the lowest. Predation was the main cause of nest failure (>90% on average, n=12 species). DSR differed among years, being higher in the driest year (1995) and lower in the most humid year (1997). Nests initiated in the first half of the breeding season had higher DSR than those initiated in the second half. Closed nests had similar survival rates to open nests. DSR differed according to species of plant that supported the nest. Nest predation in the central Monte Desert may be an important selective pressure in birds' life history, which is in accordance with other results for South American temperate birds.     

Influence of seed size and seed nature on recruitment in the polymorphic harvester ant Messor barbarus

In the polymorphic harvester ant Messor barbarus, the colony modulates response during foraging according to seed characteristics. The easiness of picking up small seeds (oat fragments or canaryseeds) shortens the time lapse between food discovery and the return of scouts to the nest, favouring the onset of recruitment and higher mobilisation rates than bigger seeds (whole oat). However, size is not the only criterion, as canaryseeds are preferred among small ones. A modulation of the trail laid by the first scouts also plays a role in the shaping of harvesting patterns and in the enhancement of recruitment towards small and/or preferred seed. This flexibility extends to the relative participation of the three worker sizes classes, which differs according to seed. Media are the most numerous at the foraging arena and largely responsible for trail-laying. Minor participate in seed harvesting and trail-laying, mostly when they can be efficient in carrying seeds, such as oat fragments. Major scarcely participate in harvesting and trail-laying, but are involved in the exploitation of bigger and/or preferred seed species. When faced with different seed species, M. barbarus seems thus to collectively adopt a “foraging time minimisation” rather than an “energetic gain per item harvested maximisation” strategy. Dynamics of foraging and division of labour between foragers of different sizes are related to both the workers’ ability to carry seed items, and the transfer of information to nestmates by trail-laying, all parameters that vary according to seed characteristics.     

Effect of plant cover on seed removal by rodents in the Monte Desert (Mendoza,Argentina)

Abstract In desert areas, predation risk is one of the highest costs of foraging and is a major influence on animal behaviour. Several strategies are used by foragers for surviving and reproducing in desert areas. The foraging strategies of the small mammals of South American deserts are still poorly known. In this study, we investigated the foraging strategies of rodents of the Monte Desert in response to distance from seed sources to sheltered sites (i.e. shrubs) during two different seasons (wet and dry). We evaluated the relative rates of removal of two species of seeds (millet and sunflower) by rodents at two sites by establishing 80 seed sources, 40 in unsheltered microhabitats and 40 in sheltered microhabitats. We recorded both the number of caches and seed consumption for each source. We found that plant cover affected the foraging activity of rodents of the sand dunes in the Monte Desert because both consumption and numbers of caches constructed from sheltered seed sources were higher than those from unsheltered ones. Consumption of sunflower was higher in the wet season than it was in the dry season, when millet consumption increased. Sunflower was the preferred seed both from sheltered or unsheltered sources. We discuss the possible causes of the different foraging strategies used by rodents of the Monte Desert.     

Seed hoarding by rodents of the Monte Desert,Argentina

Food storage is an important adaptation of several animal species to the temporally variable or unpredictable food supplies that are typical of desert environments. In the present study, whether Eligmodontia typus and Graomys griseoflavus inhabiting sand dunes in the Monte desert displayed scatter‐hoarding was investigated. Both rodent species prepared surface caches by digging small holes (1–3 cm deep) in the soil. Caches were partially covered with sand and dry leaves, which meant that all of the cache sites at each station could be found. Most caches were found below shrubs, that is, in a sheltered microhabitat rather than near the food source. The mean distance between caches and seed sources was significantly higher than the mean distance from food sources to shrubs. The proportion of caches was significantly higher in shrubs with high safety cover. The adaptive significance of this foraging behaviour could be a competitive tactic for the rapid sequestering of food from a rich and ephemeral source. Such behaviour would reflect predation risk, which constitutes an important foraging cost in deserts, and could also influence plant community dynamics.     

Effect of spatial and temporal heterogeneity on granivory in the Monte Desert,Argentina

Abstract An important aspect of arid regions is the spatial heterogeneity resulting from differences among plant communities. There are process differences among different vegetation patches increasing variability in the functioning of the ecosystem. The purpose of this study was to estimate granivory, by studying variation of seed removal rates among patches at the local scale and variation according to seed type. We carried out experiments during four seasons in three plant communities in the Monte Desert, Argentina: ‘peladal’, mesquite forest and creosotebush. Seed trays were offered to ants, birds and rodents. Two types of commercial seeds (sunflower and millet) were used. We found that seed removal rate was significantly different among plant communities, among taxa (birds, ants and murid rodents), and among seasons, and that removal rate was higher for sunflower. Seed removal rates by murid rodents were higher than by birds or ants, and occasionally as high as those found in other deserts. The heterogeneity of seed removal patterns at the local level was as strong as that found between continents. Further studies may show similar local heterogeneity in other deserts of the world.     

A note on the selection of seed types by harvester ants in northern Australia

Seed harvesting ants (mainly Pheidole spp.) in the Katherine area of the Northern Territory have been observed to actively forage seed in both native and sown pasture areas. Using discrete mixtures of seeds it was found that ants from colonies in both sown and native pastures would preferentially remove seed of the native grass species, Themeda australis and Digitaria ciliaris, to the introduced legume Stylosanthes humilis. This difference in selection could not be explained solely on a basis of size or weight differences between the species. The preference for grass species over introduced legumes indicates that ant predation is not the main factor in the low seed survival to germination in introduced pastures.     

Native seed preferences of shrub-steppe rodents,birds and ants: the relationships of seed attributes and seed use

Summary This study established the preferences of shrubsteppe granivores among seeds of 6 common sagebrushsteppe plants and related the preferences observed to physical and nutritional attributes of the seeds. Seeds of big sagebrush (Artemisia tridentata), cheatgrass (Bromus tectorum), Indian ricegrass (Oryzopsis hymenoides), western wheatgrass (Pascopyrum smithii), bitterbrush (Purshia tridentata) and green needlegrass (Stipa viridula) were placed in groups of petri dishes designed such that seed removal could be ascribed to either diurnal vertebrates, nocturnal vertebrates or ants. Though absolute quantities of seeds removed varied among the 3 granivore classes, calculations of preference based on weights of each seed species removed by each granivore class indicated that all 3 ranked the seeds similarly. Preference hierarchies of the 3 granivore classes were highly positively correlated with both calories per seed and % soluble carbohydrate of the seeds. The first correlation supports a basic prediction of optimal foraging theory —that foragers should maximize energy intake per unit time spent foraging. Both correlations emphasize the role of seed nutritional qualities in granivore seed selectivity in that soluble carbohydrate is a water-efficient energy source and its percentage is a good indicator of the digestible energy available in a food item. A corollary experiment comparing granivore use of an exotic seed (millet [Panicum miliaceum]) and a preferred native seed (Oryzopsis) demonstrated a distinct preference for the exotic. Since millet seeds are particularly high in % soluble carbohydrate, this result reinforced the apparent value of this nutritional attribute as a predictor of granivore seed preference. Among many seed resource characteristics upon which granivore seed selectivity might operate, our results indicate that individual species' nutritional composition may be particularly important. Thus, inferences about seed selectivity and resource partitioning among arid-land granivores should be interpreted with caution, especially those based on experiments using seed introductions, since the influence of seed nutritional attributes has not been widely acknowledged heretofore.     

Niche overlap and diet composition of three sympatric coastal dolphin species in the southwest Atlantic Ocean

Sympatric species are expected to differ in ecological requirements to minimize niche overlap and avoid competition. Here we assess the trophic interactions among three coexisting dolphin species from southern Brazil: the franciscana dolphin (Pontoporia blainvillei), the Guiana dolphin (Sotalia guianensis), and the Lahille's bottlenose dolphin (Tursiops truncatus gephyreus). We evaluated temporal variation in carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of bone collagen to examine potential dietary shifts resulting from increased fishing activity over the past three decades. We estimated the degree of niche overlap among these species and the contribution of potential prey sources to their diet. δ¹⁵N values were consistent among species and across years, while δ¹³C values increased for Guiana dolphins and decreased for bottlenose dolphins, suggesting changes in diet and/or foraging habitats through time. The similar δ¹³C and δ¹⁵N values and the high niche overlap between Guiana and bottlenose dolphins indicate that these species are primarily feeding on demersal prey. The franciscana diet is primarily composed of pelagic prey, resulting in a lower niche overlap in comparison with the other dolphin species. Our study provides further information about the foraging ecology of this unique dolphin community in southern Brazil with implications for its management and conservation.     

Effects of wild boar disturbance on vegetation and soil properties in the Monte Desert,Argentina

The wild boar (Sus scrofa) is an exotic agent of disturbance that arrived in the Ñacuñán Reserve of Argentina in the 1980s. When foraging, the wild boar overturns extensive areas of soil leaving them bare of vegetation. Knowledge is scarce about the boar's impact on vegetation composition and soil properties in the Monte Desert, Argentina. The objective of our study was to determine the short terms effects of wild boar rooting on vegetation and on soil physical, chemical and microbiological properties. Our results indicate that rooting activities significantly reduced the plant cover of herbs, perennial grasses and shrubs, and decreased plant richness and diversity. Disturbed soils showed less compaction, more moisture, a low C/N ratio, and high content of mineral nitrogen. These new soil characteristics could be responsible for a reduced plant cover and less soil bulk density, which could increase soil degradation by wind erosion.     

Vulnerability of small and medium-sized prey mammals in relation to their habitat preferences,age classes and locomotion types in the temperate Monte Desert,Argentina

In this study we examined predation by small felids upon small and medium-sized prey mammals in the central region of the Monte Desert. We analyzed the degree of vulnerability of prey mammals in relation to their habitat use patterns, modes of locomotion and age classes. Medium and small rodents were the predominant prey in the diet of small cats. The most consumed small mammal was Akodon molinae, which inhabits areas of dense cover, while species occurring in open habitats such as Eligmodontia typus were less consumed. Among medium-sized mammals, Galea leucoblephara that depends on patches with high plant cover for refuge was more consumed than Microcavia australis that lives in colonies and displays a complex set of antipredator strategies. Prey selectivity on small rodents showed that E. typus, which uses bipedal locomotion and an erratic escape behaviour, was consumed in greater proportion than its availability. We suggest that although its specialized locomotory mode would diminish predator attacks, its preference for open habitats would increase the probability of predator–prey encounter. Consumption of small mammals focuses mostly on adult individuals. However, we found a significant consumption of senile individuals of A. molinae, which points out that vulnerability tends to be high towards the age of maturity in this species.     

Influence of diet composition on feeding and water excretion by the tsetse fly, Glossina morsitans

Adult Glossina morsitans fed on aqueous salt solutions containing phagostimulant ATP in an in vitro feeding system gave an optimal feeding response only over a narrow pH range equivalent to that of vertebrate blood. There was much less discrimination on the basis of molar concentration.The rate and extent of water excretion by the fly was found to depend on the concentration of Na⁺ ions in the food medium: an active transport mechanism is indicated which enables water to pass from the meal through the anterior midgut wall and into the haemocoele. A favourable osmotic gradient assisted water transport in the presence of Na⁺ ions: the system could not operate efficiently in the presence of Na⁺ ions if the osmotic pressure of the food medium was higher than that of vertebrate blood, nor could it operate efficiently in any solution lacking Na⁺ ions.Normal transfer of a meal from the crop to the anterior midgut occurred only when the food medium was isotonic with vertebrate blood or in the presence of Na⁺ ions if hypotonic. Normal transfer of isotonic solutions was prevented in the presence of excess K⁺ ions, and hypertonic solutions were not transferred normally even in the presence of Na⁺ ions. Thus the rate of water excretion was reduced.Tsetse flies fed on blood in an in vitro feeding system excreted water at a significantly lower rate than flies fed on a living animal. Evidence suggests that this is due to a combined effect of changes in viscosity, effective ionic composition, and osmotic pressure, upon the normal rate and extent of food uptake and manipulation of the meal prior to digestion. The implications of this are discussed in terms of future developments of in vitro feeding techniques for haematophagous insects.     

Locomotion and Escape Modes in Rodents of the Monte Desert (Argentina)

Modes of locomotion and escape tactics are attributes that affect the structure of animal communities, promoting exploitation of different microhabitats and the coexistence of different species. Bipedal locomotion is considered to be more effective than a quadrupedal gait in escaping attacks by predators because it allows for higher speed, a faster response to attack, sudden changes of direction and better detection of aerial raptors. The aim of this study was to determine the type of locomotion used at the moment of escape by three rodent species of the Monte desert –Eligmodontia typus, Akodon molinae and Graomys griseoflavus. The study was carried out in three plant communities of the Ñacuñán Reserve (Mendoza). All three species showed differences in both mode of escape and locomotory pattern. Graomys griseoflavus exhibited the highest proportion of escapes using quadrupedal saltation. The mode of locomotion employed by E. typus varied according to the type of plant communities it inhabited. Those occurring at open sites (Medanal community) exhibited a greater propensity to jump during escapes than those from more sheltered habitats (Algarrobal community). Akodon molinae relied primarily on a quadrupedal gait when fleeing from predators, which would explain its greater dependence on plant cover. Therefore, the morphological and behavioural characteristics of these species are related to their mode of locomotion and the strategies they employ to diminish the risk of predation.     

The interactive effects of climate, life history, and interspecific neighbours on mortality in a population of seed harvester ants

Abstract.  1. Fifteen years of census data from a population of the red harvester ant ( Pogonomyrmex barbatus ), 5 years of census data from populations of Aphaenogaster cockerelli and Myrmecocystus mimicus at the same site, and long-term precipitation data from a nearby weather station were used to examine how nearest interspecific neighbours, climate, and life history influence P. barbatus colony mortality.
2. It was found that more P. barbatus colonies die following dry summers than following wet summers, but there is no relationship between the establishment of new colonies and rainfall.
3.  Aphaenogaster cockerelli , but not M. mimicus , nearest neighbours influence P. barbatus colony mortality in some years and for some ages.
4. The effects of nearest A. cockerelli neighbours are usually most severe following dry summers, when resource availability is reduced.     

Activity Patterns of Harvester Ants Pogonomyrmex pronotalis and Pogonomyrmex rastratus in the Central Monte Desert, Argentina

Seed-eating ants could have a significant effect on plant communities in deserts and semiarid zones. This effect is mediated through spatial and temporal foraging patterns, and seed selection within patches. Foraging patterns of harvester ants in South American deserts are almost unknown. The purpose of this work is to determine the temporal variations in the activity levels of Pogonomyrmex pronotalis and P. rastratus in the central Monte desert, and how these patterns may be related to abiotic factors, particularly to soil temperature. Activity levels and soil surface temperature were recorded at hourly intervals in five colonies for each species during the activity season (October, December, February, and April) in both 1999–2000 and 2000–2001 periods (except for October 1999). Surface ant activity starts in October, increases between December and February, and then ceases by April. Surface ant activity is diurnal throughout the season and usually has a unique peak during midday in October and April, and two peaks in the morning and the afternoon from December to February. The proportion of the activity budget devoted to nest's maintenance activity was similar for both species. Activity levels of foraging workers tended to be higher in P. pronotalis than in P. rastratus. P. pronotalis is active between 20 and 59°C, with higher levels of activity between 35 and 45°C, whereas P. rastratus shows activity between 18 and 58°C, with higher levels of activity between 30 and 40°C. Our results suggest that temporal changes in surface activity respond mainly to soil temperature fluctuations. However, at intermediate temperatures (those probably encompassing the thermal tolerance range of these ant species), temperature appears not to be a good predictor of daily and seasonal activity fluctuations.