Harvester Ant Species Research Articles

Increased cover of native and exotic plants on the rims of harvester ant (Hymenoptera: Formicidae) nests under grazing and drought.

Harvester ants create habitats along nest rims, which some plants use as refugia. These refugia can enhance ecosystem stability to disturbances like drought and grazing, but their potential role in invasion ecology is not yet tested. Here we examine the effects of drought and grazing on nest-rim refugia of 2 harvester ant species: Pogonomyrmex occidentals and P. rugosus. We selected 4 rangeland sites with high harvester ant nest densities in northern Arizona, USA, with pre-existing grazing exclosures adjacent to heavily grazed habitat. Our objective was to determine whether nest refugia were used by native or exotic plant species for each site and scenario of drought and grazing. We measured vegetation cover on nest surfaces, on nest rims, and at 3 distances (3, 5, and 10 m) from nests. At each site, we sampled 2 treatments (grazed/excluded) during 2 seasons (drought/monsoon). We found that nest rims increased vegetation cover compared with background levels at all sites and in almost all scenarios of treatment and season, indicating that nest rims provide important refugia for plants from drought and cattle grazing. In some cases, plants enhanced on nest rims were native grasses such as blue gramma (Bouteloua gracilis) or forbs such as sunflowers (Helianthus petiolaris). However, nest rims at all sites enhanced exotic species, particularly Russian thistle (Salsola tragus), purslane (Portulaca oleracea), and bull thistle (Cirsium vulgare). These results suggest that harvester ants play important roles in invasion ecology and restoration. We discuss potential mechanisms for why certain plant species use nest-rim refugia and how harvester ant nests contribute to plant community dynamics.

Seasonal effects of granivory on broadcast seeding restoration in a degraded semiarid grassland

Granivores, such as rodents and harvester ants, are common in dryland ecosystems. In intact dryland ecosystems, the effects of such granivores on soil seed banks, plant community structure, and ecosystem functioning are well established. However, the effects of granivores on dryland restoration have received much less attention. In this study, we evaluated the seasonality of seed removal by harvester ants, seed preference of harvester ants, and effects of granivore exclosure on the soil seed bank at four sites in southeastern Utah. We studied two species of harvester ants, Pogonomyrmex rugosus and P. occidentalis. The foraging behavior of Pogonomyrmex spp. was temperature‐dependent; they removed no seeds during winter months, and up to 80% of seeds during high activity in the summer. Despite seasonal differences in seed removal, Pogonomyrmex spp. did not show significant preferences for seed species. However, their seasonal foraging behavior translated into measurable differences in an experimental soil seed bank. Over the summer months, ants reduced the number of seeds in the seed bank by approximately 20%, but when ants were excluded during the winter, the number of seeds in the seed bank remained the same. This finding supports the timing of business‐as‐usual seeding practices of applying seeds in late fall. If seeds germinate in the first season after application, ant granivory is unlikely to pose a large barrier to establishment. However, if seeds remain dormant during the first year, as is common in many dryland species, seeds are vulnerable to ant granivory.

Harvester ant nest architecture is more strongly affected by intrinsic than extrinsic factors.

Behavior is shaped by genes, environment, and evolutionary history in different ways. Nest architecture is an extended phenotype that results from the interaction between the behavior of animals and their environment. Nests built by ants are extended phenotypes that differ in structure among species and among colonies within a species, but the source of these differences remains an open question. To investigate the impact of colony identity (genetics), evolutionary history (species), and the environment on nest architecture, we compared how two species of harvester ants, Pogonomyrmex californicus and Veromessor andrei, construct their nests under different environmental conditions. For each species, we allowed workers from four colonies to excavate nests in environments that differed in temperature and humidity for seven days. We then created casts of each nest to compare nest structures among colonies, between species, and across environmental conditions. We found differences in nest structure among colonies of the same species and between species. Interestingly, however, environmental conditions did not have a strong influence on nest structure in either species. Our results suggest that extended phenotypes are shaped more strongly by internal factors, such as genes and evolutionary history, and are less plastic in response to the abiotic environment, like many physical and physiological phenotypes.

Endangered populations of Messor arenarius ants in the Northern Coastal Plain of Israel

Abstract Messor arenarius (Fabricius) is a harvester ant species, which is mainly distributed in deserts of the Middle East and North Africa. Ants of this species dig their nests in loess or sandy soils. In Israel, this ants’ species is distributed in the Negev Desert and in sandy soils along the Mediterranean Coastal Plain. In this work, 12 sites of natural habitats in the Northern Coastal Plain of Israel were surveyed for surface activity of M. arenarius ants. These ants were found in 3 out of these 12 natural habitats. Two of these sites where M. arenarius ants were found were subjected to disturbance, caused by development works during the period of this survey. It is concluded, that the populations of M. arenarius in the Northern Mediterranean Coastal Plain of Israel are endangered. It is also concluded, that the presence of either Artemisia monosperma or Retama raetam bushes in certain habitats in the Northern Coastal Plain of Israel, are not necessary or sufficient conditions for presence of M. arenarius in these same habitats.

Above- and below-ground effects of an ecosystem engineer ant in Mediterranean dry grasslands.

Within a local assemblage, ecosystem engineers can have major impacts on population dynamics, community composition and ecosystem functions by transforming or creating new habitats. They act as an ecological filter altering community composition through a set of environmental variables. The impact of ants on their environment has been widely studied, but their multi-component effects (both trophic and non-trophic) have been rarely addressed. We investigated the roles of Messor barbarus, one of the commonest harvester ant species in south-western European Mediterranean grasslands. We analysed soil physico-chemical parameters, above-ground vegetation (e.g. species richness, plant community, micro-local heterogeneity, plant biomass) and above- and below-ground fauna (macrofauna, Collembola, Acari and nematodes). A clear and strong local impact of M. barbarus on soil, vegetation and fauna compartments emerges. The environmental filter is altered by modifications to soil physico-chemical properties, and the biotic filter by changes to plant communities and altered above- and below-ground fauna abundance, occurrence and community structure. The engineering activity of M. barbarus affects not only these separate ecosystem components but also the trophic and non-trophic relationships between them. By altering ecological filters at a local scale, M. barbarus creates habitat heterogeneity that may in turn increase ecological niches in these highly diverse ecosystems.

Predator–Prey Battle of Ecological Icons: Horned Lizards (Phrynosoma spp.) and Harvester Ants (Pogonomyrmex spp.)

Horned lizards, Phrynosoma spp., and harvester ants, Pogonomyrmex spp., could be in a predator–prey arms race in which the lizards are specialists that feed on harvester ants, and ants have highly toxic venom and other defenses to help deter predacious horned lizards. All 23 examined species of harvester ants possess venoms that are highly lethal to mice, but the venoms of the tested ant species were nearly inactive toward horned lizards. Blood plasma of Phrynosoma cornutum contains a factor (or factors) that neutralizes the ability of harvester ant venom to kill mice, but does not neutralize the venoms of honeybees, a rattlesnake, Russell's viper, or a cobra. A species of harvester ant present only in southern South America was used to test the predictions that the lethality of harvester ant venom evolved in response to predation pressure from horned lizards, and that horned lizard plasma does not neutralize the lethality of this species of harvester ant. This ant species did not overlap in range with horned lizards, which have a range from Guatemala to Canada. Not only was the venom of the South American ant species the most lethal of all tested harvester ant venoms, the venom's lethal activity was neutralized by horned lizard plasma. These results indicate that horned lizards responded to the lethality of their invertebrate prey's venom, but that the harvester ant venom lethality did not evolve in response to predation pressures by present day horned lizards.

Myrmecicultoridae, a New Family of Myrmecophilic Spiders from the Chihuahuan Desert (Araneae: Entelegynae)

The new genus and species Myrmecicultor chihuahuensis Ramirez, Grismado, and Ubick is described and proposed as the type of the new family, Myrmecicultoridae Ramirez, Grismado, and Ubick. The species is ecribellate, with entelegyne genitalia, two tarsal claws, without claw tufts, and the males have a retrolateral palpal tibial apophysis. Some morphological characters suggest a possible relationship with Zodariidae or Prodidomidae, but the phylogenetic analysis of six markers from the mitochondrial (12S rDNA, 16S rDNA, cytochrome oxidase subunit I) and nuclear (histone H3, 18S rDNA, 28S rDNA) genomes indicate that M. chihuahuensis is a separate lineage emerging near the base of the Dionycha and the Oval Calamistrum clade. The same result is obtained when the molecular data are combined with a dataset of morphological characters. Specimens of M. chihuahuensis were found associated with three species of harvester ants, Pogonomyrmex rugosus, Novomessor albisetosis, and Novomessor cockerelli, and were collected in pitfall traps when the ants are most active. The known distribution spans the Big Bend region of Texas (Presidio, Brewster, and Hudspeth counties), to Coahuila (Cuatro Cienegas) and Aguascalientes (Tepezala), Mexico.

Structural changes over time in individual‐based networks involving a harvester ant, seeds, and invertebrates

1. Harvester ants perform important ecological functions, such as seed predation and redistribution of nutrients in the soil, through a complex of ecological interactions. Most studies are static descriptions of network structure, while their temporal organisations and the factors that modulate it have been neglected.2. This study describes the temporal organisation and the influence of climatic variables (e.g. temperature and rainfall) on network structure between a harvester ant species and its food resources (seeds and invertebrates) in the Brazilian savanna over a 1‐year period. In addition, considering the ant–seed networks, the relationship between the size of seeds collected by ants and the distance travelled was examined, as well as how this relationship behaves over time.3. For all network types (ant–seed, ant–invertebrate, and combined) there was great structural variation throughout the year. Among climatic variables, only rainfall influenced the properties of ant–invertebrate networks, presenting a positive influence on diversity of interactions and a negative influence on network specialisation. Finally, this study shows that harvester ants tend to collect larger seeds in the vicinity of the nest (< 5 m) and smaller seeds at greater distances, and this relationship appears to be constant over time.4. This study highlights that ecological networks are useful tools with which to understand the temporal organisation of ecological functions. In addition, climatic changes, mainly in the rainfall regime, may interfere directly in the availability of resources and ultimately in the interactions between harvester ants and their food resources.

Within-colony genetic diversity differentially affects foraging, nest maintenance, and aggression in two species of harvester ants

There is accumulating evidence that genetic diversity improves the behavioral performance and consequently the fitness in groups of social animals. We examined the behavioral performance of colonies of two co-occurring, congeneric harvester ant species (Messor arenarius and a non-described Messor sp.) in fitness-related behaviors, pertaining to foraging performance, nest maintenance, and aggression. We linked these behaviors to the colonial genetic diversity, by genotyping workers, using six and five microsatellite markers for M. arenarius and M. sp., respectively. Correlations of genetic diversity with colony performance and aggression level contrasted between the two species. In M. arenarius, genetic diversity was correlated with foraging performance and nest maintenance but not with the overall aggression level, while in M. sp., genetic diversity was correlated with the overall aggression level, but not with foraging performance or nest maintenance. The two species exhibited similar specific aggression levels, with higher aggression shown towards heterospecifics and lower towards non-nestmate conspecifics and nestmates. However, M. sp. workers displayed a tendency to interact for longer with heterospecifics than did M. arenarius. We speculate that the different foraging strategies, group vs. individual foraging, and possibly also the different mating systems, contribute to the differences found in behavior between the two species.

Exploring the vegetation: Seed harvester ants climb and remove seeds from a giant cactus in a semiarid environment

The red harvester ant (Pogonomyrmex barbatus) was observed in situ for the first time climbing and directly removing seeds from different individuals of the giant cactus Neobuxbaumia tetetzo in a Neotropical semiarid environment in central Mexico. Here, we explain how this harvester ant species, by overcoming a vertical barrier (reaching up 3.46 m), has amplified the two-dimensional spatial dimensions in which its worker ants may forage. We discuss the ecological traits that could drive this unusual behavior and its possible implications for the cactus.

Ecological drivers and reproductive consequences of non-kin cooperation by ant queens.

The fitness consequences of joining a group are highly dependent on ecological context, especially for non-kin. To assess the relationships between cooperation and environment, we examined variation in colony reproductive success for a harvester ant species that nests either solitarily or with multiple, unrelated queens, a social strategy known as primary polygyny. We measured the reproductive investment of colonies of solitary versus social nesting types at two sites, one with primarily single-queen colonies, and the other with a majority of polygynous nests. Our results were consistent with the hypothesis that cooperative nesting by unrelated ant queens is likely a selection response to difficult environments, rather than a strategy to maximize reproduction under favorable conditions. Fewer colonies at the primarily polygynous site reproduced than at the site with primarily single queen nests, and those that did had lower reproductive investment, as measured by number and total mass of reproductives. Assessment of ecological conditions also support the harsh environment hypothesis. Colony density in the multi-queen population was higher, and nearest neighbor distances were lower for non-reproducing than reproducing colonies. To more directly test the hypothesis that colony reproduction was ecologically constrained, we experimentally supplemented food resources for a subset of colonies at the primary polygyny site. Supplemented colonies increased reproductive investment levels to equal that of colonies at the single-queen population, further indicating that environmental pressures are severe where primary polygyny is dominant, and may drive the evolution of non-kin cooperation in this context.

Consistent differences in foraging behavior in 2 sympatric harvester ant species may facilitate coexistence.

The co-occurrence of 2 similar species depends on their ability to occupy different ecological niches. Here, we compared the consistency of different aspects of foraging behavior in 2 co-occurring harvester ant species (Messor ebeninus and Messor arenarius), under field conditions. The 2 species are active concomitantly and display a similar diet, but M. arenarius features smaller colonies, larger workers on average, and a broader range of foraging strategies than M. ebeninus. We characterized the flora in the 2 species’ natural habitat, and detected a nesting preference by M. arenarius for more open, vegetation-free microhabitats than those preferred by M. ebeninus. Next, we tested the food preference of foraging colonies by presenting 3 non-native seed types. Messor arenarius was more selective in its food choice. Colonies were then offered 1 type of seeds over 3 days in different spatial arrangements from the nest entrance (e.g., a seed plate close to the nest entrance, a seed plate blocked by an obstacle, or 3 plates placed at increasing distances from the nest entrance). While both species were consistent in their foraging behavior, expressed as seed collection, under different treatments over time, M. ebeninus was more consistent than M. arenarius. These differences between the species may be explained by their different colony size, worker size, and range of foraging strategies, among other factors. We suggest that the differences in foraging, such as in food preference and behavioral consistency while foraging, could contribute to the co-occurrence of these 2 species in a similar habitat.

Seed selection and coexistence in two sympatric Messor harvester ant species (Hymenoptera: Formicidae)

Selective seed consumption by harvester ants may be affected by several seed attributes, amongst which seed size and environmental availability play a prominent role. In the present study, we considered the effects of seed size and seed availability on the coexistence and diet preference of two Messor species (Messor wasmanni and Messor minor). M. wasmanni colonies collected the most abundant resource in the environment, grass seeds (Poaceae), according to their availability whilst M. minor showed a nonselective process. In addition, the two ant species showed a different seed size preference, with M. wasmanni adopting a selective strategy and M. minor a generalist strategy. However, competition for foraging resources between the two ant species (assessed by null model algorithms) seems not to affect their foraging behaviour. The lack of a competitive structure within the ant assemblage suggests, in fact, that resources are not a limiting factor for species coexistence.

The cost of being queen: Investment across Pogonomyrmex harvester ant gynes that differ in degree of claustrality

The role of the ant colony largely consists of non-reproductive tasks, such as foraging, tending brood, and defense. However, workers are vitally linked to reproduction through their provisioning of sexual offspring, which are produced annually to mate and initiate new colonies. Gynes (future queens) have size-associated variation in colony founding strategy (claustrality), with each strategy requiring different energetic investments from their natal colony. We compared the per capita production cost required for semi-claustral, facultative, and claustral gynes across four species of Pogonomyrmex harvester ants. We found that the claustral founding strategy is markedly expensive, costing approximately 70% more energy than that of the semi-claustral strategy. Relative to males, claustral gynes also had the largest differential investment and smallest size variation. We applied these investment costs to a model by Brown and Bonhoeffer (2003) that predicts founding strategy based on investment cost and foraging survivorship. The model predicts that non-claustral foundresses must survive the foraging period with a probability of 30–36% in order for a foraging strategy to be selectively favored. These results highlight the importance of incorporating resource investment at the colony level when investigating the evolution of colony founding strategies in ants.

Spatio-temporal Foraging Dynamics in Two Coexisting Harvester Ants (Hymenoptera: Formicidae)

Different aspects of the foraging strategies of two harvester ant species, Messor wasmanni and M. minor, were investigated in a Mediterranean dry grassland area. Baits were used to evaluate the existence of a trade-off between resource discovery and domination as well as the effect of three variables (air temperature, relative humidity and distance) on the trade-off. Baits were also utilized to explore random vs non random use of time by colonies. Random vs non random utilization of space was instead evaluated by mapping the daily foraging area of colonies in a grid of 900 plots of 1 m2 each. Results revealed that species coexistence is not preferentially supported by a trade-off in resource utilization with no overall effect of the examined variables. The foraging activity of the two species widely overlapped whilst a clear competition for space occurred. The observed space partitioning could represent an advantageous strategy for the coexistence of the two ant species.

Context-dependent navigation in a collectively foraging species of ant, Messor cephalotes

More than 100 years of scientific research has provided evidence for sophisticated navigational mechanisms in social insects. One key role for navigation in ants is the orientation of workers between food sources and the nest. The focus of recent work has been restricted to navigation in individually foraging ant species, yet many species do not forage entirely independently, instead relying on collectively maintained information such as persistent trail networks and/or pheromones. Harvester ants use such networks, but additionally, foragers often search individually for food either side of trails. In the absence of a trail, these ‘off-trail’ foragers must navigate independently to relocate the trail and return to the nest. To investigate the strategies used by ants on and off the main trails, we conducted field experiments with a harvester ant species, Messor cephalotes, by transferring on-trail and off-trail foragers to an experimental arena. We employed custom-built software to track and analyse ant trajectories in the arena and to quantitatively compare behaviour. Our results indicate that foragers navigate using different cues depending on whether they are travelling on or off the main trails. We argue that navigation in collectively foraging ants deserves more attention due to the potential for behavioural flexibility arising from the relative complexity of journeys between food and the nest.

Forager abundance and its relationship with colony activity level in three species of South American Pogonomyrmex harvester ants

The proportion of foragers in ant colonies is a fairly constant species-specific characteristic that could be determined by intrinsic or extrinsic factors. If intrinsic factors are relevant, species with similar life history characteristics (e.g., colony size and foraging strategies) would be expected to have a similar proportion of foragers in their colonies. Within the genus Pogonomyrmex, North American species can vary largely in their colony size, whereas only species with small colonies are known in South America. We studied the characteristics of the foraging subcaste in three sympatric South American species of Pogonomyrmex harvester ants, and compared it with the available information on other species of the same genus. We used two mark-recapture methods and colony excavations to estimate the number and proportion of foragers in the colonies of P. mendozanus, P. inermis, and P. rastratus, and to test the relationship between forager external activity levels and abundance per colony. Forager abundance in the three studied species was lower than in most North American species. The percentage of foragers in their colonies ranged 7–15 %, more similar to North American species with large colonies than to those with small colony size. Foraging activity was positively correlated with forager abundance in all three species, implying that colony allocation to number of foragers allows for higher food acquisition. Further comparative studies involving a wider range of traits in South and North American species would allow to unveil the role of environmental factors in shaping each species’ particular traits.

Differences in the nesting sites microhabitat characteristics of two syntopic species of Messor harvester ants in a phytosociological homogeneous grassland area

Physiological tolerance of species to temperature, moisture or chemical-physical properties of the soil could be important in determining the distribution and abundance of ant nests. In the present study we investigated the possible differences in the nesting site microhabitat characteristics of two syntopic species of harvester ants of the genus Messor living in a Mediterranean homogeneous grassland area belonging to a single phytosociological association known as “Vulpio ligusticae-Dasypyretum villosi”. We tested to see whether the activity of the colonies of the two species directly altered the microhabitat characteristics of the nesting sites. Microhabitat characteristics were assessed quantifying several abiotic factors (light, temperature, soil moisture, soil pH, nitrogen) by means of the Ellenberg Bioindication Model. The model represents a simple way of interpreting the vegetation pattern in terms of ecological factors from the perspective of the plants and can be considered an effective and promising approach to link animal and plant ecology. Our data showed significant differences in the nesting site microhabitat characteristics partially due to a different capacity of the two species to alter nesting site proprieties. Possible differences in the physiological tolerance of these species to moisture gradients could be crucial in determining the distribution and abundance of their nests.

Competition for foraging resources and coexistence of two syntopic species of Messor harvester ants in Mediterranean grassland

1. Diet composition of two syntopic species of Messor seed-harvester ants (M. wasmanni Krausse and M. minor André) was evaluated during different periods over the year (May, July, October), by analysing food type (plant parts and species) and food size (weight, length, width). Morphological traits of foragers (head width and femur length) considered important features promoting diet partitioning were measured. 2. We used two robust randomisation algorithms (RA2 and RA3), adopted in niche overlap studies, to check for random vs non random utilisation of resources at intra- and interspecific level for the different periods. 3. Analyses showed high levels of overlap in the diet of the two species and no evidence of interspecific competition during most of the activity season. In particular, there was an aggregated use of resources in summer, whilst niche partitioning and evidence of competition when resources decreased in autumn. Intraspecifically, no evidence of competition was found. 4. Results suggest two different mechanisms for minimising competition: when food resources are abundant (summer), ants collected the same plant species but selected different sizes; when food resource is scarce (autumn), ants foraged on different plants. 5. The importance of different factors (morphological, behavioural, ecological) possibly affecting competition and coexistence are discussed.

Mixed colonies and hybridisation of Messor harvester ant species (Hymenoptera: Formicidae)

The Mediterranean harvester ant species Messor minor, M. cf. wasmanni, and M. capitatus can co-occur in the same habitat. In Italian populations, we encountered colonies that contained workers from more than one species as identified via standard morphology, as well as colonies with workers that appeared to be morphologically intermediate between species. This unusual finding required further analysis. We analysed such colonies using microsatellites, mitochondrial DNA and refined morphometrics, and a simple inference key for the colony-level interpretation of data from the three sources combined. We infer that Messor minor and M. cf. wasmanni engage in bidirectional interspecific gene flow. Hybrids between these two species are inferred to produce fertile offspring, which would indicate that barriers to hybridisation do not exist or can be completely overcome. This is unexpected, given that they are non-sister species and broadly sympatric in nature. Our findings also indicate the possible occurrence of hybrid-hybrid crosses, a phenomenon rarely observed in ants. We cautiously interpret the data at hand as in support of the interspecific gene flow considerably shaping the genetic makeup of populations, raising the question about a potential adaptive value of this hybridisation. Messor capitatus mixes with hybrids of the other two species, but we found no indication of hybridisation involving this species. We discuss various hypotheses on the causations of colony mixing and hybridisation in the three Messor species at the proximate and ultimate level.