Harvester Ant Nests 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.

Tree canopy cover and elevation affect the distribution of red harvester ant nests in a peri-urban setting.

With an increase in human population over the past 30 years, regional land use in south Texas has shifted from grassland and shrubland to a peri-urban matrix. Despite this shift from natural areas to more anthropogenically modified habitats, native red harvester ants (Pogonomyrmex barbatus) have maintained nest sites within parts of these matrices. To determine which habitat characteristics in a peri-urban landscape may play a role in red harvester ant nest site selection, we mapped the location of nests in 2020 and 2021. We then evaluated nest presence and absence relative to elevation, percentage of surrounding impervious surfaces, distance to roadways, and tree canopy cover (using NDVI). For a sub-sample of the study site, we also measured soil moisture and estimated the potential foraging area per colony with Voronoi tessellation. We found that nests were clustered together near high human-use areas such as athletic fields, lawns, sidewalks, and railroad tracks. Nests were more likely to be found in areas with higher elevation and lower tree canopy cover, with no impact from surrounding impervious surfaces or soil moisture. In fact, many nests were observed immediately adjacent to roadways and in paved parking lots. Red harvester ants are highly adept at nesting in disturbed, urbanized matrices, but still appear to be constrained by certain environmental factors like shading, potential flood risk (elevation), and access to food resources (foraging area).

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.

Harvester ant seed removal in an invaded sagebrush ecosystem: Implications for restoration.

A better understanding of seed movement in plant community dynamics is needed, especially in light of disturbance‐driven changes and investments into restoring degraded plant communities. A primary agent of change within the sagebrush‐steppe is wildfire and invasion by non‐native forbs and grasses, primarily cheatgrass (Bromus tectorum). Our objectives were to quantify seed removal and evaluate ecological factors influencing seed removal within degraded sagebrush‐steppe by granivorous Owyhee harvester ants (Pogonomyrmex salinus Olsen). In 2014, we sampled 76 harvester ant nests across 11 plots spanning a gradient of cheatgrass invasion (40%–91% cover) in southwestern Idaho, United States. We presented seeds from four plant species commonly used in postfire restoration at 1.5 and 3.0 m from each nest to quantify seed removal. We evaluated seed selection for presented species, monthly removal, and whether biotic and abiotic factors (e.g., distance to nearest nest, temperature) influenced seed removal. Our top model indicated seed removal was positively correlated with nest height, an indicator of colony size. Distance to seeds and cheatgrass canopy cover reduced seed removal, likely due to increased search and handling time. Harvester ants were selective, removing Indian ricegrass (Achnatherum hymenoides) more than any other species presented. We suspect this was due to ease of seed handling and low weight variability. Nest density influenced monthly seed removal, as we estimated monthly removal of 1,890 seeds for 0.25 ha plots with 1 nest and 29,850 seeds for plots with 15 nests. Applying monthly seed removal to historical restoration treatments across the western United States showed harvester ants can greatly reduce seed availability at degraded sagebrush sites; for instance, fourwing saltbush (Atriplex canescens) seeds could be removed in <2 months. Collectively, these results shed light on seed removal by harvester ants and emphasize their potential influence on postfire restoration within invaded sagebrush communities.

Mucilage‐bound sand reduces seed predation by ants but not by reducing apparency: a field test of 53 plant species

Seed mucilage, a coating on seeds or fruit that becomes slimy and sticky when wet, has evolved convergently many times across plants. One common consequence of having seed mucilage is that sand and dirt particles stick to wet seeds and remain tightly bound to the seed surface after the mucilage dries. Here, we test the hypothesis that a mucilage-bound sand coating protects the seed from seed predators; either as a physical barrier or by reducing apparency of the seed (i.e., camouflage). We experimentally manipulated the sand coating on seeds of 53 plant species of 13 families and assayed the defensive benefit of the sand coating in feeding "depots" near harvester ant nests in California's Central Valley. Consistent with a defensive function, sand coating reduced ant predation on seeds in 48 of the 53 species examined. To test whether this striking benefit was due to reduced apparency, we conducted an addition experiment using flax seeds in which we factorially manipulated the color of both the background substrate and the sand coating, creating visually apparent and unapparent seeds. Our results did not support the reduced apparency hypothesis; seeds coated in background-matched sand were removed at the same rate as seeds coated in unmatched sand. The defensive benefit of a sand coating was not well-predicted by seed mass, entrapped sand mass, or sand mass scaled by seed mass. Together, our results demonstrate that seed mucilage is a phylogenetically widespread and effective seed defensive trait and point to the physical barrier, not reduced apparency, as a mechanism.

Harvester ant nest distribution depends on soil disturbance regime

Nest densities of harvester ants (Messor barbarus) are high in rain-fed cereal fields in north-eastern Spain where the ants remove large quantities of seeds, contributing to reductions in weed populations. The distribution of harvester ant nests within a field can influence the effectiveness of ants as weed seed predators because areas with low ant nest density have lower weed seed removal rates. Tillage can disturb or even kill ant colonies and may be an important factor explaining the distribution of nests within fields. During the summers of 2011–2013, the number of nests in a 50 × 50 m area in 4 tilled and 3 no-till fields were counted. Tilled fields were disturbed twice a year, in November before cereal seeding and in July, after cereal harvest, whereas no-till fields had no soil disturbance. Ant nests were evenly spaced in no-till fields whereas nests were randomly distributed in tilled fields. Our results provide evidence that no-till in cereal fields promotes a more even distribution of M. barbarus nests, which should result in higher and more regular levels of weed seed predation across the field.

Transition of Vegetation States Positively Affects Harvester Ants in the Great Basin, United States

Invasions by non-native plants can alter ecosystems such that new ecological states are reached, but less is known about how these transitions influence animal populations. Sagebrush (Artemisia tridentata) ecosystems are experiencing state changes because of fire and invasion by exotic annual grasses. Our goal was to study the effects of these state changes on the Owyhee and western harvester ants (Pogonomyrmex salinus Olsen and P. occidentalis Cresson, respectively). We sampled 358 1-ha plots across the northern Great Basin, which captured unburned and burned conditions across 1−≥31 years postfire. Our results indicated an immediate and consistent change in vegetation states from shrubland to grassland between 1 and 31 years postfire. Harvester ant occupancy was unrelated to time since fire, whereas we observed a positive effect of fire on nest density. Similarly, we discovered that fire and invasion by exotic annuals were weak predictors of harvester ant occupancy but strong predictors of nest density. Occupancy of harvester ants was more likely in areas with finer-textured soils, low precipitation, abundant native forbs, and low shrub cover. Nest density was higher in arid locations that recently burned and exhibited abundant exotic annual and perennial (exotic and native) grasses. Finally, we discovered that burned areas that received postfire restoration had minimal influence on harvester ant occupancy or nest density compared with burned and untreated areas. These results suggest that fire-induced state changes from native shrublands to grasslands dominated by non-native grasses have a positive effect on density of harvester ants (but not occupancy), and that postfire restoration does not appear to positively or negatively affect harvester ants. Although wildfire and invasion by exotic annual grasses may negatively affect other species, harvester ants may indeed be one of the few winners among a myriad of losers linked to vegetation state changes within sagebrush ecosystems.

Ecosystem Engineering of Harvester Ants: Effects on Vegetation in a Sagebrush-Steppe Ecosystem

Abstract. Harvester ants are influential in many ecosystems because they distribute and consume seeds, remove vegetation, and redistribute soil particles and nutrients. Understanding the interaction between harvester ants and plant communities is important for management and restoration efforts, particularly in systems altered by fire and invasive species such as the sagebrush-steppe. Our objective was to evaluate how vegetation cover changed as a function of distance from Owyhee harvester ant (Pogonomyrmex salinus) nests within a sagebrush-steppe ecosystem. We sampled 105 harvester ant nests within southern Idaho, USA, that occurred in different habitats: annual grassland, perennial grassland, and native shrubland. The influence of Owyhee harvester ants on vegetation was larger at the edge of ant nests, but the relationship was inconsistent among plant species. Percent cover was positively associated with distance from harvester ant nests for plant species that were considered undesirable food sources an...

Effect of patch size on seed removal by harvester ants

SummaryThe harvester ant Messor barbarus can be responsible for high weed seed losses in dry land cereals in Spain. Because weeds occur in patches, harvester ants have to be able to find and exploit patches. However, seed patches can differ in size and may, therefore, differ in the probability of being discovered and exploited. Here, 90 patches varying in size from 0.25 to 9 m2 were created in three 50 × 50 m subareas in a cereal field. Oat seeds were sown as weed seed surrogates in the patches at 2000 seeds m−2. After 24 h, those remaining were collected and the exploitation rate (the percentage of seeds removed per patch discovered by ants) was estimated. Harvester ant nests and the location of the seed patches were georeferenced and used to estimate distances between them. The patch encounter rate (the proportion of patches discovered by the ants) decreased slightly, but significantly, with decreasing patch size, though not the exploitation rate, which was lowest in the smallest patches (78–94%) and highest in the largest (99–100%). Seed patches that were not found or partially exploited were mostly located in subareas with a lower ant nest density or a longer distance away from the nearest nest than seed patches that were fully exploited. The results of this study indicate that the interaction between the spatial distribution of ant nests and the patchy distribution of seeds can create opportunities for seeds to be subjected to lower levels of predation.

Sampling animal sign in heterogeneous environments: How much is enough?

Animal ecologists often use animal sign as a surrogate for direct observation of organisms, especially when species are secretive or difficult to observe. Spatial heterogeneity in arid environments makes it challenging to consistently detect and precisely characterize animal sign, which can bias estimates of animal abundance or habitat use. Piute ground squirrels (Urocitellus mollis) and Owyhee harvester ants (Pogonomyrmex salinus) live in arid environments and are fossorial, which can make them difficult to observe directly. Their relative abundance can be assessed using sign (i.e., burrows and nests). We implemented an over-sampling framework (i.e., recorded an excessive amount of information) with two observers to 1) identify a sampling intensity that balanced precision with our resource constraints, and 2) assess classification and detection of squirrel burrows and ant nests across vegetation conditions. We sampled 20 1-ha plots for ground squirrel burrows and ant nests using six 4 m×100 m belt transects. Analyses of precision and sampling effort indicated that three belt transects covering 1200 m2 per ha provided sufficient precision, while minimizing effort. Regardless of vegetation conditions, counts by two observers were strongly correlated for ground squirrel burrows (r = 0.99, P < 0.001, df = 18; slope = 0.92) and harvester ant nests (r = 0.99, P < 0.001, df = 18; slope = 1.01) indicating observer consistency and perhaps high detection probability. These findings illustrate an approach for evaluating sampling designs in many ecological contexts.

Utilization of harvester ant nest sites by Persian goitered gazelle in steppes of central Iran

Ants are among the most important elements in many ecosystems and known as famous ecosystem engineers. By changing physical and chemical properties of soil, ants may provide suitable habitats for other species. Based on previous observations, we hypothesized that Persian goitered gazelles (Gazella subgutturosa subgutturosa) exhibit a preference for utilizing sites close to seed harvester ant (Messor spp.) nests. We tested our hypothesis by (1) mapping the occurrence of harvester ant nests and aggregated gazelle pellet groups along 31 strip transects, (2) monitoring pellet group accumulation bimonthly at 56 pairs of permanent plots established on ant nests and at adjacent control sites for a complete year, and (3) comparing vegetation and soil parameters between ant nest sites used by gazelles and paired control plots without ant nests. Although the area of Messor spp. nest sites covered only about 0.29% of the sampled transects, 84% of the gazelle pellet group aggregation sites were positioned upon ant nests, suggesting that gazelles actively selected Messor spp. nest sites. Pair-wise comparisons between ant nest plots and paired control plots also confirmed higher use of ant nest sites by gazelles compared to sites without ant nests in all time periods. Percent soil organic matter, percent cover of gravel, and annual herb vegetation significantly differed between ant nest and paired control plots in all the vegetation communities. We suggest that the alterations brought about by harvester ants on soil and vegetation make these sites attractive to gazelles. Gazelle territoriality behaviour and use of ant nests as bedding sites may be the reasons for selection of ant nest sites by gazelles.

Florida Harvester Ant Nest Architecture, Nest Relocation and Soil Carbon Dioxide Gradients

Colonies of the Florida harvester ant, Pogonomyrmex badius, excavate species-typical subterranean nests up the 3 m deep with characteristic vertical distribution of chamber area/shape, spacing between levels and vertical arrangement of the ants by age and brood stage. Colonies excavate and occupy a new nest about once a year, and doing so requires that they have information about the depth below ground. Careful excavation and mapping of vacated and new nests revealed that there was no significant difference between the old and new nests in any measure of nest size, shape or arrangement. Colonies essentially built a replicate of the just-vacated nest (although details differed), and they did so in less than a week. The reason for nest relocation is not apparent. Tschinkel noted that the vertical distribution of chamber area, worker age and brood type was strongly correlated to the soil carbon dioxide gradient, and proposed that this gradient serves as a template for nest excavation and vertical distribution. To test this hypothesis, the carbon dioxide gradient of colonies that were just beginning to excavate a new nest was eliminated by boring 6 vent holes around the forming nest, allowing the soil CO2 to diffuse into the atmosphere and eliminating the gradient. Sadly, neither the nest architecture nor the vertical ant distribution of vented nests differed from either unvented control or from their own vacated nest. In a stronger test, workers excavated a new nest under a reversed carbon dioxide gradient (high concentration near the surface, low below). Even under these conditions, the new and old nests did not differ significantly, showing that the soil carbon dioxide gradient does not serve as a template for nest construction or vertical worker distribution. The possible importance of soil CO2 gradients for soil-dwelling animals is discussed.

The effects of harvester ant (Messor ebeninus Forel) nests on vegetation and soil properties in a desert dwarf shrub community in north-eastern Arabia

In desert ecosystems, harvester ants have been shown to be important granivores and seed dispersers. Because many desert plants exist as seeds in the ground for long periods, harvester ants may greatly influence plant population dynamics. In this study, we examine the effects of harvester ant nests on vegetation and soil properties in a desert ecosystem dominated by the dwarf shrub Rhanterium epapposum in Kuwait, north-eastern Arabia. Soil properties were greatly modified in the circular refuse zone around the nests of the harvester ants, with elevated levels of nutrients (except nitrogen) and organic matter. Plant species richness and productivity were also significantly enhanced in the refuse zone. The vast majority of species inhabiting this zone were desert annuals, but there is no evidence to suggest that the species involved have any distinct association with ant nests. Harvester ants therefore contribute substantially to small-scale spatial heterogeneity in this aridland plant community. It is also suggested that the significance of ant nests in providing favourable sites for plant growth is enhanced in drier years, when many species in off-nest sites may die prematurely due to drought. Furthermore, because ants occasionally abandon their nests and create new colonies, the addition of nutrient-rich patches to the landscape over time could represent an important mechanism for maintaining fertility of desert soils, possibly with long-term implications for plant biodiversity. Harvester ants can therefore be regarded as a key ecosystem engineer in this relatively undisturbed desert ecosystem of Kuwait.

Busy Builders (review)

Reviewed by: Busy Builders Elizabeth Bush Munro, Roxie . Busy Builders; written and illus. by Roxie Munro. Cavendish, 2012. 40p. Trade ed. ISBN 978-0-7614-6105-0 $17.99 E-book ed. ISBN 978-0-7614-6106-7 $17.99 Ad Gr. 2-4. Eight insects and one arachnid are showcased here for their interesting approaches to domestic architecture. The honeybee community in its waxen hive will be most familiar, but the red harvester ants in their underground burrows, organ-pipe mud daubers in their tubes, pine processionary caterpillars in their solar-heated silk nests, and paper hornet hives of wood slivers and spit are equally intriguing. Munro's chosen format sends a perplexing mixed message regarding the intended audience, though. Each creature is introduced in a detailed, colored-ink image that extends across an oversized double spread with a simple header: "This is a Honeybee. Where does it live?" The subsequent spread offers a close-up (also oversized) that seems to carry over the visual appeal to a preschool/primary-aged crowd. The paragraph of accompanying text, however, is quite a bit more sophisticated in both concepts and vocabulary, evident in text such as "Patrol ants are stationed at the entrance, while other ants go out to forage for food. One harvester ant nest reportedly extended fifteen feet (4.6 m) below the ground and contained 436 chambers." Newly independent readers who have already developed an interest in critters that buzz, crawl, and creep may have sufficient background to handle the terminology with minimal assistance, but younger listeners may need definitions and explanations along the way from an adult reader. A brief glossary and an even briefer list of further resources are included. Copyright © 2012 The Board of Trustees of the University of Illinois

Spatial Patterns of Grass Seedling Recruitment Imply Predation and Facilitation by Harvester Ants

Extended drought on the Edwards Plateau, TX, causes high mortality in grass species' populations and their replacement by recruitment from the seeds of survivors. Harvester ant nests may act as refugia for perennial plants during drought, as evidenced by a high survivorship of plants directly adjacent to nests. Because grasses tend to disperse seeds short distances and drought eliminated seed banks, we hypothesized that the spatial pattern of grass seedlings and adults would be clumped at small scales around these refugia. In contrast, nests would be regular at small to moderate scales. We studied the spatial patterns of seedlings, adults, and ant nest locations using a modified Ripley's K-function technique for transect data. Point data were collected along a 1-km transect. Spatially regular patterns were found for nests from 1.0- to 22.0-m scales. We found that two species' seedlings and adults were significantly clumped with ant nests, and two species had significantly regular spatial patterns with nests. Surprisingly, patterns of clumped seedlings occurred up to 30-m scales with nests. Nests appear to affect seedling establishment at multiple scales. These results suggest that nests may facilitate colonization for some grass populations, but regular spatial patterns in other species imply that forager seed harvest may inhibit establishment.

Effects of tillage and irrigation in cereal fields on weed seed removal by seed predators

Summary 1. Agricultural intensification can cause a huge increase in productivity. However, associated costs in terms of reduced, self-regulation and increased reliance on external inputs for the control of pests, diseases and weeds are seldom taken into account or acknowledged. A pro-active approach in which ecosystems services are documented and potential effects of changes in agricultural practices evaluated may lead to more informed decisions prior to implementation. 2. We investigated the effects of management of cereal production in a semi-arid region on weed seed mortality caused by predators. Seed losses have a greater impact on weed population size than any other life cycle process and should therefore be of significance for natural weed control. We hypothesized that the conversion from rain-fed to irrigated production should lead to reduced and the adoption of no-till techniques to increased seed predation. 3. Seed removal and seed predator populations were monitored in irrigated ( N = 3) and rain-fed cereal fields ( N = 6) and field margins. Of the dryland fields half was conventionally tilled and the other half no-till. Seed removal (g g − 1 2-days − 1 ) was followed from April 2007 until June 2008, using Petri-dishes and exclosure cages. Populations of harvester ants were estimated by direct nest counts; rodent populations by Sherman live traps. 4. Seed removal in dryland cereals, mainly by harvester ants Messor barbarus was high from mid April to mid October, and should cause a strong weed suppressive effect. Seed removal in irrigated cereals, mainly by granivorous rodents Mus spretus , was low. 5. Seed removal was higher in no-till than in conventional fields and corresponded to differences in harvester ant nest densities. 6. Synthesis and applications . Our results show that tillage and irrigation in a semi-arid cereal production system results in a reduction and total annihilation of granivorous harvester ants, respectively. The concurrent decline in weed seed mortality could lead to increased herbicide use and dependency. In particular, in areas where economic margins are small or the environmental costs of tillage and irrigation high, the increased costs of chemical weed control may exceed the benefits. Here, preserving biodiversity to enhance natural weed control is a viable alternative to agricultural intensification.

Effects of harvester ant (Messor spp.) activity on soil properties and microbial communities in a Negev Desert ecosystem

Harvester ants (Messor spp.) function as an essential link between aboveground resources and below-ground biota such as the microbial community. We examined changes in soil microbial biomass and functional diversity resulting from harvester ant (Messor spp.) activity in the Negev Desert, Israel. Abiotic and biotic soil parameters were recorded during two seasons—wet and dry—also representing food availability periods for the ants (low and high seed availability, respectively). Soil samples were collected monthly from the 0- to 10- and 10- to 20-cm soil layers: (1) near the nest entrance, (2) under chaff piles, and (3) at a 2-m radius from the nest entrance (control). Harvester ant activity increased the percentage of organic matter, total soluble nitrogen, and microbial activity in nest-modified soils in comparison to the control soils. Higher CO2 evolution was recorded in the low-seed season in ant nest soils than in the control soils. During the high-seed season, higher carbon dioxide evolution was recorded only at the nest entrance locations. There were no differences in microbial biomass between the low- and high-seed seasons, but highest microbial biomass was found under chaff in low-seed season and in nest soils in high-seed season. Microbial functional diversity was higher in nest-modified soils than in the control soils. This study suggests that the effect of harvester ant nests on soil fertility is due to increased microbial biomass and microbial activity in ant nest-modified soils.

Harvester Ant Nests Improve Recovery Performance of Drought Impacted Vegetation in Grazing Regimes of Semiarid Savanna, Texas

Vegetation composition is often dictated by grazing intensity in semiarid savannas; recovery following drought may depend on pre drought species composition. Nests of the red harvester ant, Pogonomyrmex barbatus, affect the dynamics, composition and recovery of post drought communities due to their larger size, greater seed production and higher perennial grass richness. We hypothesized that vegetation at ant nests would survive drought and recover faster than vegetation in the surrounding grasslands, but that individual and population recovery would depend on plant species composition, which, in turn, would be influenced by grazing intensity. We assessed nest influence on density, cover, number of inflorescences and dynamics of grass and forb species. Disk margins (area encircling the ant nest) were compared with grassland locations in unreplicated heavy, light and ungrazed treatments before, immediately after and one year after a severe drought. Significantly greater aerial and basal cover of grasses was found on disk margins compared to grasslands in each treatment. Grass cover and number of inflorescences increased faster on disk margins compared to grasslands. Fastest grass growth was seen on margins in the ungrazed treatment. There was greater diversity in ungrazed treatments of grazing intolerant mid-grasses compared to the grazed treatments, suggesting that mid-grasses may persist belowground, leading to faster productivity in the ungrazed treatment. Grass densities were generally higher and increased faster in grasslands resulting in smaller grasses compared to the large, more robust grasses on disk margins. Forbs showed significantly lower abundance and cover on margins compared to grasslands. These observations suggest that red harvester ant nests may serve as drought refugia for grass survival and a seed source for recovering grass populations after drought in semiarid savannas.

Native harvester ants threatened with widespread displacement exert localized effects on serpentine grassland plant community composition

Seed‐harvesting ants can influence the abundance and distribution of plant species through both the selective harvesting of seeds and the construction of nutrient‐rich nest mounds, but the relative contributions of these two mechanisms have not been addressed by previous studies. Furthermore, the impact of ant seed harvesting in California serpentine grasslands remains unresolved because of divergent results from several previous experiments. This study investigates the influence of harvester ants on serpentine grassland plant species composition by examining two potential signatures of seed harvesting ants on plant community composition: species composition on versus off ant nest mounds, and species abundance as a function of distance from nest mounds. Of the 28 plant species identified in this study, 22 exhibited spatial patterns consistent with effects of seed harvesting, nest construction, or both. Although most species showed significant gradients in abundance with distance from a nest, there were no clear relationships between plant species distributions and previously reported harvester ant seed foraging patterns. Harvester ant nest mounds supported plant communities that were distinct from the surrounding serpentine grassland, with notably higher densities of legumes and invasive annual grasses. Comparison of our results with those of previous studies indicates that the patterns we observed are generally consistent over time, but affect more species and a larger fraction of the grassland than previously reported. Unaffected areas of the grassland seem likely to serve as important refuges for some plant species.

Effects of a common harvester ant ( Messor andrei) on richness and abundance of soil biota

Soil biota and chemistry were compared between harvester ant ( Messor andrei) nests and adjacent, non-ant soil from a semi-arid, serpentine grassland. The major soil groups (bacteria, fungi, nematodes, miscellaneous eukaryotes, and microarthropods) were quantified. All soil groups examined were more abundant and diverse in ant nests than in non-ant soil, and all soil nutrients were similarly enriched inside nests. A principal component analysis indicated that the enrichment of bacteria and fungi inside ant nests explained the majority of variation in the other response variables. Thus, it appears that M. andrei performs some activity and/or enriches one or more nutrients (e.g. phosphorous), which heavily impacts the base of the soil foodweb (bacteria and fungi). This suggests that these ant nests exert significant effects on the resident soil biota via bottom-up processes.