Alkyl-Dimethylpyrazines in Mandibular Gland Secretions of Four Odontomachus Ant Species (Formicidae: Ponerinae).

Ponerine ants are known to contain mixtures of pyrazines in their mandibular glands. We analyzed the mandibular gland contents of four ponerine species (Odontomachus chelifer, O. erythrocephalus, O. ruginodis, and O. bauri) by gas chromatography coupled with mass spectrometry, and found that each species contains specific mixtures of trisubstituted alkylpyrazines among other volatiles. Attempts to identify alkylpyrazines solely by mass spectral interpretation is unrealistic because spectra of positional isomers are indistinguishable. To avoid misidentifications, we synthesized a large number of reference compounds and compared their mass spectral and gas chromatographic properties with those present in the Odontomachus species under investigation. Most of the compounds identified were 2-alkyl-3,5-dimethylpyrazines. Interestingly, when the third substituent was an isopentyl group, the two methyl groups were found to be located at the 2 and 5 ring positions. Using our data, we recognized several misidentifications in previous publications.

A global database of ant species abundances.

What forces structure ecological assemblages? A key limitation to general insights about assemblage structure is the availability of data that are collected at a small spatial grain (local assemblages) and a large spatial extent (global coverage). Here, we present published and unpublished data from 51 ,388 ant abundance and occurrence records of more than 2,693 species and 7,953 morphospecies from local assemblages collected at 4,212 locations around the world. Ants were selected because they are diverse and abundant globally, comprise a large fraction of animal biomass in most terrestrial communities, and are key contributors to a range of ecosystem functions. Data were collected between 1949 and 2014, and include, for each geo-referenced sampling site, both the identity of the ants collected and details of sampling design, habitat type, and degree of disturbance. The aim of compiling this data set was to provide comprehensive species abundance data in order to test relationships between assemblage structure and environmental and biogeographic factors. Data were collected using a variety of standardized methods, such as pitfall and Winkler traps, and will be valuable for studies investigating large-scale forces structuring local assemblages. Understanding such relationships is particularly critical under current rates of global change. We encourage authors holding additional data on systematically collected ant assemblages, especially those in dry and cold, and remote areas, to contact us and contribute their data to this growing data set.

Climate mediates the effects of disturbance on ant assemblage structure.

Many studies have focused on the impacts of climate change on biological assemblages, yet little is known about how climate interacts with other major anthropogenic influences on biodiversity, such as habitat disturbance. Using a unique global database of 1128 local ant assemblages, we examined whether climate mediates the effects of habitat disturbance on assemblage structure at a global scale. Species richness and evenness were associated positively with temperature, and negatively with disturbance. However, the interaction among temperature, precipitation and disturbance shaped species richness and evenness. The effect was manifested through a failure of species richness to increase substantially with temperature in transformed habitats at low precipitation. At low precipitation levels, evenness increased with temperature in undisturbed sites, peaked at medium temperatures in disturbed sites and remained low in transformed sites. In warmer climates with lower rainfall, the effects of increasing disturbance on species richness and evenness were akin to decreases in temperature of up to 9°C. Anthropogenic disturbance and ongoing climate change may interact in complicated ways to shape the structure of assemblages, with hot, arid environments likely to be at greatest risk.

Assembling an ant community: species functional traits reflect environmental filtering.

Species should only persist in local communities if they have functional traits that are compatible with habitat-specific environmental conditions. Consequently, pronounced regional environmental gradients should produce environmental filtering, or a trait-based spatial segregation of species. It is critical to quantify the links between species' functional traits and their environment in order to reveal the relative importance of this process to community assembly and promote understanding of the impacts of ongoing environmental changes. We investigated this relationship using epigaeic ants in an environmentally heterogeneous region of Florida. We found evidence for environmental filtering as environmental conditions such as groundcover, surface temperature, vapor pressure deficit, and plant diversity were strongly correlated with assemblage composition. Certain species traits appeared particularly important to persistence: (1) ants in environments with less groundcover have relatively longer legs but do not differ in size, (2) ants in hotter environments exhibit greater thermal tolerances, and (3) ants in hotter and drier environments do not exhibit greater desiccation resistance. These findings show surface complexity and temperature may interact with morphology and physiology to impact the spatial distribution of ants and underscore the importance of climate change. Climate warming is predicted to alter assemblage composition, competitive dynamics, and consequently impact ecosystem processes. We suggest environmental filters acting at regional scales, as shown here, act in tandem with more frequently studied local-scale competitive interactions to delimit ant community assemblages.

Fast food in ant communities: how competing species find resources.

An understanding of foraging behavior is crucial to understanding higher level community dynamics; in particular, there is a lack of information about how different species discover food resources. We examined the effect of forager number and forager discovery capacity on food discovery in two disparate temperate ant communities, located in Texas and Arizona. We defined forager discovery capacity as the per capita rate of resource discovery, or how quickly individual ants arrived at resources. In general, resources were discovered more quickly when more foragers were present; this was true both within communities, where species identity was ignored, as well as within species. This pattern suggests that resource discovery is a matter of random processes, with ants essentially bumping into resources at a rate mediated by their abundance. In contrast, species that were better discoverers, as defined by the proportion of resources discovered first, did not have higher numbers of mean foragers. Instead, both mean forager number and mean forager discovery capacity determined discovery success. The Texas species used both forager number and capacity, whereas the Arizona species used only forager capacity. There was a negative correlation between a species' prevalence in the environment and the discovery capacity of its foragers, suggesting that a given species cannot exploit both high numbers and high discovery capacity as a strategy. These results highlight that while forager number is crucial to determining time to discovery at the community level and within species, individual forager characteristics influence the outcome of exploitative competition in ant communities.

Identification of alkylpyrazines by gas chromatography mass spectrometry (GC-MS).

Pyrazines are an important group of natural products widely used as food additives and fragrants. Gas chromatography-mass spectrometry (GCMS) is the most widely applied analytical technique for characterization of alkylpyrazines. However, mass spectra of many positional isomers of alkylpyrazines are very similar. Consequently, an unambiguous identification of each positional isomer by spectral interpretation or database search protocols is practically unfeasible. In fact, there are many misidentifications in literature. To identify alkylpyrazines, chemists often resort to gas chromatographic retention indices (RIs). Although there are many compilations of retention indices of alkylpyrazines, these databases are often incomplete and the values reported are sometimes inconsistent. Herein, we present retention indices of fifty-six alkylpyrazines recorded on DB-1, ZB-5MS, DB-624, and ZB-WAXplus stationary phases, and compare them with those available in the NIST-2017 MS-RI database. Furthermore, we demonstrate how RI values can be used, together with mass spectral interpretations, to identify certain alkylpyrazines unambiguously.

Geographic variation in resource dominance-discovery in Brazilian ant communities.

A predictive framework for the ecology of species invasions requires that we learn what limits successful invaders in their native range. The red imported fire ant (Solenopsis invicta) is invasive in the United States, Puerto Rico, Australia, New Zealand, and China. Solenopsis invicta appears to be a superior competitor in its introduced range, where it can cause the local extirpation of native species, but little is known about its competitive ability in its native range in South America. Here we examine the competitive ability of S. invicta for food resources in three widely separated Brazilian ant communities. Each of these communities contains 20-40 ant species, 8-10 of which were common and frequently interacted with S. invicta. S. invicta at all three sites was attacked by several species-specific phorid parasitoids, and at one site, two other species were attacked by their own specialized parasitoids. We examined interactions in these local communities for evidence that trade-offs among ant species between resource dominance and resource discovery, and between resource dominance and parasitoid vulnerability facilitate local coexistence. The trade-off between resource dominance and resource discovery was strong and significant only at Santa Genebra, where parasitoids had no effect on the outcome of confrontations at resources. At Bonito, parasitoids significantly reduced the ability of S. invicta, which was the top-ranked behavioral dominant, from defending and usurping food resources from subordinate species. In the Pantanal, S. invicta ranked behind three other ant species in a linear hierarchy of behavioral dominance, and lost the majority of its interactions with a fourth more subordinate species, Paratrechina fulva, another invasive species. Parasitoids of S. invicta were uncommon in the Pantanal, and did not affect its low position in the hierarchy relative to the other two sites. Parasitoids, however, did affect the ability of Linepithema angulatum, the top-ranked behavioral dominant in this community, from defending and usurping resources from behavioral subordinates. These results indicate that both interspecific competition and trait-mediated indirect effects of phorid parasitoids affect the ecological success of the red imported fire ant in its native range, but that the relative importance of these factors varies geographically.

Linked indirect effects in ant-phorid interactions: impacts on ant assemblage structure.

Interaction modifications arise when the presence of one species alters the behavior of a second thereby altering that species' interactions with a third. Species-specific phorid parasitoids that attack ants at food resources can modify the competitive interactions between their host and competing ant species. This study examines whether interaction modifications created during interactions between the phorid fly parasitoid, Apocephalus sp. 8, its host, Pheidole diversipilosa, and ant species with which P. diversipilosa competes determine P. diversipilosa's foraging success. We also explore how varying resource size affects the ability of P. diversipilosa to capture resources and if this factor changes the nature or magnitude of the interaction modifications. We find that conflict between the host ant and competitor species at resources greatly improves the ability of the parasitoid to locate its host. In addition, the presence of a parasitoid at a food resource strongly reduces the ability of the host ant to capture that food resource when competing with other ant species. A wide variety of competitor species benefit from these linked interaction modifications and do so in accordance with their abundance in the assemblage. Finally, the size of the food resource in dispute affects the ability of the host ant to capture the resource, but parasitism and resource size affect resource turnover independently. These results are relevant to understanding the importance of interaction modifications in determining structure of ant assemblages.

Foraging behavior and morphology: seed selection in the harvester ant genus, Pogonomyrmex.

Optimally foraging animals can be behaviorally or morphologically adapted to reduce the energetic and time costs of foraging. We studied the foraging behavior and morphology of three seed harvester ant species, Pogonomyrmex barbatus, P. desertorum, and P. occidentalis, to determine the importance of behavioral strategies and morphological features associated with load carriage in reducing the costs of foraging. We found that none of five morphological features we measured had a significant impact on seed selection. Also, body size did not influence running speed, an important variable in time costs of foraging. Temperature had the largest effect on running speed in these species. Our results show that these species have foraging strategies which minimize the time costs of traveling with seeds. We also describe a pattern where the running speed in individual-foraging species is less affected by increasing seed size than in trunk-trail foragers, when temperature and body mass are held constant. These results support previous work which showed that time costs are most important in seed selection for Pogonomyrmex, and suggest that central place foraging theory may need to accommodate variation in foraging strategy to more accurately predict optimal seed size selection in harvester ants.

Effect of treefall gaps on the patchiness and species richness of Neotropical ant assemblages.

Natural formation of treefall gaps plays an integral role in the ecological and evolutionary dynamics of many tropical forests, affecting the spatiotemporal distribution of plants and the animals that interact with them. This study examines the impact of treefall gaps on the spatial and temporal patchiness of ant assemblages in a moist lowland forest in Panama. Using pitfall traps and honey baits, we compared ant assemblages in five 1 to 2-year-old treefall gaps (ca 100 m2) and five adjacent plots (ca 100 m2) in undisturbed forest understory at three different times of year (late wet season, late dry season, and early wet season). We found little evidence that ant assemblages respond dramatically to the formation of treefall gaps and the differences in habitat qualities they produce. Ant abundance, species richness, species composition, and rates of resource discovery did not differ between gaps and forest understory. However, we did find significant differences in numerical abundance related to forest stratum (ground vs vegetation) and resource type in pitfall traps (oil-cockroach vs honey), and significant differences in ant species richness and rates of resource discovery across seasons. While habitat effects by themselves were never statistically significant, habitat and seasonal differences in species richness interacted significantly to produce complex, season-dependent differences among gap and forest habitats. These results suggest that the formation of natural treefall gaps has less of an effect on Neotropical ant assemblages compared to other groups of organisms (e.g., plants, birds) or other causes of patchiness (e.g., ant mosaics, moisture availability, army ant predation). The results of our study also have important implications for the underlying causes of habitat differences in the distribution of ant-defended plants.

Habitat complexity modifies ant-parasitoid interactions: implications for community dynamics and the role of disturbance.

Species must balance effective competition with avoidance of mortality imposed by predators or parasites to coexist within a local ecological community. Attributes of the habitat in which species interact, such as structural complexity, have the potential to affect how species balance competition and mortality by providing refuge from predators or parasites. Disturbance events such as fire can drastically alter habitat complexity and may be important modifiers of species interactions in communities. This study investigates whether the presence of habitat complexity in the form of leaf litter can alter interactions between the behaviorally dominant host ants Pheidole diversipilosa and Pheidole bicarinata, their respective specialist dipteran parasitoids (Phoridae: Apocephalus sp. 8 and Apocephalus sp. 25) and a single species of ant competitor (Dorymyrmex insanus). We used a factorial design to manipulate competition (presence/absence of competitors), mortality risk (presence/absence of parasitoids) and habitat complexity (presence/absence of leaf litter). Parasitoid presence reduced soldier caste foraging, but refuge from habitat complexity allowed increased soldier foraging in comparison to treatments in which no refuge was available. Variation in soldier foraging behavior correlated strongly with foraging success, a proxy for colony fitness. Habitat complexity allowed both host species to balance competitive success with mortality avoidance. The effect of fire on habitat complexity was also studied, and demonstrated that the immediate negative impact of fire on habitat complexity can persist for multiple years. Our findings indicate that habitat complexity can increase dominant host competitive success even in the presence of parasitoids, which may have consequences for coexistence of subordinate competitors and community diversity in general.

When trade-offs interact: balance of terror enforces dominance discovery trade-off in a local ant assemblage.

1. Trade-offs underpin local species coexistence. Trade-offs between interference and exploitative competitive ability provie a mechanism for explaining species coexistence within guilds that exploit overlapping resources. 2. Omnivorous, leaf litter ants exploit a shared food base and occur in species-rich assemblages. In these assemblages, species that excel at usurping food items from other species are poor at finding food items first. In assemblages where some members are attacked by phorid fly parasitoids, host species face an additional trade-off between defending themselves against parasitic attack and maximizing their competitive abilities. Host species thus face two trade-offs that interact via the trait-mediated indirect interaction generated by phorid defence behaviour. 3. In this study we test for the existence of these trade-offs and evaluate the predictions of a model for how they interact in an assemblage of woodland ants in which two behaviourally dominant members are attacked by phorid fly parasitoids as they attempt to harvest food resources. 4. The major findings are that unparasitized species in the assemblage follow a dominance-discovery trade-off curve. When not subject to attack by phorid flies, host species violate that trade-off by finding resources too quickly for their level of behavioural dominance. In contrast, when attacked by their phorid parasitoids, the host species dominance drops such that they fall into the assemblage trade-off. 5. These results match the predictions of the balance of terror model, which derives the optimal host response to parasitism, indicating that the host species balance the competing fitness costs of reduced competitive dominance and loss of workers to parasitism. This result supports the view that understanding the structure of ecological communities requires incorporating the indirect effects created by trait plasticity.