Filippo Frizzi

Competition trade‐offs in the organisation of a Mediterranean ant assemblage

Abstract 1. The organisation of an ant assemblage inhabiting an olive orchard in central Italy was analysed and patterns of dominance among ant species were described in order to assess (i) the relationship between thermal dependency and degree of behavioural dominance, and (ii) the relationship between dominance and discovery ability.

Spatial patterns of the ant Crematogaster scutellaris in a model ecosystem

1. The spatial arrangement of individuals and populations may have deep influences on all the biotic interactions within a community.

The Rules of Aggression: How Genetic, Chemical and Spatial Factors Affect Intercolony Fights in a Dominant Species, the Mediterranean Acrobat Ant Crematogaster scutellaris

Nest-mate recognition plays a key role in the biology of ants. Although individuals coming from a foreign nest are, in most cases, promptly rejected, the degree of aggressiveness towards non nest-mates may be highly variable among species and relies on genetic, chemical and environmental factors. We analyzed intraspecific relationships among neighboring colonies of the dominant Mediterranean acrobat ant Crematogaster scutellaris integrating genetic, chemical and behavioral analyses. Colony structure, parental relationships between nests, cuticular hydrocarbons profiles (CHCs) and aggressive behavior against non nest-mates were studied in 34 nests located in olive tree trunks. Bayesian clustering analysis of allelic variation at nine species-specific microsatellite DNA markers pooled nests into 14 distinct clusters, each representing a single colony, confirming a polydomous arrangement of nests in this species. A marked genetic separation among colonies was also detected, probably due to long distance dispersion of queens and males during nuptial flights. CHCs profiles varied significantly among colonies and between nests of the same colony. No relationship between CHCs profiles and genetic distances was detected. The level of aggressiveness between colonies was inversely related to chemical and spatial distance, suggesting a ‘nasty neighbor’ effect. Our findings also suggest that CHCs profiles in C. scutellaris may be linked to external environmental factors rather than genetic relationships.

Changes in ground-foraging ant assemblages along a disturbance gradient in a tropical agricultural landscape

Changes in ground-foraging ant assemblages occurring along an agricultural intensification gradient were analysed in Nicaragua (Central America). Five habitat types were compared: secondary forest fragments, organic shade coffee and cocoa plantations, corn fields and active pastures. Ants were captured by tuna baiting following standardised protocols. Species richness and diversity decreased along the gradient from forest to the pasture, and each of the agricultural systems (cocoa, coffee and corn plantations) significantly differed in diversity. Species composition also differed along the disturbance gradient, as shown by multivariate analysis. Forest and pasture were at the extremes of the gradient and shared no common species. The ant assemblages associated with coffee plots, although poorer in species, were more similar to those of the forest than to those of the cocoa plantations. The observed trends confirm that shaded agro-forestry plantations can help to maintain high biodiversity levels in tropical landscapes.

Characterization of polymorphic microsatellite loci in the ant Crematogaster scutellaris

Nine polymorphic microsatellite loci were isolated and characterized in the ant Crematogaster scutellaris from Italy. Allelic variation was tested for individuals collected from 26 nests in an olive grove in Tuscany, where ant–homopteran mutualistic interactions are under study to control damages to the olive crop. Allelic diversity was 12.4 and the number of alleles per locus ranged from 8 to 17. Mean observed and expected heterozygosity were 0.826 (range, 0.760–0.923) and 0.829 (range, 0.736–0.869), respectively. This novel set of loci will be used to assess population structure, dispersal strategies and mating system of C. scutellaris, and in particular to determine genetic relationships among individuals engaged in intraspecific conflicts.

Modeling warfare in social animals: a "chemical" approach.

We present here a general method for modelling the dynamics of battles among social animals. The proposed method exploits the procedures widely used to model chemical reactions, but still uncommon in behavioural studies. We applied this methodology to the interpretation of experimental observations of battles between two species of ants (Lasius neglectus and Lasius paralienus), but this scheme may have a wider applicability and can be extended to other species as well. We performed two types of experiment labelled as interaction and mortality. The interaction experiments are designed to obtain information on the combat dynamics and lasted one hour. The mortality ones provide information on the casualty rates of the two species and lasted five hours. We modelled the interactions among ants using a chemical model which considers the single ant individuals and fighting groups analogously to atoms and molecules. The mean-field behaviour of the model is described by a set of non-linear differential equations. We also performed stochastic simulations of the corresponding agent-based model by means of the Gillespie event-driven integration scheme. By fitting the stochastic trajectories with the deterministic model, we obtained the probability distribution of the reaction parameters. The main result that we obtained is a dominance phase diagram, that gives the average trajectory of a generic battle, for an arbitrary number of opponents. This phase diagram was validated with some extra experiments. With respect to other war models (e.g., Lanchesters ones), our chemical model considers all phases of the battle and not only casualties. This allows a more detailed description of the battle (with a larger number of parameters), allowing the development of more sophisticated models (e.g., spatial ones), with the goal of distinguishing collective effects from the strategic ones.

Combined effects of temperature and interspecific competition on the mortality of the invasive garden ant, Lasius neglectus: A laboratory study

The invasive garden ant, Lasius neglectus, is a dominant species due to its capacity to form large supercolonies. This species was assumed to possess a wide thermal niche since it is able to adapt to cold climates, which is a factor that boosted its rapid expansion from south to many central-northern European Countries. However, the effect of variations in environmental temperatures on its competitive ability against other species has still not been investigated. In this paper, we analyzed the change in survival ability of Lasius neglectus during encounters with two Mediterranean dominant ants (Crematogaster scutellaris and Tapinoma nigerrimum) at four different temperatures (15, 20, 25 and 30°C). Firstly, control tests were performed to provide the baseline survival ability of the three species at different temperatures. Secondly, competition tests were carried out at the same temperatures. Lasius neglectus survival was negatively affected by high temperature (30°C) in control tests, and this impairment was much more pronounced in competition tests. On the contrary, the two opponent species were only marginally affected by temperatures in control tests. Crematogaster scutellaris was a better competitor than L. neglectus, particularly at high temperatures. Tapinoma nigerrimum was a weaker competitor and was always outcompeted by L. neglectus, particularly at low temperatures. This result could suggest that L. neglectus is at a disadvantage during interspecific encounters when temperatures are high and that the predicted future increase in environmental temperatures may potentially enhance this handicap.

Effect of water and resource availability on ant feeding preferences: a field experiment on the Mediterranean ant Crematogaster scutellaris

Ants’ feeding habits are affected by individual and collective needs, which may vary among seasons. In this study, we tested the food preferences of the Mediterranean ant Crematogaster scutellaris toward sucrose and amino acid solutions at increasing concentrations, during spring and summer, by analyzing individual choices and mass recruitment in the field. Given that water may be limiting in summer, we also analyzed the role of water as a resource itself. Finally, to investigate how previous short-term availability of resources affects feeding choices, we over-supplied colonies with a continuous flux of amino acids, sucrose and water, before conducting individual tests. As for sucrose, only the most concentrated solutions were largely accepted during the spring, whereas all the solutions were equally taken during the summer. On the contrary, the average acceptance of all amino acid solutions was high only during summer and low in spring. Similar results emerged for recruitments on both nutrients. Amino acid supplementation had no effect on resource acceptance, whereas both sucrose and water supplementation affected the acceptance of all other resources. This study provides hints on the factors affecting seasonal variations in the uptake of carbohydrates and amino acids, and more importantly, clearly shows how food choice is affected by water availability, a factor frequently overlooked in the study of nutritional ecology of ants.

Molecular identification of imported red wood ant populations in the Campigna Biogenetic Nature Reserve (Foreste Casentinesi National Park, Italy)

During the second half of the XIX century, several species of red wood ants (Formica rufa group) were used as biological control agents and introduced to the Italian Apennine mountains where they were formerly absent. Among these, Formica lugubris was the most widely introduced alien species. In 1996, however, a taxonomic revision defined two sibling species, F. lugubris and F. paralugubris, each with distinct ecological and behavioural characteristics that differently affect their spatial distribution. The first one can be both monodomous and polydomous with a low number of interlinked nests, and it is obliged to mate by nuptial flight. Conversely, F. paralugubris is usually unicolonial, with a considerable portion of intranest mates. Discrimination between the two species using morphological characters is possible but requires considerable taxonomic expertise. Restriction analysis, whereby presence/absence of a BamHI site in the mitochondrial DNA COI gene results in different restriction profiles, provides an alternative efficient method to unambiguously distinguish F. lugubris from F. paralugubris. We applied this method to identify ant species introduced to several locations in the Campigna Forest of the Foreste Casentinesi, Monte Falterona and Campigna National Park, one of the Italian locations mostly affected by alien ant introductions. This area has considerable ecological relevance. Given the potential impact of ants on native arthropod communities, it is of the most importance to identify which species has been introduced. Contrary to previous assumptions, results of our analysis clearly show that only F. paralugubris is present in the National park.

Chronicle of an impact foretold: the fate and effect of the introduced Formica paralugubris ant

Starting in 1958, red wood ants (Formica rufa group) from the Alps were transplanted to several Apennine forests along the Italian peninsula to be employed as biological control agents for tree insect pests. In the Campigna Biogenetic Nature Reserve, central Italy, hundreds of mounds of the dominant Formica paralugubris were repeatedly introduced, creating several populations that still survive today. In this study, we analyzed the temporal dynamics and the ecological impact of five of these populations. Their present state was assessed by censusing the total number of nest mounds and their volumes, while past changes were reconstructed from literature records. We also evaluated the impact of this species on autochthonous arthropod communities by comparing impacted and non-impacted areas and performing predation experiments. The density of nests and their volume remained stable or declined for a few years after transplant, and then they began to grow steadily. Local arthropods were severely affected, since almost all collected groups were significantly less represented in impacted than in non-impacted sites. Additionally, experiments with live bait demonstrated that potential prey have a significantly greater probability of being consumed in the areas occupied by F. paralugubris. These results prompt a thorough assessment of the fate of the introduced red wood ant populations, since their role as biological control agents has to be traded against the ecological impact on native arthropod communities. This is particularly relevant for highly biodiverse areas, such as the Campigna forest, that are home of several invertebrate species with conservation interest.