Riitta Savolainen

A competition hierarchy among boreal ants: impact on resource partitioning and community structure.

Differences in social organization and behaviour rank ant species into a competition hierarchy (starting with superior competitors): territorials (e.g., the Formica rufa -group red wood ants), encounterers (e.g. Camponotus, Lasius niger), and submissives (e.g., Formica fusca). Territorials and encounterers behave aggressively against individuals of alien colonies; these species are not expected to cooccur. Submissives behave recessively and may coexist with stronger species, but their forager numbers and nest densities should decrease. If such small-scale behavioural processes structure the ant community, predictable larger-scale nest-distribution pattern of the species is expected. We tested the expectations with bait experiments and nest mapping, and modelled the results by multiway contingency tables. Submissives showed complementary abundances with territorials in terms of forager numbers on the baits, and their nest densities within the territory increased toward its periphery. Pressure by territorial and encounter species on the baits caused the submissive species to shift from protein to carbohydrate. Territorials and encounterers had complementary occurrences on the baits. The nests of territorials were far apart, with only occasional nests of encounter species at the outskirts of the territory. In the late successional habitats of the boreal taiga biome superior territorial competitors, especially the polycalic red wood ant species, assume the role of organizing centers of ant species assemblages.

Niche differentiation of ant species within territories of the wood ant Formica polyctena

The question if dominant ant species affect habitat use of other ant species was studied around two mounds of the territorial Formica polyctena. We sampled foraging ants at 10, 30 and 60 m from the mounds in four vertical layers on rocky outcrop interspersed with small vegetation patches and in neighbouring forest. We tested expected interactions among ant species on the basis of a linear competition hierarchy concept consisting of three levels: territorial (top competitors), encounter (aggressive but nonterritorial) and submissive (nonaggressive) species. We focussed on resource partitioning by space (not by food) and shifts in use of vertical layers of the habitat in presence of the territorial species. F polyctena was present everywhere except in the litter. Its numbers decreased with distance from its mound, although its activity was substantially patchy within each distance zone. The encounter species occurred occasionally in places where F. polyctena was scarce. The submissive F. fusca, morphologically similar to the top dominant, did not respond by layer shifts; but its numbers decreased toward the mound of F polyctena. The submissive Myrmica shifted from surface of ground to litter and shrub layers at high densities of the dominant. Small colonies and short foraging distances of the submissive species allow coexistence within the territory in lowdensity patches of F polyctena. Our ant community consists of three functional guilds corresponding to taxonomic and morphological guilds: the larger above-ground Formicinae, the smaller and compacter Formicinae, and the small litter-inhabiting Myrmicinae. Interference competition is stronger and more effective among the Formicinae than among the Myrmicinae or between the subfamilies, but the top dominant affects all ant species of the community. Coexistence between the submissives and the top dominant is facilitated by niche differentiation and behavioural responses in the presence of the top dominant.

Sympatric speciation through intraspecific social parasitism

Sympatric speciation through intraspecific social parasitism has been proposed for the evolution of Hymenopteran workerless parasites. Such inquilines exploit related host taxa to produce their own sexual offspring. The relatedness of inquilines to their hosts has been generalized in Emerys rule, suggesting that social parasites are close or the closest relatives to their host species. If the closest relative of each parasite is its host, then multiple independent origins of the parasite species are implied even within a single genus, probably through sympatric speciation. To test the plausibility of sympatric speciation in inquilines, we conducted a mitochondrial DNA phylogenetic analysis in three inquiline–host pairs of Myrmica ant species. We show that congeneric inquilines have originated independently several times. We also show that two of the inqulines are more closely related to their hosts than to any other species. Our results suggest sympatric speciation of Myrmica inquilines. Sympatric speciation is probably facilitated by the social biology and ecology of Myrmica, with polygyny as a prerequisite for the evolution of intraspecific parasitism.

Sex Investment in a Social Insect: The Proximate Role of Food

In eusocial Hymenoptera, relatedness asymmetries lead to conflict between parents and offspring over sex investment; workers strive for a 3:1 female-biased ratio of sex allocation, and queens strive for a 1:1 ratio. Many studies support this genetic relatedness hypothesis, but the variation in allocation ratios in natural populations is great and remains mostly unexplained. In this paper, we examined whether food supply determines sex in- vestment of the ant Formica podzolica. We compare reproductive parameters of colonies and populations across habitats, and compare sex allocation of fed and unfed colonies. Nest density, worker size, sex ratio, and sex allocation were all greater along forest edges than in meadows, and these patterns were associated with natural food abundance. Furthermore, there was a strong tendency for individual colonies to produce either all-male or all-female sexuals, and worker size was greatest in colonies producing all females and smallest in those producing all males. Most important, sex investment was greatly affected by a sup- plemented diet, as the population investment ratio, R (i.e., ratio of males to males plus females), was female biased (0.36) for fed colonies and male biased (0.62) for unfed, control colonies. Finally, investment ratios were more male biased in polygynous than in monog- ynous colonies as predicted by a genetic relatedness hypothesis. These results demonstrate that food supply has an important proximate influence on sex investment, and may explain much of the natural variation in sex investment in populations of eusocial Hymenoptera.

Role of food supply in structuring a population of Formica ants

1. In Elk Island National Park, Alberta, Canada, we studied the role of food supply in structuring a population of the ant Formica podzolica by examining the association between natural food levels and abundance, distribution and alate production of colonies, and comparing alate production of fed and unfed colonies. 2. Nest densities and reproductive output of colonies were greatest along forest edges, intermediate in grazed meadows and lowest in overgrazed meadows. These patterns among habitats were associated positively with natural food levels. 3. Colonies were dispersed uniformly on plots along forest edges, but dispersed either uniformly or randomly on plots in grazed and overgrazed meadows

Colony success of the submissive ant Formica fusca within territories of the dominant Formica polyctena

Abstract. 1. Twenty‐three nests of the submissive ant Formica fusca L. were sampled in two adjacent territories of the dominant wood ant Formica polyctena Först. The nests were dug up at different distances from the wood‐ant mounds. Distance is assumed to be inversely related to the extent of disturbance of F. fusca by F. polyctena.

Interference by wood ant influences size selection and retrieval rate of prey by Formica fusca

SummaryDominance relations between the territorial wood ant Formica polyctena and the submissive F. fusca generate testable predictions on the mechanisms of their coexistence. Here I tested the influence of interference competition by the dominant F. polyctena on the foraging of F. fusca. In the presence of F. polyctena, the activity and the retrieval rate of items of F. fusca decreased significantly. When F. fusca were given a choice between small chironomids and flies a hundred times heavier they selected nearly always flies in the absence of wood ants; when disturbed F. fusca took proportionately more chironomids. In nature, irrespective of distance from the wood-ant mound the size distribution of potentially available food items was the same. F. fusca collected smaller items close to the wood-ant mound, where the density of interfering wood ants was high, than far from it. Also, F. polyctena carried larger protein items from the outskirts of its territory than from the center. The items of F. fusca were on average smaller than those of F. polyctena although the item-size overlap was substantial. When encountering a F. polyctena, those F. fusca workers carrying a fly always lost their booty to the dominant but always managed to bring the chironomids to the nest. F. fusca and F. polyctena were equally efficient in detecting single chironomids placed on the surface of ground, but the presence of either species decreased the discovery rate of the other. This implies mutual exploitation competition between the species for the locally most abundant protein resource. F. polyctena found single flies faster than F. fusca. In an earlier study I showed that close to the wood-ant mound the colony size and production of sexual offspring of F. fusca were reduced. I suggest that the suppressed colony success of F. fusca is attributable to the diminished size and decreased retrieval rate of prey items close to the woodant mound abounding with interfering wood-ant foragers.

Phylogeny, divergence-time estimation, biogeography and social parasite-host relationships of the Holarctic ant genus Myrmica (Hymenoptera: Formicidae).

We reconstructed a molecular phylogeny of the ant genus Myrmica, tested reciprocal monophyly of the Nearctic and Palearctic representatives, and inferred social parasite-host relationships for five workerless inquilines and four temporary parasites. We sequenced six gene fragments of 106 specimens (17 not identified to species), analysed the data with Bayesian phylogenetic inference and maximum likelihood, and estimated divergence times using penalized likelihood. Our well resolved phylogeny supported most morphologically defined species groups. The Nearctic and Palearctic species were not reciprocally monophyletic, which suggested repeated species interchange across the Beringian land bridge. Parasitism evolved several times in Myrmica. Three inquilines and one temporary parasite were closest relatives of their host, two inquiline species and one temporary parasite clustered basally to their host(s), and two temporary parasites more distantly. Myrmica probably diversified following drastic climatic cooling at the Eocene-Oligocene boundary ca. 34 Ma, the oldest species groups being rugosa and ritae in central and southeastern Asia. The oldest inquiline, Myrmica karavajevi, was estimated at 17 Ma, the youngest species M. hirsuta at 0.8 Ma, whereas the microgyne of M.rubra is an intraspecific inquiline.

OPERATIONAL SEX RATIOS AND MATING CONFLICT BETWEEN THE SEXES IN THE WATER STRIDER GERRIS LACUSTRIS

This is the first study to highlight the importance of individual operational sex ratio (OSR) experience in altering mating behavior and intersexual conflict over mating in insects. Reproductively active Gerris lacustris were held under highly male- or female-biased sex ratios and then tested in various treatment combinations with either male- or female-biased sex ratios. With our manipulations we disentangled the effect of experienced and ambient (mating environment) OSR, separately for females and males, which allowed the assessment of the differing contributions of male persistence and female reluctance in mating. Only male OSR experience affected the duration of mating: males from male-biased environments increased the length of copulation and postcopulatory guarding and the proportion of guarding of total duration of mating. Female-biased experience of both sexes resulted in short matings with practically no guarding, and males dismounted before females started struggling. Males from male-biased and females from female-biased environments mated a relatively long time, but 95% of these matings were terminated by the female. Females with female-biased experience resisted mating attempts more vigorously than females with male-biased experience, and males failed to mate more frequently with females from female-biased than with females from male-biased environments. Ambient OSR differences did not affect mating behavior. We conclude that theory on the impact of OSR on mating behavior should be expanded to include OSR experience, especially when males and females come from OSR environments biased toward their own sex.

Mechanisms of competition in a guild of formicine ants

Competition between colonies often explains patterns of nest spacing in social insects, but the way in which it produces regular spacing has rarely been studied experimentally. We conducted three field experiments in Elk Island National Park, Alberta, Canada, to examine potential spacing mechanisms across four species of Formica ants. Space preemption of founding queens and asymmetric interference between established colonies were key processes in all species but F. podzolica. The relative importance of these mechanisms was contingent upon the neighboring species, and understood in terms of their activity levels, aggressiveness and development of recruitment systems. Exploitative competition for food may account in part for abundance and distribution of F. podzolica. However, a removal experiment failed to produce significant differences in reproductive output and sex allocation between control and neighbor-removed colonies