Kari Vepsäläinen

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.

Ant assemblages in the taiga biome: testing the role of territorial wood ants

SummaryAnts were collected with sets of pitfall traps in four coniferous-forest habitats in southern Finland. A three-level competition hierarchy concept was used to generate predictions on ant community structure. The levels of the hierarchy, and the respective predictions, from top to bottom were: (1) The dominant territorial wood ants (Formica rufa-group species), expected to exclude each other. (2) The other aggressive species, likely to be excluded by the F. rufa-group. (3) The submissive species, non-aggressive and defending only their nest, and thus likely to coexist with the dominants but in reduced numbers. As expected, the species of the F. rufa-group excluded each other, and the species number of the other aggressive ants was significantly cut down in the presence of the F. rufa-group. The aggressive species F. sanguinea and Camponotus herculeanus showed complementary occurrences with the F. rufa-group, and Lasius niger reduced occurrences. The number of the submissive species was not significantly affected by the F. rufa-group. However, pairwise correlation coefficients were significantly more often negative than positive between presence of the F. rufa-group and average proportion of pitfalls per set with a submissive species, each analyzed in turn. The result indicates that the F. rufa-group also reduced the colony densities of the submissive species. We conclude that in the taiga biome territorial wood ants are, after adjusting for physical vicissitudes of the environment, the major structuring force of ant species assemblages.

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.

Why are there so many species? Spatio-temporal heterogeneity and northern bumblebee communities

It is proposed that (1) the different body sizes and proboscis lengths of bumblebee species lead to different optimal foraging patterns in different species. The morphological differences affect niche relations, depending on the pattern of the resource environment. Tongue morphology allows only a basic set of three to four species per community, differing by proboscis length and frequency in indulging in robbery. To explain considerably higher community species numbers (frequently between six and eleven species), (2) the effect of superabundant resources was examined. Data suggest that species-rich bumblebee communities exist locally by the Schlaraffenland effect. As few of the bumblebee communities are located at fields of superabundant resources, or in non-competitive environments, other coexistence mechanisms were sought for. (3) Intensified intraspecific competition relative to interspecific competition may theoretically allow coexistence, but supporting evidence is still lacking. (4) Spatio-temporal heterogeneity was advocated as the main coexistence mechanism: the energy available for colony growth changes seasonally and depends to a large extent on the location of the nest relative to dynamically changing resources. This may lead to temporal reversions in the competitive relations between the colonies and species. All three tests available suggested that spatio-temporal heterogeneity suffices in explaining the high species numbers in northern bumblebee communities. However, specific features of bumblebee foraging behaviour call for measurements of withinand between-colony competition.

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.

The Structure of Ant Assemblages in an Urban Area of Helsinki, Southern Finland

We collected ants in ten replicated habitat types of an urban island and described their assemblages using Non-metric Multidimensional Scaling and Multivariate Regression Trees. Lasius niger was the most abundant species, followed by Myrmica rubra, Formica fusca, L. flavus and L. platythorax; these species comprised 87% of all 1133 nests of the 16 species found. Ant assemblages changed gradually from open habitats to sites with closed tree canopy. Species most tolerant to urban pressure were L. niger, L. flavus and M. rugulosa, whereas forest-associated species were scarce or absent. Successful urban species had extensive (Palaearctic) or more limited (Euro-Siberian) distribution. Common and abundant habitat generalists were overrepresented relative to rural areas; these were also efficiently dispersing pioneer species with independent colony founding. Lack of suitable (micro)habitat apparently hampered colonization of many species. Competitively superior, territorial species were rare or absent, as were species dependent on dead wood, and many species depending on other ants. The indicated reorganization of interspecific competitive relationships may be due to selective impoverishment of the urban species pool. Comparison with other urban studies suggested that in ants, faunal homogenization has not taken place on a European scale.

Phylogeography of the ant Myrmica rubra and its inquiline social parasite

Widely distributed Palearctic insects are ideal to study phylogeographic patterns owing to their high potential to survive in many Pleistocene refugia and—after the glaciation—to recolonize vast, continuous areas. Nevertheless, such species have received little phylogeographic attention. Here, we investigated the Pleistocene refugia and subsequent postglacial colonization of the common, abundant, and widely distributed ant Myrmica rubra over most of its Palearctic area, using mitochondrial DNA (mtDNA). The western and eastern populations of M. rubra belonged predominantly to separate haplogroups, which formed a broad secondary contact zone in Central Europe. The distribution of genetic diversity and haplogroups implied that M. rubra survived the last glaciation in multiple refugia located over an extensive area from Iberia in the west to Siberia in the east, and colonized its present areas of distribution along several routes. The matrilineal genetic structure of M. rubra was probably formed during the last glaciation and subsequent postglacial expansion. Additionally, because M. rubra has two queen morphs, the obligately socially parasitic microgyne and its macrogyne host, we tested the suggested speciation of the parasite. Locally, the parasite and host usually belonged to the same haplogroup but differed in haplotype frequencies. This indicates that genetic differentiation between the morphs is a universal pattern and thus incipient, sympatric speciation of the parasite from its host is possible. If speciation is taking place, however, it is not yet visible as lineage sorting of the mtDNA between the morphs.

DNA barcoding as a heuristic tool for classifying undescribed Nearctic Myrmica ants (Hymenoptera: Formicidae)

The Palearctic species of the ant genus Myrmica are well studied. In contrast, the taxonomy of the Nearctic species is outdated, making identification impossible. We collected Myrmica samples in the Holarctic and investigated their diversity using mtDNA data. We analysed a barcode sequence of the Cytochrome Oxidase I gene for 57 Palearctic and 293 Nearctic Myrmica samples. We used sequences of known Palearctic species to search for Myrmica barcode patterns. All but one Palearctic species groups were recovered. The Nearctic diversity was much higher than known. We retrieved the punctiventris, crassirugis and incompleta groups, and established nine additional tentative species groups. Genetic distance analysis revealed a large overlap of intra‐ and inter‐specific distances in Palearctic species and species groups. We could not find a variation gap to separate Nearctic sequences into species with COI data only. Variation in scape morphology divided two genetic groups further. Scape morphology correlated with most molecular groups, except three specimens. Our results illustrate that barcoding, using only a limited amount of genetic information, cannot serve as a universal proxy for taxonomy and species demarcation. It should be considered a first step in understanding the taxonomic diversity of an unknown group of organisms.