Sources of variation in the incidence of ant–aphid mutualism in boreal forests

• 1 The mutualism between wood ants of the Formica rufa group and aphids living in the canopy of trees is a widespread phenomenon in boreal forests, and it can affect tree growth. However, not all trees in the forest are involved in this interaction.
• 2 To assess the incidence of host trees involved in this ant–aphid mutualism and its spatial distribution in boreal forests, we inventoried sample plots with a radius of 10–15 m around wood ant mounds in 12 forest stands of two age classes (5–12‐year‐old sapling stands and 30–45‐year‐old pole stands) and two dominant tree species (Scots pine and silver birch) in Eastern Finland from 2007 to 2009.
• 3 The proportion of trees visited by ants out of all trees on the individual study plots were in the range 4–62%, and 1.5–39% of the trees on the plots were consistently visited by ants during all 3 years. The percentage of host trees increased with the ant mound base area on the plots. Trees visited by ants were larger and closer to the mound than trees not visited by ants. Within the group of visited trees, more ants were found on bigger trees and on trees close to the ant mounds.
• 4 Extrapolated from plot to stand level, we estimated that 0.5–6.6% of the trees were host trees in at least one of the three study years, and that only 0.01–2.3% of all the trees were consistently visited by ants during all 3 years. It is concluded that ant–aphid mutualism is a minor occurrence at the stand level.

     

Distribution of ant species and mounds (Formica) in different-aged managed spruce stands in eastern Finland

Mound‐building ants (Formica spp.), as key species, have large impacts on organisms and ecosystem functions in boreal Eurasian forests. The density, sizes and locations of ant mounds determine the magnitude and the spatial distribution of ant activities in forest ecosystems. Clear‐cutting can destroy wood ant colonies, and the species, abundance, dimensions and locations of ant mounds may change as forest stand structure changes with stand age. We compared ant species composition, ant mound numbers and dimensions, and the spatial distribution of mounds in Norway spruce [Picea abies (L.) Karst.] stands of different age (5, 30, 60 and 100 years) in eastern Finland. The mound density of Formica aquilonia Yarr. was greater in the two oldest stand age classes, while most mounds of Formica rufa L., Formica polyctena Först., Formica lugubris Zett., Formica exsecta Nyl. and Formica pressilabris Nyl. were found in the two youngest age classes. The mean volume, the volume per area and height/diameter ratio of F. aquilonia mounds increased with stand age. In the oldest stand age class, mounds were slightly smaller in well‐lit locations than in shade and near stand edges than further from the edges indicating that new mounds are established in well‐lit locations. Similarly, the longest slopes of the mounds faced south, indicating the importance of exposure to the sun. F. aquilonia mounds were concentrated near stand edges, and the spatial distribution of the mounds was aggregated in some stands. At the ecosystem level, the aggregation of ant mounds near stand edges may increase the edge productivity, as mounds concentrate resources to the edges and release nutrients after abandonment.     

Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis

• 1 Ants that protect food resources on plants may prey on (or deter) herbivores and thereby reduce damage. Red wood ants (of the Formica rufa group) are dominant ants in boreal forests of Eurasia and affect the local abundance of several herbivorous species.
• 2 The pine weevil Hylobius abietis (L.) is a herbivore that causes severe damage by feeding on the bark of coniferous seedlings within areas of forest regeneration.
• 3 We investigated whether ants can protect conifer seedlings from pine weevil feeding. In a manipulative experiment, ants were attracted to sugar baits attached to spruce seedlings and the damage caused by pine weevils was compared with control seedlings without ant‐baits.
• 4 The feeding‐scar area was approximately one‐third lower on the seedlings with ant‐baits compared with the controls. Besides red wood ants, Myrmica ants were also attracted in high numbers to the ant baits and the relative effects of these species are discussed.
• 5 The results obtained in the present study support the trophic cascade hypothesis (i.e. damage to herbivores is suppressed in the presence of predators). The decreased pine weevil feeding on the baited seedlings was probably a result of nonconsumptive interactions [i.e. the presence of (or harassment by) ants distracting pine weevils from feeding].
• 6 Understanding the role of ants may have important implications for future strategies aiming to control pine weevil damage. For example, maintaining suitable conditions for ants after harvesting stands may be an environmentally friendly but currently unexploited method of for decreasing weevil damage.

     

Stand type is more important than red wood ant abundance for the structure of ground‐dwelling arthropod assemblages in managed boreal forests

• 1 The relationships between red wood ants (Formica rufa group) and other ground‐dwelling arthropods were studied in young managed forests stands in Eastern Finland. The main objectives were: (i) to test the influence of stand type (dominant tree species; age: sapling versus pole stage) and numbers of red wood ants on the occurrence of other ground‐dwelling arthropods and (ii) to study the occurrence of red wood ants versus other arthropods on a distance gradient from ant mounds. We used pitfall traps set in 5–14‐year‐old sapling stands and 30–45‐year‐old pole‐stage stands of Scots pine (Pinus sylvestris L.) and birch (Betula spp.) forests.
• 2 Pitfall trap catches of red wood ants did not vary significantly between the forest stand types, although some groups of other arthropods showed clear responses to stand type (e.g. catches of other Formicinae and Gnaphosidae were higher in sapling stands than in pole‐stage stands). The number of red wood ants clearly explained less of the variation in assemblages of other ground‐dwelling arthropods than the forest stand type.
• 3 Red wood ant numbers decreased significantly with distance from the mounds, but the other ground‐dwelling arthropods were insensitive to this gradient or even showed a preference for proximity to ant mounds and high ant activity.
• 4 The results obtained in the present study suggest that wood ants do not have strong effects on several other ground‐dwelling arthropod groups in young managed forests other than in the immediate vicinity of their mounds.

     

Effects of ants on the foraging of birds in spruce trees

I experimentally excluded ants from randomly selected spruce trees Picea abies near colonies of the wood ant Formica aquilonia. Foraging activity of birds in these trees was then compared to the foraging activity of birds in neighboring spruce trees, where ants were allowed to continue foraging. Birds which foraged in the foliage showed the effects of competition with ants: they visited the trees without ants more frequently, and for longer periods. In addition, the insects and spiders that they utilized as food were more abundant in the foliage of trees without ants. Cone-foraging birds, however, which fed on seeds in cones at the tops of the trees, did not show a preference for trees without ants. The differences of tree usage between foliage-gleaning and coneforaging birds can be explained by alteration of the birds' food supply by wood ants: ants did not feed on seeds in cones, and so did not compete with cone-foraging birds. However, foraging wood ants did feed on arthropods living in the foliage, thus reducing the amount of food available to birds there.     

The effect of red wood ant abundance on feeding damage by the pine weevil Hylobius abietis

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The size–distance relationship in the wood ant Formica rufa

1. The size–distance relationship among honeydew‐collecting foragers of the red wood ant Formica rufa was investigated. Within the colony territory, the size (as measured by head width) and fresh weight of samples of foragers were determined for ants ascending and descending trees near, and farther from, the central nest mound. 2. The mean size of the ants was significantly higher at far trees than at near trees in six out of the seven colonies investigated, confirming the general presence of the size–distance relationship. 3. In three colonies, a load–distance relationship was also found. For a given head width, honeydew‐carrying ants descending far trees were significantly heavier than those descending near trees (i.e. they were carrying heavier loads from trees farther away from the central nest mound). 4. This is the first time that both load–distance and size–distance relationships have been reported in foraging workers from the same ant colony. 5. The combined effects of these characteristics suggest that colony foraging efficiency is enhanced by far trees being visited by the larger workers that then return with heavier loads of honeydew.     

Experimental investigation of information transmission in Formica pratensis (Hymenoptera, Formicidae) using “binary tree” maze

Organization of foraging and information transmission in the ant Formica pratensis were studied using the “binary tree” maze. The ants were shown to use distant homing. They were able to memorize and transmit up to four bits of information. The foraging mode was shown to depend on the colony size: an increase in the number of ants was accompanied by switching from solitary to group foraging. A relay mode of information transfer (an ant relays the information to several others, etc.) was revealed. The organization of work and information flows in the colonies of Formica polyctena and F. pratensis were found to be essentially different. The communicative systems in dominant ant species are variable and species-specific.     

Interference competition,not predation,explains the negative association between wood ants (Formica rufa) and abundance of ground beetles (Coleoptera: Carabidae)

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

Summary Ants 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.     

Genetic differentiation in sympatric wood ants, <Emphasis Type="Italic">Formica rufa</Emphasis> and <Emphasis Type="Italic">F. polyctena</Emphasis>

Summary Direct observations have suggested that the closely related wood ants Formica polyctena and F. rufa represent different social organizations, with high queen number in F. polyctena and a high frequency of monogynous nests in F. rufa. We examined social organization and genetic population structure in a setup where populations of the two species are sympatric and gene flow between the species is possible. Our aim was to compare social organization in the species, and study evolutionary relationships between them. The observed relatedness among colony workers suggested that the difference in the level of polygyny is quantitative rather than qualitative, with a higher queen number in F. polyctena. The observed difference in polygyny was not accompanied by a difference in spatial genetic differentiation which was weak in both species. The genetic distance between the species is consistent with limited interspecific gene flow. Identification of a few possible F. rufa migrants in F. polyctena populations suggests potential interspecific gene flow. Thus, reproductive isolation of the species may not be complete when they are sympatric.Received 14 March 2003; revised 10 October 2003; accepted 20 October 2003.     

A genetic model for disruptive selection on colony social organisation,reproduction, and ecotype distribution in wood ants inhabiting different woodland habitats

Summary Wood ants of theFormica rufa species complex are typically bound to woodland habitats and their eventual colonization of more open habitats depends on the presence of a sufficient number of woody plants. Computer simulations of the long-term development of large wood ant populations in woodland systems of differing structure were performed. A cellular, stochastic automation model simulated the alternatives of a compact, coherent woodland system and a fragmented, coarse-grained woodland system. The simulation of a compact woodland system gave evidence that disruptive selection alone may be sufficient to produce two distinct ectotypes, which are known asFormica rufa andFormica polyctena, even if there is a full fertility of the crosses. The simulation of a fragmented, coarse-grained woodland system has supported the view, that a high local frequency of nests with intermediate phenotypes might be explainable by a particular woodland pattern which favour mixed strategies. It was shown that queen dominance and intraspecific social parasitism are likely to be important factors in the dynamics of large wood ant populations.     

Influence of the wood ant, Formica polyctena, on soil nutrient and the spruce tree growth

The effect of Formica polyctena ant nests on the distribution of soil nutrients, soil pH and the growth of Norway spruce trees was studied in the southern part of the Czech Republic. Soil nutrient content (exchangeable P, N, K and pH) and growth of mature spruce trees were measured at four distances from the nearest ant hill (0–1, 3–5, 10–50 and >200 m). Trees at all distances were visited by ants, except for those >200 m from the nearest nest. Soil pH and of P, K and NO₃ concentrations were higher near ant nests (<1 m), and pH and K at distances of 3–5 m, when compared with distances of 10–50 and >200 m from the nests, where no significant differences in these variables were detected. In contrast, tree ring analyses (1974–2004) showed that trees >200 m from the ant nests grew significantly faster than trees at other distances, followed by trees within 1 m of the nests. No growth differences were found between the growth of trees at 3–5 and 10–50 m from ant nests. We postulate that nutrient and carbohydrate removal of honeydew collected by ant‐tended aphids are slowing growth of tree. However, trees may partly compensate for this depletion by having access to a larger supply of soil nutrients near ant nests.     

Die systematischen Grundlagen der Waldameisenvermehrung in Deutschland

Résumé Les différences morphologiques entre les deFormica rufa L. et celles deF. polyctena f?rst. sont décrites. Les deux espèces sont distinguées par le nombre des poils. En outre les nids de chaque espèce ont une odeur spéciale. Les nids deF. rufa L. sont en partie monogynes et en partie polygynes, ceux deF. polyctena f?rst., le plus souvent polygynes. Jusqu'à présent on a multiplié les deux espèces ensemble. En se rencontrant, elles se combattent. L'auteur propose de multiplier désormais seulementF. polyctena f?rst, parce que cette espèce est plus souvent polygyne queF. rufa.

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Riassunto Le differenze morfologiche delle diFormica rufa L. eF. polyctena f?rst. sono descritte brevemente. Si possono separare le due specie per mezzo della pelosità del torace e della testa. Oltre a ci? tutte le due hanno un odore specifico. I nidi diFormica rufa L. sono in parte monogini e in parte poligini, quelli diFormica polyctena f?rst. sempre poligini. Finora le due specie sono state trapiantate insieme. Esse si combattono quando si incontrano. L'Autore propone di usare per i trapianti solamente laFormica polyctena f?rst. poichè questa specie è in Germania più abbondante dellaFormica rufa poligina.

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Die Arbeiten wurden mit Unterstützung der Deutschen Forschungsgemeinschaft durchgeführt.     

Recognition of Individuals from Mixed Colony by Formica sanguinea and Formica polyctena Ants

It was examined whether Formica polyctena and F. sanguinea ants from a mixed colony elicit higher levels of aggression of conspecific ants in comparison to ants from homospecific colonies. Individuals were confronted in an experimental arena and their behavior was recorded. It was found that F. polyctena workers behaved more aggressively toward ants from a mixed colony. This pattern, however, was not confirmed in F. sanguinea. Moreover, both species clearly discriminated between conspecific and allospecific ants from a mixed colony. It seems that as a result of social interactions both species exchanged cuticular hydrocarbons, which caused their recognition labels to adjust to some extent. Results of the present study support the idea that that F. sanguinea is able to form mixed colonies in which species-specific recognition cues are probably still retained.     

Observer ants: Storers of foraging area information inFormica rufa L. (Formicidae,Hymenoptera)

Summary The structure of a foraging area in Formica s. str. is maintained by specific foraging roads and by experienced ants familiar with the foraging area, namely foragers and observer ants which used to be foragers. After all the foragers have been removed, observer ants of a F. rufa nest experimental column resume foraging, rapidly restoring the initial structure of the foraging area. For a nest population to lose completely information of the foraging area of a column, not only foragers but also observer ants must be removed and direct access from the nest to experimental road blocked.

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Les fourmis observatrices: des stockeuses d'informations sur l'aire d'affouragement chez Formica rufa L.

Resume La structure de l'aire d'affouragement chezFormica rufa est maintenue grâce à des pistes d'affouragement spécifiques et des Fourmis expérimentées qui sont familiarisées avec l'aire d'affouragement: des fourrageuses et des observatrices qui sont habituellement des fourrageuses. Lorsque toutes les fourrageuses sont enlevées, les observatrices d'un nid expérimental deFormica rufa reprennent l'affouragement et rétablissent rapidement la structure initiale de l'aire d'affouragement. Pour que la population d'un nid perde toute information sur l'aire d'affouragement d'une colonne, il faut que non seulement les fourrageuses mais aussi les observatrices soient enlevées et que l'accès du nid à la piste soit empêché.

     

Comportement desFormica devant un obstacle «infranchissable»

Summary Formica polyctena can send information along their path about an obstacle. A simple mechanism (abnormal workers agitation) help the ants to find another way behind the obstacle.Formica rufa can dig a tunnel under a large circular wall that blocks all accesses to the nest. They can also open a hole in the wall, passing building materials through. These materials can be carried also underground, to repair the central part of the nest whose access is blocked. That is another proof of the extreme plasticity of the building activity among red ants.     

The effect of a single colony of the red wood ant,Formica polyctena,on the spider fauna (Araneae) of a beech forest floor

Summary The predation on spiders in a forest ecosystem by a colony of red wood ants, Formica polyctena, was estimated using a barrier to isolate the colony. Of the ants' total prey, 4.6% were spiders. In order to estimate the effect of F. polyctena within their hunting area on the spider population, the spiders' population density was studied inside and outside the hunting area. Samples of the forest floor were taken, spider webs were counted and pitfall traps were used. No significant difference was found in density or composition of the spider fauna inside and outside the hunting area.     

Effect of host species,host nest density and nest size on the occurrence of the shining guest ant <Emphasis Type="Italic">Formicoxenus nitidulus</Emphasis> (Hymenoptera: Formicidae)

Understanding habitat requirements of species is important in conservation. As an obligate ant nest associate, the survival of the globally vulnerable shining guest ant, Formicoxenus nitidulus, is strictly tied to that of its hosts (mound building Formica ants). We investigated how host species, nest density, inter-nest distance and nest mound size relate to the occurrence of F. nitidulus. In total, 166 red wood ant nests were surveyed in SW Finland (120 Formica polyctena, 25 F. rufa, 14 F. aquilonia, 5 F. pratensis, and 2 F. lugubris). Overall, F. nitidulus was found in 60% of the nests. For the actual analysis, only F. polyctena and F. rufa nests were included due to the small number of other nests. F. nitidulus was more likely to be found among F. polyctena than F. rufa. Also, while inter-nest distance was not important, a high nest density, commonly found in polydomous (multi-nest) wood ant colonies, was beneficial for F. nitidulus. The guest ant was also more likely to be found in large host nests than small nests. Thus, our results show that the best habitat for the guest ant is a dense population of host nest mounds with a high proportion of large mounds. Conservation efforts should be directed at keeping the quality of the red wood ant habitats high to preserve their current populations and to increase colonization. This will not only benefit the guest ant, but also a plethora of other species, and help in maintaining the biodiversity of forests.     

Population studies on hill building wood-ants of the Formica rufa-group

Summary Population studies on hill building wood-ants of the Formica rufa-group.Total inventories of inhabited ant hills of the Formica rufa-group were carried out in a part of the Black Forest amounting to 1,640 ha (4,100 acres) near Freiburg im Breisgau on five occasions during the period 1966–1978. Nest densities of 12.7 to 19.1 per 100 ha were recorded. Mean annual nest mortality for the three most abundant species (F. rufa, F. polyctena, F. pratensis) was 21–33% (Table 2); it remained almost the same during all intervals between inventories for all age classes studied (Fig. 2). The mean annual nest natality varied between 31 and 51%. It was therefore concluded that changes in the numbers of ant hills (Fig. 1) resulted from nest natality rather than from the reasonably stable nest mortality. Nest size was assessed by height and diameter. The development of nest size showed a marked increase with age for F. rufa and F. polyctena reaching a maximum at 2–5 years. Mean life expectancy of small nests was markedly lower than that of larger nests (Fig. 3). Consequently in the large size classes the numbers of old nests were significantly greater than the numbers of newly founded nests (Fig. 4). Results are compared with those recorded in the literature for palearctic and nearctic species belonging to the Formica rufa-group.Supported by Ministerium für Ernährung, Landwirtschaft und Umwelt Baden-Württemberg (MELU) and Gesellschaft zur Förderung der Forst- und Holzwirtschaftlichen Forschung Freiburg