Wood ants are wood ants: deforestation causes population declines in the polydomous wood ant Formica aquilonia

Abstract.  1. One of the main themes in ecology is adaptation for survival in different habitats and the potential of the environment to regulate populations.
2. The effects of clear-cutting on nest-abandonment rate and local population sizes in the polydomous wood ant Formica aquilonia was studied, using uncut forest stands as controls.
3. The nest-abandonment rate was clearly higher in clear-cuts than in forest interiors. In clear-cuts, 39% of pre-deforestation nests and 73% of new bud-nests were abandoned 4–5 years after deforestation, whereas in forest interiors fewer than 2% of nests were abandoned at the same time period. Local population size decreased 30% in clear-cuts, but fewer than 2% in forest interiors.
4. The results demonstrate that despite modern logging practices in which mechanical harming of nest mounds is reduced, nest mounds are abandoned at high rate, and despite frequent establishment of new bud-nests, populations start to decline.
5. The likely reason for the high nest-abandonment rate in clear-cuts is a combination of changed abiotic conditions, resource limitation, and disturbed reproduction.
6. Species that are sensitive to changes in the size of habitat patch, such as F. aquilonia , likely are harmed by logging, even employing biodiversity oriented management practices. Hence there is a need for conservation actions that are based on the size of protection areas.     

Community structure in north temperate ants: temporal and spatial variation

Summary Ant communities in Vermont and New York woods were sampled in four time periods to determine species composition, relative abundances, and nest locations in space. The Vermont community was richer, containing more species and higher nest densities than New York. Both communities followed the geometric distribution of species abundances, suggesting that a single resource was mediating competition. The resource most clearly implicated was suitable nest sites, principally pre-formed plant cavities. Nonrandom species associations, underdispersion in every season, and the occurrence of incipient nests overwintering aboveground all implicated shortage of such cavities. Furthermore, microhabitat differences which produce suitable nest sites occur over a very small scale in these communities.     

Colony structure and spatial distribution of food resources in the polydomous meat ant <Emphasis Type="Italic">Iridomyrmex purpureus</Emphasis>

Polydomous social insects may reduce the costs of foraging by the strategic distribution of nests throughout their territory or home-range. This efficiency may most likely be achieved if the resources are relatively stable in place and time, and the colonies and nests are distributed in response to the location of the resources. However, no study has investigated how the distribution of food sources influences the spatial patterns of nests within polydomous colonies under natural conditions. Our two year study of 140 colonies of the Australian ant Iridomyrmex purpureus revealed that the decentralization of nests within colonies is associated with the distribution of trees containing honey-dew producing hemiptera. We show there is a positive correlation between the maximum distance between trees containing hemiptera and the maximum distance between nests within a colony. In addition, we demonstrate the mechanism by which this pattern may arise; new nests are built nearer to trees containing hemiptera than existing nests. Further, the distance between trees containing hemiptera and the nearest nests was negatively correlated with the length of exploitation of that tree. Finally, we show that most food is delivered to the nearest nest after which other ants redistribute it between the nests. Combined, these data suggest that foraging efficiency may be an important selection pressure favouring polydomy in I. purpureus. Received 6 April 2006; revised 29 September; accepted 4 October 2006.     

Cytoplasmic and nuclear markers reveal contrasting patterns of spatial genetic structure in a natural Ipomopsis hybrid zone

Spatial variation in natural selection may play an important role in determining the genetic structure of hybridizing populations. Previous studies have found that F1 hybrids between naturally hybridizing Ipomopsis aggregata and Ipomopsis tenuituba in central Colorado differ in fitness depending on both genotype and environment: hybrids had higher survival when I. aggregata was the maternal parent, except in the centre of the hybrid zone where both hybrid types had high survival. Here, we developed both maternally (cpDNA PCR-RFLP) and biparentally inherited (nuclear AFLP) species-diagnostic markers to characterize the spatial genetic structure of the natural Ipomopsis hybrid zone, and tested the prediction that the majority of natural hybrids have I. aggregata cytoplasm, except in areas near the centre of the hybrid zone. Analyses of 352 individuals from across the hybrid zone indicate that cytoplasmic gene flow is bidirectional, but contrary to expectation, most plants in the hybrid zone have I. tenuituba cytoplasm. This cytotype distribution is consistent with a hybrid zone in historical transition, with I. aggregata nuclear genes advancing into the contact zone. Further, nuclear data show a much more gradual cline than cpDNA markers that is consistent with morphological patterns across the hybrid populations. A mixture of environment- and pollinator-mediated selection may contribute to the current genetic structure of this hybrid system.     

How ants find each other; temporal and spatial patterns in nuptial flights

Reproduction is a key factor in understanding population ecology and therefore species occurrence. However, patterns in reproductive behaviour for distinct ant species remain insufficiently known. In this paper strategies in mate finding are studied for six ant species (Lasius niger, Lasius umbratus, Temnothorax nylanderi, Myrmica rubra, Myrmica ruginodis, Stenamma debile) in a forest – forest edge – agricultural field gradient. Using window traps, we studied whether these species had a restricted nuptial flight season, displayed swarming behaviour, and whether the alates aggregated at the forest edge. The flight season was limited to one month or less for L. niger, T. nylanderi, M. rubra, M. ruginodis and S. debile. Swarming behaviour occurred in all but one (L. umbratus) species. Although none of the six species seemed to have highest nest density at the forest edge, three of them, M. rubra, M. ruginodis and S. debile, showed male aggregations there, indicating this to be the main reproduction site. This last finding could be due to a more suitable micro-climate, but most likely, edges are conspicuous land marks which are used by ants to meet mates. The behavioural patterns of ant sexuals at the forest edge can influence dispersal possibilities in fragmented landscapes, reproductive success and nest densities. Received 27 November 2007; revised 27 March; accepted 5 April 2008.     

Stridulations reveal cryptic speciation in neotropical sympatric ants

The taxonomic challenge posed by cryptic species underlines the importance of using multiple criteria in species delimitation. In the current paper we tested the use of acoustic analysis as a tool to assess the real diversity in a cryptic species complex of Neotropical ants. In order to understand the potential of acoustics and to improve consistency in the conclusions by comparing different approaches, phylogenetic relationships of all the morphs considered were assessed by the analysis of a fragment of the mitochondrial DNA cytochrome b. We observed that each of the cryptic morph studied presents a morphologically distinct stridulatory organ and that all sympatric morphs produce distinctive stridulations. This is the first evidence of such a degree of specialization in the acoustic organ and signals in ants, which suggests that stridulations may be among the cues used by these ants during inter-specific interactions. Mitochondrial DNA variation corroborated the acoustic differences observed, confirming acoustics as a helpful tool to determine cryptic species in this group of ants, and possibly in stridulating ants in general. Congruent morphological, acoustic and genetic results constitute sufficient evidence to propose each morph studied here as a valid new species, suggesting that P. apicalis is a complex of at least 6 to 9 species, even if they present different levels of divergence. Finally, our results highlight that ant stridulations may be much more informative than hitherto thought, as much for ant communication as for integrative taxonomists.     

Latitudinal gradients in colony size for social insects: termites and ants show different patterns

On the basis of a comparison of Nearctic and Neotropical ants, social insects have been proposed to show a latitudinal gradient in colony size. Further, the "fasting endurance hypothesis," which predicts larger colonies in areas with extended periods of low food availability, was proposed as the mechanism driving the gradient. To test the generality of the pattern and its mechanism, we examined the relationships between termite colony size and both latitude and annual evapotranspiration, a measure of plant productivity. We found no evidence that colony size increases with increasing latitude or decreasing plant productivity. We conclude that the pattern identified for ants cannot be generalized to include social insects as a whole. As is the case for ecogeographic gradients in insect body sizes, a pattern that is reported for one taxon may not be consistent for other taxa at the global level.     

Colony size predicts division of labour in attine ants

Division of labour is central to the ecological success of eusocial insects, yet the evolutionary factors driving increases in complexity in division of labour are little known. The size–complexity hypothesis proposes that, as larger colonies evolve, both non-reproductive and reproductive division of labour become more complex as workers and queens act to maximize inclusive fitness. Using a statistically robust phylogenetic comparative analysis of social and environmental traits of species within the ant tribe Attini, we show that colony size is positively related to both non-reproductive (worker size variation) and reproductive (queen–worker dimorphism) division of labour. The results also suggested that colony size acts on non-reproductive and reproductive division of labour in different ways. Environmental factors, including measures of variation in temperature and precipitation, had no significant effects on any division of labour measure or colony size. Overall, these results support the size–complexity hypothesis for the evolution of social complexity and division of labour in eusocial insects. Determining the evolutionary drivers of colony size may help contribute to our understanding of the evolution of social complexity.     

Genetic analysis of colony structure in polydomous and polygynous ant populations

Most interactions between individuals of social insects occur in colonies. The correct identification of colonies is therefore crucial for most empirical studies which aim to test evolutionary theories based on properties at the colony level. In many ant populations, the identification of colonics is hampered by polydomy, i.e. that single colonics occupy several, spatially separated nests. Only few attempts have been made so far to develop genetical methods for analysing the structure of specific colonics. Three methods to solve this problem are presented: rare genotype sisterhoods (tracking rare genotypes or alleles), G -distance (a measure of genotypic heterogeneity derived from G -statistics), and neighbour relatcdness (estimates of genetic relatcdness for specific nest pairs). Our methods quantify the likelihood of nest pairs being con-colonial or non-colonial, and given sufficient genetical resolution, statistical tests can be applied. The methods proposed here arc applied to a highly polygynous population of the red ant, Myrmica sulcinodis. In this population single colonics are found to inhabit 1–4 nests, and both monodomous and polydomous colonies coexist in dense clusters of nests. This result is discussed with respect to the functional significance of polydomy. Further, the general application of the methods for determination of colony structure is discussed.     

Colony genetic structure and queen mating frequency in fire ants of the subgenus Solenopsis (Hymenoptera: Formicidae)

Colony genetic structure was studied in natural populations of three fire ant taxa, Solenopsis richteri Forel, S. geminata (Fabr.), and hybrid S. invicta/richteri , using allozyme markers. All colonies studied exhibited arrays of female genotypes predicted under a model of monogyny (single functional queen) and monoandry (single insemination of queens). Males produced in the colonies appear to originate exclusively from the foundress queen, rather than from any virgin females present in the colonies. Thus these social insect colonies represent simple, albeit enormous, family groups. Single insemination and foundress parentage of males appear to be conserved reproductive traits in the subgenus Solenopsis , whereas another major determinant of colony genetic structure, the number of functional queens, is evolutionarily labile in this group.     

Linking traits of foraging animals to spatial patterns of plants: social and solitary ants generate opposing patterns of surviving seeds

Foraging traits of seed predators are expected to impact the spatial structure of plant populations, community dynamics and diversity. Yet, many of the key mechanisms governing distance- or density-dependent seed predation are poorly understood. We designed an extensive set of field experiments to test how seed predation by two harvester ant species interact with seed dispersal in shaping the spatial patterns of surviving seeds. We show that the Janzen–Connell establishment pattern can be generated by central-place foragers even if their focal point is located away from the seed source. Furthermore, we found that differences in the social behaviour of seed predators influence their sensitivity to seed density gradients and yield opposing spatial patterns of surviving seeds. Our results support the predictions of a recent theoretical framework that unifies apparently opposing plant establishment patterns, and suggest that differences in foraging traits among seed predators can drive divergent pathways of plant community dynamics.     

Dispersion movements in ants: spatial structuring and density-dependent effects

This paper examines whether the characteristics of individual dispersion movements in ants are changed when workers are moving solitarily or in a group. We analyzed the trajectories of workers of the species Messor sancta moving solitarily or in groups of different size (5, 10, 15 individuals), tested for density-dependent effects on their trajectory characteristics and investigated through resampling techniques whether ants are able to spatially structure their movements through direct (e.g. contact) or indirect (pheromone deposited on the ground) interactions. In addition to group size, the effects of the nutritional state of the colony and of the state of the area on which ants were dispersing were also examined. Solitary ants moved faster and had more sinuous trajectories than ants moving in a group. We found however no significant differences in trajectory characteristics between groups of different size. Whatever the group size, ants from starved colonies moved more slowly and had more direct trajectories than their counterpart coming from fed colonies. On the other hand, the state of the area on which ants were moving had no direct significant effect on dispersion movement. Ants dispersing in a group moved independently and did not coordinate their movements through direct or indirect interactions. However, the geometry of their path was changed not only through the effect of random encounters with other workers but also through an active modification of their movement when they perceived directly or indirectly the presence of nearby workers.     

Small-scale spatial structure within patterns of seed dispersal

A large proportion of dispersing propagules land near their maternal plant, even in species that have evolved structures which enhance dispersal. For these propagules, their post-dispersal spatial pattern is likely to reflect the overall shape and scale of the parental plant canopy and, especially in poorly dispersing species, aggregation of propagules on the plant prior to dispersal. Localised patterns within seed shadows are also likely to be affected by secondary movement after dispersal, leading to either more or less small-scale aggregation, depending on the mechanism. Our general aim was to study the small-scale spatial structure within patterns of seed dispersal of Raphanus raphanistrum L. to generate hypotheses about the sequence of processes and events leading to the spatial pattern of dispersal in this species. More specifically, we determined the sizes of small-scale structures within the seed shadows on the ground after dispersal and the extent to which these match the sizes of pre-dispersal aggregations within the parental canopy. Variation in plant size and shape was provided by four levels of inter-specific competition resulting from differing wheat crop densities. Positions of propagules were determined using a three-dimensional digitizer, and the data for each plant were analysed using point pattern analysis. Not surprisingly, larger plants, growing at lower plant density, had larger seed shadows, showing an overall influence of maternal plant size. The pattern of propagules exhibited significant small-scale aggregates, with similar sizes on the plant and on the ground. There was no evidence that aggregation size was greater on the ground or increased with time, but the strength of the aggregation increased with length of time on the ground.     

Specific patterns of excavation in isolated ants

Summary The tunnels dug by isolated worker ants [Myrmica ruginodis Nylander,M. scabrinodis Nylander,Formica lemani Bondroit,F. fusca L. andLasius niger (L.)] have been studied photographically. Although there was much variation within species, certain interspecific differences in tunnel shape appeared. The tunnels ofL. niger are more spreading, longer and more convoluted than those of the other species, and often branch beneath the surface. The tunnels ofM. scabrinodis show the same features to a smaller degree. The tunnels ofF. fusca andF. lemani are straight, unbranched and vertical.

---

Résumé Nous avons étudié par un moyen photographique les tunnels que creusent les ouvrières isolées deMyrmica ruginodis Nyl.,M. scabrinodis Nyl.,Formica fusca L.,F. lemani Bondroit etLasius niger (L.). Quoiqu'il y ait une variation intraspécifique très importante, nous avons constaté plusieurs différences entre ces espèces. Les tunnels deL. niger sont plus longs, plus ramifiés et moins rectilignes que chez les autres espèces. Les tunnels deM. scabrinodis montrent les mêmes caractéristiques, mais d'une façon moins prononcée. Les tunnels deFormica fusca etF. lemani sont plus on moins verticaux et rectilignes et ne se divisent pas.

     

Colony variation in the collective regulation of foraging by harvester ants

This study investigates variation in collective behavior in a natural population of colonies of the harvester ant, Pogonomyrmex barbatus. Harvester ant colonies regulate foraging activity to adjust to current food availability; the rate at which inactive foragers leave the nest on the next trip depends on the rate at which successful foragers return with food. This study investigates differences among colonies in foraging activity and how these differences are associated with variation among colonies in the regulation of foraging. Colonies differ in the baseline rate at which patrollers leave the nest, without stimulation from returning ants. This baseline rate predicts a colony's foraging activity, suggesting there is a colony-specific activity level that influences how quickly any ant leaves the nest. When a colony's foraging activity is high, the colony is more likely to regulate foraging. Moreover, colonies differ in the propensity to adjust the rate of outgoing foragers to the rate of forager return. Naturally occurring variation in the regulation of foraging may lead to variation in colony survival and reproductive success.