Dominant Ants Research Articles

Contrasting fire‐related resilience of ecologically dominant ants in tropical savannas of northern Australia

ABSTRACTThis paper examines the role of fire in mediating the relative abundance of two of the world's major ecologically dominant ant genera, Iridomyrmex and Oecophylla, where they coexist across the tropical savanna landscapes of northern Australia. These taxa have contrasting biogeographical histories, which are predicted to lead to contrasting responses to fire. Iridomyrmex is an autochthonous Australian genus that has radiated primarily in the arid zone; as such, its abundance is predicted to be promoted by frequent fire because this maintains an open habitat. In contrast, Oecophylla is a genus of leaf‐nesting ants occurring in the canopies of Old World tropical rainforest, and is a recent arrival to Australia in geological time; the abundance of these ants is predicted to decline under frequent fire. We test these predictions using results from a landscape‐scale fire experiment, where three experimental fire regimes (including no fire) were applied to replicated subcatchments over a 5‐year period. Using sweep nets, ants were sampled in the grass layer (the habitat layer of greatest overlap between Iridomyrmex and Oecophylla) in eucalypt woodland (canopy cover < 30%) and open eucalypt forest (canopy cover about 50%) habitats. A total of 27 species from 11 genera were collected during the study; eight were common enough for statistical analysis, and the abundances of four of these were significantly affected by fire treatment. As predicted, the abundance of Iridomyrmex was promoted by fire, whereas that of Oecophylla declined. These changes occurred only under late‐season (relatively high intensity) fires, and for Oecophylla occurred only in open forest (not woodland) habitat. This fire‐mediated relationship between Iridomyrmex and Oecophylla mirrors the much broader, ecosystem‐wide dynamic between eucalypt‐dominated savanna and rainforest in tropical Australia, with savannas dominated by fire‐resistant sclerophyll elements of Australian origin, and rainforest dominated by fire‐sensitive mesophyll elements of South‐East Asian origin.

Assessing habitat and resource availability for an endangered desert bird species in eastern Morocco: the Houbara Bustard

In Morocco we tested the consistency between an a priori habitat classification based on topography, hydrology, vegetation structure and composition, and an a posteriori classification based on arthropod assemblages, in a plain supporting wild endangered Houbara Bustards. According to vegetation structure, we defined seven a priori habitats that differed significantly in perennial cover and height. A multivariate multiple regression analysis showed a significant relationship between arthropod assemblages and vegetation structure. Canonical Analyses of Principal Coordinates, conducted simultaneously on direct searches of arthropods and trapping data, showed significant differences between assemblages in both cases, and produced two similar constrained ordinations of six a posteriori habitats: esparto grass (Stipa tenacissima), temporarily flooded areas, fields, “reg” with short perennials, “reg” with tall perennials and wadis. The two sampling methods reflected a dominance of ants and beetles. Arthropod biomasses increased significantly in spring and then decreased significantly in summer for beetles, and in autumn for ants. No strong differences appeared between habitats within seasons, especially in spring, indicating a uniform distribution of food resources during the Houbara breeding season. The “reg” with short perennials had the highest ant biomass in summer. This “reg” and fields also supported the highest arthropod biomass in autumn. Variation in arthropod biomass was a pertinent factor that should be integrated into Houbara habitat selection studies. The definition of habitat availability based on easily identifiable landscape units, combined with empirical tests on arthropod communities provided an accurate classification for habitat selection studies and conservation planning.

Ant patchiness: a spatially quantitative test in coffee agroecosystems

Arboreal ants form patchy spatial patterns in tropical agroforest canopies. Such patchy distributions more likely occur in disturbed habitats associated with lower ant diversity and resource availability than in forests. Yet, few studies have quantitatively examined these patchy patterns to statistically test if ants are non-randomly distributed or at what scale. Coffee agroecosystems form a gradient of management intensification along which vegetative complexity and ant diversity decline. Using field studies and a spatially explicit randomization model, I investigated ant patchiness in coffee agroecosystems in Chiapas, Mexico varying in management intensity to examine if: (1) coffee intensification affects occurrence of numerically dominant ants, (2) numerical dominants form statistically distinguishable single-species patches in coffee plants, (3) shade trees play a role in patch location, and (4) patch formation or size varies with management intensity. Coffee intensification correlated with lower occurrence frequency of numerically dominant species generally and of one of four taxa examined. All dominant ant species formed patches but only Azteca instabilis was patchy around shade trees. Ant patchiness did vary somewhat with spatial scale and with strata (within the coffee layer vs around shade trees). Patchiness, however, did not vary with management intensity. These results provide quantitative evidence that numerically dominant ants are patchy within the coffee layer at different scales and that shade tree location, but not coffee management intensity, may play a role in the formation of patchy distributions.

The effect of a dominant ant, Iridomyrmex purpureus, on resource use by ant assemblages depends on microhabitat and resource type

Abstract Behaviourally and ecologically dominant ants are thought to structure ant communities through their monopolization of resources. Effects of a dominant ant, Iridomyrmex purpureus, on resource use by ant communities were tested using an exclusion experiment on sandstone outcrops near Sydney, in south‐eastern Australia. The success of functionally different ants at honey and mealworm baits placed in rock and vegetation microhabitats was measured in a series of surveys before and after exclusion. I. purpureus was successfully excluded from the outcrops and patterns of resource use following I. purpureus exclusion were consistent with those of sites without I. purpureus and procedural control sites. At sites with I. purpureus, resources were discovered more rapidly, however, other ants were less successful, particularly on rock substrates. Iridomyrmex spp. were more successful on rock than in vegetation, however, in the presence of I. purpureus, the success of other Iridomyrmex on rock substrates and at honey baits was reduced. Camponotine and myrmicine ants had low success at baits and tended to be less successful in the presence of I. purpureus. In contrast, the opportunist Rhytidoponera metallica was not affected by the presence of I. purpureus and was more successful in the vegetation habitat. These findings indicate that the dominant ant affects the success of other ants at baits, particularly the ecologically similar Iridomyrmex. However, effects of I. purpureus on other taxa differ between habitats differing in complexity and resources differing in composition and defensibility.

Spatial structure of litter-dwelling ant distribution in a subtropical dry forest

Understanding the spatial patterns of species distribution is essential to characterize the structure of communities, to optimize species inventories and to evaluate the impact of biotic and abiotic variables. Here we describe the spatial structure of the distribution of leaf litter ant species, and of biotic factors that could explain it, in a subtropical semi-deciduous forest of the Argentinian Chaco, characterized by a dense understorey of shrubs and terrestrial bromeliads. Environmental variables (leaf litter quantity and ground bromeliad density) were measured and ants were collected in 1 m2 quadrats distributed along two 200 m transects at intervals of 1.25 m. Overall 87 species were collected. Sixteen positive associations and a single negative association were observed between the 11 most frequent species taken pair-wise. Our results suggest that the spatial distribution of leaf litter ants was determined at two different scales. At a small scale (period below 10 m) a periodic spatial structure, likely due to intraspecific competition, produced a succession of peaks of abundance separated by gaps. At a larger scale (period around 50 m), periodically distributed environmental factors induced aggregates of colonies of species responding positively to these factors. A high quantity of leaf litter and, to a lesser extent, a high density of ground bromeliads promoted a high density and a high species richness of ants. Numerically dominant ants being generally positively associated, interspecific competition was apparently weak. All ant species whose abundance was correlated with an environmental factor were not completely spatially structured by it. This suggests that some other factors, such as intraspecific competition, may have counter-effects.

Ecology of an Improbable Association: The Pseudomyrmecine Plant‐antTetraponera tessmanniand the Myrmecophytic LianaVitex thyrsiflora(Lamiaceae) in Cameroon1

ABSTRACTIn young individuals of the obligate myrmecophytic lianaVitex thyrsiflora, several species of ants and other arthropods compete for resources offered by the plant. In mature individuals, the only inhabitant is the ant speciesTetraponera tessmanni, which is completely restricted toVitexlianas as its sole host. Established colonies of this ant provide effective defense against herbivores. The association betweenV. thyrsifloraandT. tessmanniis unusual in two respects. First, the climbing life form is rare among myrmecophytes. Secondly, it is surprising that a pseudomyrmecine should be the obligate associate of a liana. Pseudomyrmecine plant‐ants often prune vegetation contacting their host plant. This behavior functions in part to protect against invasion of the host by ecologically dominant ants. In contrast,T. tessmannidoes not prune and is associated with a plant whose success, and thus that of its resident ant colony, depends on contacts with many other plants. Several traits ofV. thyrsifloraandT. tessmannicombine to make the colonization of host plants by potential competitors very difficult. These include behavioral and morphological filters restricting entrance into the plant and exploitation of the resources it can supply; plant anatomical organization that enablesT. tessmanniworkers to carry out all activities, except leaf patrolling, within a single, branched private nesting space within which all food resources offered by the plant are produced; and polygyny, permitting the colony to monopolize a large, rapidly growing and long‐lived territory.

Reduced Diversity and Complexity in the Leaf‐Litter Ant Assemblage of Colombian Coffee Plantations

Abstract: Coffee agroecosystems have recently undergone a dramatic intensification in Colombia, a megadiverse country, especially in terms of the nature of shade cover. We tested for changes in the composition, ecological associations, and diversity of ants (Hymenoptera: Formicidae) along a gradient of intensification of coffee production in the Colombian Andes. We surveyed 16 farms in two regions, classified into four management types: (1) forest (no agriculture), (2) organic polygeneric shaded coffee, (3) monogeneric shaded coffee, and (4) sun coffee (unshaded). Forty sampling units (20 1‐m2 plots on the ground and 20 coffee bushes) were established at each farm between 2001 and 2002. We sampled with a mini‐winkler litter extraction technique and through visual searching. Organic polygeneric shaded‐coffee plantations contained significantly higher ant species richness, and their ant assemblages resembled the forest patches more than any other management type. The number of statistically significant associations among ant species dropped with production intensification, as did the number of ant species involved in such associations. The network of ant associations in shaded systems transformed into an extremely simplified network in sun coffee, with a few dominant ants extending almost entirely throughout the crop. Intensification of coffee agriculture not only caused loss of litter ant species (especially forest species) but also a reduction in the complexity of the ant assemblage in the leaf litter of this agroecosystem.

Constraint and Competition in Assemblages: A Cross‐Continental and Modeling Approach for Ants

The mechanisms leading to structure in local assemblages are controversial. On the one hand, assemblage structure is thought to be the outcome of local interactions determined by the properties of species and their responses to the local environment. Alternatively, this structure has been shown to be an emergent property of assemblages of identical individuals or of random sampling of a regional assemblage. In ants at baits, a combination of environmental stress and interspecific competition is widely held to lead to a unimodal relationship between the abundance of dominant ants and species richness. It is thought that in comparatively adverse environments, both abundance and richness are low. As habitats become more favorable, abundance increases until the abundance of dominant ants is so high that they exclude those that are subordinate and so depress richness. Here we demonstrate empirically that this relationship is remarkably similar across three continents. Using a null model approach, we then show that the ascending part of the relationship is largely constrained to take this form not simply as a consequence of stress but also as a result of the shape of abundance frequency distributions. While the form of the species-abundance frequency distribution can also produce the descending part of the relationship, interspecific competition might lead to it too. Scatter about the relationship, which is generally not discussed in the literature, may well be a consequence of resource availability and environmental patchiness. Our results draw attention to the significance of regional processes in structuring ant assemblages.

Bottom‐up control and co‐occurrence in complex communities: honeydew and nectar determine a rainforest ant mosaic

Complex distribution patterns of species‐rich insect communities in tropical rainforests have been intensively studied, and yet we know very little about processes that generate these patterns. We provide evidence for the key role of homopteran honeydew and plant nectar in structuring ant communities in an Australian tropical rainforest canopy and understorey. We also test the ant visitation of these resources against predictions derived from the ‘ant‐mosaic’ hypothesis. Two ant species were highly dominant in terms of territorial behaviour and abundance:OecophyllasmaragdinaandAnonychomyrma gilberti. Both dominant ant species monopolised large aggregations of honeydew‐producing homopterans. Attended homopteran species were highly segregated between these two ant species. For the use of extrafloral and floral nectar (involving 43 ant species on 48 plant species), partitioning of ant species among plant species and between canopy and understorey was also significant, but less pronounced. In contrast to trophobioses, simultaneous co‐occurrence of different nectar foraging ant species on the same plant individuals was frequent (23% of all surveys). While both dominant ant species were mutually exclusive on honeydew and nectar sources, co‐occurrence with non‐dominant ant species on nectaries was common. The proportion of visits with co‐occurrences was low for dominant ants and high for many sub‐ordinate species. These findings support the ant mosaic theory. The differential role of honeydew (as a specialised resource for dominant ants) and nectar (as an opportunistic resource for all ants including the co‐occurring non‐dominant species) provides a plausible structuring mechanism for the Australian canopy ant community studied.

Dominant meat ants affect only their specialist predator in an epigaeic arthropod community.

Ants are thought to exert an important influence on the structure of arthropod assemblages through predation and competition. I examined the effect of a dominant ant, Iridomyrmex purpureus, on epigaeic arthropod assemblages on rock outcrops using an exclusion experiment. I compared arthropod assemblages on four replicate outcrops allocated to each of the following treatments: I. purpureus present; I. purpureus absent; I. purpureus excluded; and procedural control. Nests of I. purpureus were caged in summer 2001 and epigaeic arthropod assemblages were sampled at all sites using pitfall traps in autumn and spring 2001 and summer 2002. I also collected items from foraging workers to determine the diet of I. purpureus. Exclusion cages successfully reduced the abundance of I. purpureus workers in pitfall traps by more than 97%. Exclusion of I. purpureus did not affect the size distribution, biomass or abundance of arthropod predators or non-predatory arthropods, although the total biomass of ants was greater at sites with I. purpureus. Spider biomass, species richness, abundance and composition were also not affected by the presence of I. purpureus, although the I. purpureus mimic and specialist predator, Habronestes bradleyi, became less abundant at sites from which I. purpureus was excluded. Predation by I. purpureus on other arthropods may not have a significant effect on epigaeic arthropod communities, but the complex role of I. purpureus in this ecosystem and the high diversity of species belonging to multiple trophic levels may obscure its effects in this system.

Arboreal ants as key predators in tropical lowland rainforest trees.

Ants numerically dominate the canopy fauna of tropical lowland rain forests. They are considered to be key predators but their effects in this regard have only rarely been studied on non-myrmecophytes. A conspicuously low abundance of less mobile, mainly holometabolous arthropods like Lepidoptera larvae corresponds with ant dominance, while hemimetabolous highly mobile nymphs occur regularly and in large numbers in the trees. This is in contrast to the temperate regions where ants are mostly lacking on trees and holometabolous larvae are frequent. In this study we experimentally measured ant predation in the trees by offering caterpillars as baits. Fifty-four ant species were tested, of which 46 killed caterpillars and carried them away to their nests while only eight species ignored the offered larvae. Insecticidal knockdown fogging of ten trees after finishing the prey experiments showed that on average 85% of ant individuals per tree were predacious. With the analysis of another 69 foggings and meticulous observations in many other trees this suggests that arboreal ants are responsible for the low abundance of less mobile arthropods in tropical lowland rain forest canopies. Ant predation was significantly lower in a disturbed forest indicating that human disturbance induces a change in the functional interactions in these ecosystems.

The Effect of Logging on the Ground-Foraging Ant Community in Eastern Amazonia

In order to determine the suitability of ants as indicator organisms for habitat disruption in tropical forests, we studied the effects of both high and low impact logging on ant communities in northeastern Pará State, in the Brazilian Amazon. We collected ants from logged forests and unlogged forest sites with Winkler bags throughout the 1998 rainy season (January and April) and the following dry season (July and September). Both methods of timber harvesting showed impacts on ant community composition when compared with unlogged forest, although these impacts did not include modifications in total species richness or the relative contribution of each ant subfamily to the total number of species. Instead, logging induced alterations took place at the level of species and genera. A 2-fold reduction in the dominance of ants of the highly diverse genus Pheidole was associated with forest alterations in high-impact logging sites. Thus, logging in Amazonia can be seen to promote species shifts in ant communities, without, however, altering species richness. Ants of the genus Pheidole are potentially useful indicators for forest disturbances resulting from timber extraction.

Ants that associate with Lycaeninae butterfly larvae: diversity, ecology and biogeography

Abstract.Field data on associations of butterfly larvae with ants were collated for 435 species of the subfamily Lycaeninae to analyse patterns in the identity, diversity and ecology of attendant ants. Worldwide, members of at least fifty‐three ant genera from six subfamilies associate with Lycaeninae larvae. All these ant genera also forage for other liquid carbohydrate food sources. Species of Crematogaster and Camponotus constitute by far the most important ant associates of Lycaeninae larvae. Diversity of butterfly–ant associations, as measured by Hurlbert rarefaction, is highest in the Oriental, Australian and Nearctic faunal regions, and lower in Africa and the Palaearctic. The Neotropical region could not be assessed separately due to paucity of data. Multiple regression analyses revealed that the number of Lycaeninae species associated with a particular ant genus in a facultatively mutualistic manner is largely determined by the ecological prevalence and geographical distribution, but does not correlate with species richness, of an ant genus. This opportunism is suggested to be the key variable supporting the geographically and taxonomically widespread occurrence of ant–caterpillar mutualism in the Lycaeninae. Obligate interactions are much more specific and almost exclusively involve ecologically dominant ants with long‐lived colonies. Neither species richness nor geographical distribution of an ant genus are significant predictors. In such associations, the role of ants shifts from visitors to hosts, and patterns of host use and specificity resemble those known from inquiline ant guests.

The trade‐off between thermal tolerance and behavioural dominance in a subtropical South American ant community

Summary 1. The distribution, ecology, and behaviour of ants (Hymenoptera: Formicidae) is profoundly influenced by environmental stress and competition. As in plants, trade‐offs in adaptations to these factors are the basis for functional classifications of ant taxa and communities at a global scale. 2. Theory predicts a trade‐off between stress tolerance and competitive dominance in both plants and ants. In ants, low temperature is thought to be stressful, so I hypothesized that there would be a positive relationship between temperature and behavioural dominance. I evaluated this relationship in a South American, Chaco ant community. 3 The activity and behaviour of ground‐foraging, omnivorous ants were examined at baits in open and closed, forested habitats during different seasons and times of day to characterize the responses of ant taxa to variation in microclimate and competitors. 4. Behaviourally dominant ants were most active at moderately high temperatures, whereas subordinate species were active at extreme temperatures, when they had virtually exclusive access to resources. 5. The patterns presented here and those observed in other studies suggest that there is a general trade‐off between behavioural dominance and thermal tolerance in ants. This trade‐off creates a linear relationship between temperature use and dominance for ants up to ≈ 35 °C, but extremely high temperatures may also be stressful such that the full relationship is actually unimodal.

How plants shape the ant community in the Amazonian rainforest canopy: the key role of extrafloral nectaries and homopteran honeydew

Ant-plant interactions in the canopy of a lowland Amazonian rainforest of the upper Orinoco, Venezuela, were studied using a modified commercial crane on rails (Surumoni project). Our observations show a strong correlation between plant sap exudates and both abundance of ants and co-occurrence of ant species in tree canopies. Two types of plant sap sources were compared: extrafloral nectaries (EFNs) and honeydew secretions by homopterans. EFNs were a frequent food source for ants on epiphytes (Philodendron spp., Araceae) and lianas (Dioclea, Fabaceae), but rare on canopy trees in the study area, whereas the majority of trees were host to aggregations of homopterans tended by honeydew-seeking ants (on 62% of the trees examined). These aggregations rarely occurred on epiphytes. Baited ant traps were installed on plants with EFNs and in the crowns of trees from three common genera, including trees with and without ant-tended homopterans: Goupia glabra (Celastraceae), Vochysia spp. (Vochysiaceae), and Xylopia spp. (Annonaceae). The number of ant workers per trap was significantly higher on plants offering one of the two plant sap sources than on trees without such resources. Extrafloral nectaries were used by a much broader spectrum of ant species and genera than honeydew, and co-occurrence of ant species (in traps) was significantly higher on plants bearing EFNs than on trees. Homopteran honeydew (Coccidae and Membracidae), on the other hand, was mostly monopolised by a single ant colony per tree. Homopteran-tending ants were generally among the most dominant ants in the canopy. The most prominent genera were Azteca, Dolichoderus (both Dolichoderinae), Cephalotes, Pheidole, Crematogaster (all Myrmicinae), and Ectatomma (Ponerinae). Potential preferences were recorded between ant and homopteran species, and also between ant-homopteran associations and tree genera. We hypothesize that the high availability of homopteran honeydew provides a key resource for ant mosaics, where dominant ant colonies and species maintain mutually exclusive territories on trees. In turn, we propose that for nourishment of numerous ants of lower competitive capacity, Philodendron and other sources of EFNs might be particularly important.

Short-term dynamics of an acacia ant community in Laikipia, Kenya.

In monospecific stands of Acacia drepanolobium in Laikipia, Kenya, virtually all but the smallest trees are occupied by one of four species of ants. Although trees are a limiting resource, all four ant species are maintained in this system. Three separate lines of evidence confirm a linear dominance hierarchy among these four ants: (1) experimentally staged conflicts, (2) natural transitions among 1773 tagged trees over a 6-month period, and (3) the average sizes of trees occupied by ants of different species. Short-term dynamics during a drying period reveal that many smaller trees (<1 m) occupied by dominant ants were subsequently abandoned, and that abandoned trees had grown more slowly than those that were not abandoned. Height growth increments over 6 months were generally independent of ant occupant, but increased with tree height. Among taller trees (>1 m), changes in ant occupation congruent with the dominance hierarchy (i.e., transitions from more subordinate ant species to more dominant ant species) occurred on trees that grew faster than average. In contrast, the (less frequent) changes in ant occupation "against" the direction of the dominance hierarchy occurred on trees that grew more slowly than average. Observed correlations between tree vigor and takeover direction suggest that colony growth of dominant ant species is either favored in more productive microhabitats, or that such colonies differentially seek out healthier trees for conquest. Colonies of dominant species may differentially abandon more slowly growing trees during (dry) periods of retrenchment, or suffer higher mortality on these trees. Subordinate ant species appear to move onto these abandoned trees and, to a lesser extent, colonize new recruits in the sapling class. These data reveal that within a simple linear dominance hierarchy, short-term variations exist that may reveal underlying mechanisms associated with coexistence.

Mechanisms of dispersed central-place foraging in polydomous colonies of the Argentine ant

Many species of ants occupy multiple nests, a condition known as polydomy. Because of their decentralized structure, polydomous colonies may be removed from some of the constraints associated with classic central-place foraging. We used laboratory and field experiments to assess the mechanisms involved in dispersed central-place foraging in polydomous colonies of the Argentine ant Linepithema humile, a widespread invasive species. Both in the laboratory and in the field, Argentine ants established new nests at sites located near food. Laboratory colonies of L.humile redistributed workers, brood and resources among nests in response to the spatial heterogeneity of food resources. In addition, laboratory colonies formed recruitment trails between nests in the context of foraging, providing a mechanism for the transport of material between nests. This highly flexible system of allocating nests, workers and brood throughout a colony's foraging area potentially increases foraging efficiency and competitive ability. The importance of polydomy as a determinant of competitive ability is underscored by its prevalence among ecologically dominant ants, including most, if not all, highly invasive species.

Associations between lycaenid butterflies and ants in Australia

AbstractA comprehensive and critical review of all available literature on associations between Australian lycaenid butterflies and ants was undertaken to establish an accurate database of the partners involved. Collections and observations of lycaenids and ants were used to augment this review, resulting in a significant number of newly documented association (and non‐association) records. Twenty published records considered to be erroneous or doubtful are noted, with justifications given for their deletion from the association database. In total, 265 different associations between lycaenids and ants, plus 65 non‐attendance records are documented for Australia. Nearly 80% of the lycaenid species in Australia, for which the early stages are known, are recorded associating with ants and half of these are obligately ant‐associated. Patterns of association are examined from the perspective of both lycaenids and ants, with a focus on ant systematics and ecology. Lycaenids are recorded with five ant subfamilies, including the first record of an association with the Pseudomyrmecinae. The Dolichoderinae, and to some extent the Formicinae, have a disproportionately high percentage of genera that associate with lycaenid butterflies. All ant species that tend lycaenids spend at least some portion of their time foraging on vegetation to collect plant and insect nectar. There is a robust relationship between the competitive status of ants within a community, and their frequency and degree of association with lycaenids. Obligate ant‐association is accompanied by a high degree of specificity for ant partner, but two notable exceptions,Ogyris aenoneandO. amaryllisare discussed. Facultative myrmecophiles tend to associate with a broad range of ants, although interactions with ecologically dominant ants are less frequent than might be expected based on the abundance of dominant ant species in Australian communities.

Sterilization and canopy modification of a swollen thorn acacia tree by a plant-ant

Obligate symbioses between specialized arboreal ants and plants have evolved independently in many lineages1,2. Ant-plants (myrmecophytes) typically provide hollow nest cavities and nutrition to the occupying ant colony1,3,4,5,6. In turn, resident plant-ants often protect their hosts from herbivory7,8,9,10,11 and/or overgrowth by surrounding vegetation12,13. As individual plants are rarely occupied by more than one ant colony14,15,16,17, co-occurring plant-ant species compete intensely for hosts13,14,18,19. In such multi-species systems, ecological interactions among potential partners may lead to the evolution of cheating20,21. Previous studies have revealed that some specialized plant-ants are effectively parasites of their host-plants8,18,22,23, but the selection pressures favouring such behaviours are poorly understood. Here we describe host parasitism in an east African plant-ant that prunes and sterilizes its host-tree canopies, apparently to minimize contact with competitively dominant ants occupying neighbouring trees. We propose that the high density of ant-trees and low diversity of tree species in this savanna habitat have selected for induced, parasitic pruning of host trees by this competitively subordinate ant species.

Cuticular hydrocarbons correlated with reproductive status in a queenless ant

Reproductive division of labour is regulated behaviourally in social insects lacking morphologically specialized castes. The directional nature of dominance interactions shows that recognition occurs, but little is known about its basis. In the queenless ant Dinoponera quadriceps , the top worker in the hierarchy (‘alpha’) mates and produces offspring in each colony, while other workers remain virgin. Dominant ants frequently rub one antenna of subordinates against their own cuticle, and alpha and infertile nest–mates consistently differ in their relative proportions of the cuticular hydrocarbon 9–hentriacontene (9-C 31 ). The second-ranking ‘beta’ occasionally lays unfertilized eggs and we show that she has less 9-C 31 than the alpha but more than infertile workers. To investigate further the link between 9-C 31 and ovarian activity, we experimentally removed alpha workers ( n =11 individuals) and used solid-phase microextraction (SPME) with gas chromatography to measure changes in 9-C 31 on live beta workers which attained alpha status. The proportion of 9-C 31 on the replacement alpha increased significantly after six weeks, in parallel with her gain in fecundity. We discuss whether 9-C 31 provides honest information about egg–laying ability, enabling ants to recognize the different classes of nest–mates involved in reproductive conflicts. Such fertility cues could reliably underpin the antagonistic interactions occurring in insect societies.