Do invasive ants respond more strongly to carbohydrate availability than co‐occurring non‐invasive ants? A test along an active Anoplolepis gracilipes invasion front

Invasions by non‐native insects can have important ecological impacts, particularly on island ecosystems. However, the factors that promote the success of invaders relative to co‐occurring non‐invasive species remain unresolved. For invasive ants, access to carbohydrate resources via interactions with both extrafloral nectary‐bearing plants and honeydew‐excreting insects may accelerate the invasion process. A first step towards testing this hypothesis is to determine whether invasive ants respond to variation in the availability of carbohydrate resources, and whether this response differs from that of co‐occurring, non‐invasive ants. We investigated the effect of carbohydrate subsidies on the short‐term foraging and hemipteran‐tending behaviours of the invasive ant Anoplolepis gracilipes (Formicidae) and co‐occurring ant species on an extrafloral nectary‐bearing plant by experimentally manipulating carbohydrate levels and tracking ant recruitment. We conducted experiments in 2 years at two sites: one site was invaded by A. gracilipes prior to 2007 and the other became invaded during the course of our study, allowing pre‐ (2007) and post‐invasion (2009) comparisons. Short‐term increases in carbohydrate availability increased the density of A. gracilipes workers on plants by as much as 400% and reduced tending of honeydew‐excreting insects by this species by up to 89%, with similar responses across years. In contrast, ants at the uninvaded site in 2007 showed a weak and non‐significant forager recruitment response. Across all sites, A. gracilipes workers were the only ants that responded to carbohydrate manipulations in 2009. Furthermore, ant–carbohydrate dynamics at a site newly invaded by A. gracilipes quickly diverged from dynamics at uninvaded sites and converged on those of the site with an established invasion. These findings suggest that carbohydrate resources may be particularly important for A. gracilipes invasions, and underscore the importance of species interactions, particularly putative mutualisms, in facilitating exotic species invasions.     

Do associations between native and invasive plants provide signals of invasive impacts?

Do invasive plant species act more as “passengers” or drivers of ecological change in native plant communities? Snapshot studies based on correlations at the site scale ignore longer-term dynamics and variation in how particular invaders affect particular native species. We analyzed patterns of co-occurrence between three invading species (Alliaria petiolata, Lonicera x bella, and Rhamnus cathartica) and 70 native plant species in 94 southern Wisconsin forests at two scales to test four hypotheses. Surveys at these sites in the 1950s and again in the 2000s allowed us to assess how initial plant diversity and site conditions affected subsequent patterns of invasion. Sites with more native species in the 1950s experienced fewer invasions of Lonicera and Rhamnus. However, this result may reflect the fact that more fragmented habitat patches supported both fewer species in the 1950s and more invasions. At the site-level, few negative correlations exist between invasive and native species’ abundances. Sites with higher Alliaria densities in the 2000s, however, support fewer native species and lower populations of several declining natives. Rhamnus-invaded sites support lower populations of two increasing species. Association (C-score) analyses detect more associations and more negative associations at the 1 m² scale than at the site scale. Most strong associations between invasive and increasing native species are positive while those with declining natives are often negative. Species restricted to specialized habitats rarely co-occur with invaders. Alliaria has more negative associations at fragmented sites where it is more abundant and invasions may be older. Fine-scale invasive-native associations were stronger, easier to detect, and less consistent than those detectable at the site-level. Thus, screening large numbers of local associations using observational data may allow us to identify particular invasive-native interactions worth further investigation. Although invading plants sometimes act as passive passengers, increasing in tandem with certain native plants in response to disturbed fragmented habitats, they may also contribute to the declines we observe in many native species. Monitoring invasions would allow us to assess whether local associations serve to predict subsequent invasive species impacts.     

Are ant-aphid associations a tritrophic interaction? Oleander aphids and Argentine ants

Summary Oleander aphids, (Aphis nerii), which are sporadically tended by ants, were used as a moded system to examine whether host plant factors associated with feeding site influenced the formation of ant-aphid associations. Seasonal patterns of host plant utilization and association with attendant ants were examined through bi-weekly censuses of the aphid population feeding on thirty ornamental oleander plands (Nerium oleander) in northern California in 1985 and 1986. Colonies occurred on both developing and senescing plant terminals, including leaf tips, floral structures, and pods. Aphids preferentially colonized leaf terminals early in the season, but showed no preference for feeding site during later periods. Argentine ants (Iridomyrmex humilis) occasionally tended aphid colonies. Colonies on floral tips were three to four times more likely to attract ants than colonies on leaf tips, even though the latter frequently contained more aphids. Ants showed a positive recruitment response to colonies on floral tips, with a significant correlation between colony size and number of ants. There was no recruitment response to colonies on leaf tips. These patterns were reproducible over two years despite large fluctuations in both aphid population density and ant activity. In a laboratory bioassay of aphid palatability, the generalist predator,Hippodamia convergens, took significantly more aphids reared on floral tips compared to those reared on leaf tips. The patterns reported here support the hypothesis that tritrophic factors may be important in modifying higher level arthropod mutualisms.     

Dispersal for distance? Acacia ligulata seeds and meat ants Iridomyrmex viridiaeneus

Abstract Ant seed dispersal distances are typically small, averaging less than 1 m in published studies. Here, a new record (180 m) for ant seed dispersal distance is reported, and preliminary observations are made on the interaction between meat ants Iridomyrmex viridiaeneus Viehmeyer (Hymenoptera: Formicidae) and diaspores of the sandhill wattle, Acacia ligulata A. Cunn. ex Benth. (Fabaceae) in Kinchega National Park, New South Wales (NSW), Australia. Iridomyrmex viridiaeneus moved diaspores over distances of 7–180 m (mean 93.9 m) from the source trees to their nests, removed the arils underground and discarded the seeds over a 3000‐m² area surrounding the nest. A germination trial determined that the viability of discarded seeds was 40%, with 80% of the viable seeds in a dormant condition. Although the cumulative effects of I. viridiaeneus on A. ligulata recruitment require further investigation, this study and others raise the possibility that myrmecochorous systems in the Australian arid zone may be characterized by longer dispersal distances than those in other parts of the world. Long‐distance seed movement by ants lends credence to the hypothesis that distance dispersal (in contrast to directed dispersal) could be of benefit to myrmecochorous plants.     

Do ants make direct comparisons?

Many individual decisions are informed by direct comparison of the alternatives. In collective decisions, however, only certain group members may have the opportunity to compare options. Emigrating ant colonies (Temnothorax albipennis) show sophisticated nest-site choice, selecting superior sites even when they are nine times further away than the alternative. How do they do this? We used radio-frequency identification-tagged ants to monitor individual behaviour. Here we show for the first time that switching between nests during the decision process can influence nest choice without requiring direct comparison of nests. Ants finding the poor nest were likely to switch and find the good nest, whereas ants finding the good nest were more likely to stay committed to that nest. When ants switched quickly between the two nests, colonies chose the good nest. Switching by ants that had the opportunity to compare nests had little effect on nest choice. We suggest a new mechanism of collective nest choice: individuals respond to nest quality by the decision either to commit or to seek alternatives. Previously proposed mechanisms, recruitment latency and nest comparison, can be explained as side effects of this simple rule. Colony-level comparison and choice can emerge, without direct comparison by individuals.     

How do ants assess food volume?

By comparing the behaviour of Lasius niger scouts at sucrose droplets of different volumes, we empirically identified the criterion used by each scout to assess the amount of food available as well as the rules governing its decision to lay a recruitment trail. When scouts discovered food volumes exceeding the capacity of their crop (3 or 6 μl), 90% immediately returned to the nest laying a recruitment trail. In contrast, when smaller food droplets (0.3, 0.7 or 1 μl) were offered, several scouts stayed on the foraging area, presumably exploring it for additional food. If unsuccessful, they returned to the nest without laying a trail. The droplet volume determined the percentage of trail-laying ants but had no influence on the intensity of marking when this was initiated. The key criterion that regulated the recruiting behaviour of scouts was their ability to ingest their own desired volume. This volume acted as a threshold triggering the trail-laying response of foragers. Collective regulation of foraging according to food size resulted from the interplay between the distribution of these desired volume thresholds among colony members and the food volume available. We relate some aspects of the foraging ecology of aphid-tending ants to this decision-making process. Copyright 2000 The Association for the Study of Animal Behaviour.     

Could melatonin unbalance the equilibrium between autophagy and invasive processes?

The Syrian hamster Harderian gland (HG) is a juxtaorbital organ exhibiting marked gender-associated morphological differences. Regarding contents of porphyrins, this gland is a good model for studying physiological oxidative stress effects, since both sexes present strong (in females) and moderate (in males) levels of this stress in normal conditions. We have recently showed that autophagic processes are in the Syrian hamster HG as the first result of an elevated porphyrin metabolism observed in both sexes. In this case, autophagy is not a cell death mechanism per se but a constant renovation system which allows to continuing with the normal gland activity. Moreover, we have also reported that this gland presents invasive processes, resembling to tumoral progression, and are, additionally, a consequence of a strong oxidative stress environment that is mainly observed in female Syrian hamster HG and in minor proportion in male HG. Here, we present additional data and discuss a model of melatonin action on these cited processes by which melatonin would be able to destroy the equilibrium between both detoxifying actions. We postulate that melatonin reduces oxidative stress level into HG as direct antioxidant. This decrease of free radicals produces the autophagy inhibition due to outbreak signal disappearance in HG. Under these events and regarding the huge contents of porphyrins that this gland supports, the invasive process triggers.     

How do invasive species travel to and through urban environments?

Globalisation has resulted in the movement of organisms outside their natural range, often with negative ecological and economic consequences. As cities are hubs of anthropogenic activities, with both highly transformed and disturbed environments, these areas are often the first point of entry for alien species. We compiled a global database of cities with more than one million inhabitants that data had on alien species occurrence. We then identified the most prominent pathways of introduction and vectors of spread of alien species in these cities. Most species were intentionally introduced to cities and were released or escaped from confinement. The majority of alien species then spread within cities through natural means (primarily unaided dispersal). Pathway prominence varied across the taxonomic groups of alien species: the most prominent pathway for plants and vertebrates was the escape pathway; for invertebrates the stowaway and contaminant pathways were most likely to facilitate introductions. For some organisms, pathway prominence varied with the geographical and climatic characteristics of the city. The characteristics of the cities also influenced the prominence of vectors of spread for alien species. Preventing the natural spread of alien species within cities, and into adjacent natural environments will be, at best, difficult. To prevent invasions, both the intentional and unintentional introduction of potentially harmful alien species to cities must be prevented. The pathways of introduction and vectors of spread identified here should be prioritised for management.     

Native and alien invasive plants: more of the same?

We compare the ecological and habitat characteristics of alien and native vascular plants which have recently expanded in range m England, Scotland, the Republic of Ireland and the Netherlands In the great majority of respects, expanding aliens and natives are functionally indistinguishable However, there are a few consistent differences aliens are more likely than natives to be clonal, polycarpic perennials with erect, leafy stems, and to have transient seed banks We discuss these trends m the context of the difficulties faced by aliens in invading mostly closed plant communities in relatively cool, damp climates Our results are consistent with some predictions of the attributes of'ideal'invaders, but contradict others We argue that the ecological attributes of successful alien invaders are strongly habitat-dependent     

Do exaptations facilitate mutualistic associations between invasive and native species?

As invasive species are key threats to ecosystem structure and function, it is essential to understand the factors underlying their success. Enigmatically, mutualistic organisms are often successful in colonizing novel environments even though they commonly persist only through intricate relationships with other species. Mutualistic ants, for example, protect aphids from natural enemies while collecting carbohydrate–rich honeydew. To facilitate this interaction, ants have evolved aggressive responses to aphid alarm pheromone emissions. As invasive and native mutualists have not evolved together, however, it is unclear if this form of cross-species communication exists between these two parties thereby facilitating these novel interactions. We address this hypothesis by assessing whether the invasive Argentine ant, Linepithema humile, responds to native poplar aphid, Chaitophorus populicola, alarm signals. Here, we show that interspecific signalling does exist in this newly established mutualistic interaction. Argentine ant workers exhibit increased aggression and double the number of visits to an aphid colony after an aphid alarm signal is emitted. We suggest that pre-adaptations may facilitate the emergence of mutualistic associations between many invasive and native species.     

Army ants: an evolutionary bestseller?

Army ants are characterized by a complex combination of behavioral and morphological traits. Molecular data now indicate that army ant behavior has a unique evolutionary origin and has been conserved for over more than 100 million years.     

Soil conditioning and plant–soil feedbacks affect competitive relationships between native and invasive grasses

Understanding how competition from invasive species and soil conditions individually and interactively affect native performance will increase knowledge of invasion dynamics and can be used to improve the success of restoration plans. This study, conducted in Reno NV, USA, uses a two-phase plant?Csoil feedback experiment coupled with a target-neighbor competition design to examine the individual and interactive effects of both soil conditions and invasive neighbors on native performance. Study species include invasive species (Bromus tectorum and Agropyron cristatum) and native species (Elymus elymoides and Pseudoroegneria spicata). Results indicate that both plant performance and competitive interactions were influenced by species-specific soil conditioning. Specifically, invasive B. tectorum generated a larger competitive effect on natives than invasive A. cristatum; however, only A. cristatum conditioned soil in a manner that increased competitive effects of conspecifics on natives. Native P. spicata was relatively unaffected by soil conditioning and conversely, E. elymoides was strongly affected by soil conditioning. Few previous studies have examined soil conditioning and the interaction of soil conditioning and neighbor effects that both are potentially important mechanisms in structuring plant communities and influencing plant invasion.     

An uneasy alliance:a nesting association between aggressive ants and equally fierce social wasps

Although the Neotropical territorially dominant arboreal ant Azteca chartifex Forel is very aggressive towards any intruder,its populous colonies tolerate the close presence of the fierce polistine wasp Polybia rejecta(F.).In French Guiana,83.33%of the 48 P.rejecta nests recorded were found side by side with those of A.chartifex.This nesting association results in mutual protection from predators(i.e.,the wasps protected from army ants;the ants protected from birds).We conducted field studies,laboratorybased behavioral experiments and chemical analyses to elucidate the mechanisms allowing the persistence of this association.Due to differences in the cuticular profiles of the two species,we eliminated the possibility of chemical mimicry.Also,analyses of the carton nests did not reveal traces of marking on the envelopes.Because ant forager flows were not perturbed by extracts from the wasps’Dufour’s and venom glands,we rejected any hypothetical action of repulsive chemicals.Nevertheless,we noted that the wasps"scraped"the surface of the upper part of their nest envelope using their mandibles,likely removing the ants'scent trails,and an experiment showed that ant foragers were perturbed by the removal of their scent trails.This leads us to use the term"erasure hypothesis."Thus,this nesting association persists thanks to a relative tolerance by the ants towards wasp presence and the behavior of the wasps that allows them to"contain"their associated ants through the elimination of their scent trails,direct attacks,"wing-buzzing"behavior and ejecting the ants.     

Worker brain development and colony organization in ants: Does division of labor influence neuroplasticity?

Brain compartment size allometries may adaptively reflect cognitive needs associated with behavioral development and ecology. Ants provide an informative system to study the relationship of neural architecture and development because worker tasks and sensory inputs may change with age. Additionally, tasks may be divided among morphologically and behaviorally differentiated worker groups (subcastes), reducing repertoire size through specialization and aligning brain structure with task‐specific cognitive requirements. We hypothesized that division of labor may decrease developmental neuroplasticity in workers due to the apparently limited behavioral flexibility associated with task specialization. To test this hypothesis, we compared macroscopic and cellular neuroanatomy in two ant sister clades with striking contrasts in worker morphological differentiation and colony‐level social organization: Oecophylla smaragdina , a socially complex species with large colonies and behaviorally distinct dimorphic workers, and Formica subsericea , a socially basic species with small colonies containing monomorphic workers. We quantified volumes of functionally distinct brain compartments in newly eclosed and mature workers and measured the effects of visual experience on synaptic complex (microglomeruli) organization in the mushroom bodies—regions of higher‐order sensory integration—to determine the extent of experience‐dependent neuroplasticity. We demonstrate that, contrary to our hypothesis, O. smaragdina workers have significant age‐related volume increases and synaptic reorganization in the mushroom bodies, whereas F. subsericea workers have reduced age‐related neuroplasticity. We also found no visual experience‐dependent synaptic reorganization in either species. Our findings thus suggest that changes in the mushroom body with age are associated with division of labor, and therefore social complexity, in ants. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1072–1085, 2017     

Experimental effects of habitat fragmentation on rove beetles and ants: patch area or edge?

The effects of habitat fragmentation may include the loss of species from isolated fragments or changes in species abundances among habitats that differ in area, structure, or edge characteristics. We measured the species richness and abundance of ground‐dwelling insects in a 1.14‐ha old field that was mowed to produce patches of unmowed vegetation which differed in size, degree of isolation, and the amount of habitat edge. Four treatments – ranging from unfragmented (169‐m²) to highly fragmented (1‐m²) patches – were replicated four times in a Latin square design, and insects were sampled twice during 1995 using 177 pitfall traps. Species richness showed a non‐monotonic response to fragmentation, with the fewest species occurring in the slightly fragmented treatment. Responses of rove beetles and ants, the most species‐rich and abundant taxa, respectively, were similar to the overall insect community but ants had a stronger and more consistent treatment effect in both sample months. Ordinations of ant and rove‐beetle assemblages using nonmetric multidimensional scaling showed that the slightly fragmented treatment differed from other treatments in species occurrence and abundance. The lower species richness in the slightly fragmented treatment was primarily due to a subset of ant and rove beetle species that showed a lower abundance than in other treatments, possibly because this treatment had the greatest amount of habitat edge. We hypothesize that the non‐monotonic species response to fragmentation was due to the differential effects of habitat edge on species movements across the habitat boundary between unmowed patches and mowed areas. A greater effect due to the amount of habitat edge rather than total patch area, at least among the range of patch sizes studied, suggests that the length of habitat edge may be quite important to the distribution and abundance of ground‐dwelling animals in fragmented habitats.     

How do ants stick out their tongues?

The mouthparts are very important tools for almost any task performed by ants. In particular, the labiomaxillary complex is essential for food intake. In the present study we investigated the anatomical design of the labiomaxillary complex in various ant species, focusing on movement mechanisms. Six labial and six maxillary muscles with different functions control the several joints and ensure the proper performance of the labiomaxillary complex. According to our measurements of sarcomere lengths, muscle fiber lengths and diameters, and the relative muscle volumes, the labial and maxillary muscles feature rather slow than fast muscle characteristics and do not seem to be specialized for specific tasks. Since glossa protractor muscles are absent, the protraction of the glossa, the distal end of the labium, is a nonmuscular movement. By histological measurements of hemolymph volumes we could exclude a pressure-driven mechanism. Additional experiments showed that, upon relaxation of the glossa retractor muscles, the glossa protracts elastically. This elastic mechanism possibly sets an upper limit to licking frequency, thus influencing food intake rates and ultimately foraging behavior. In contrast to many other elastic mechanisms among arthropods, glossa protraction in ants is based on a mechanism where elasticity works as an actual antagonist to muscles. We compared the design of the labiomaxillary complex of ants with that of the honeybee and suggest an elastic mechanism for glossa protraction in honeybees as well.     

Congruency of hydrocarbon patterns in heterospecific groups of ants: transfer and/or biosynthesis?

Summary In homospecific groups of ants, each species has its own hydrocarbon profile, on the epicuticle and in the postpharyngeal gland (PPG). When reared together in bispecific groups, workers of both species possess each other's hydrocarbons in both locations. The present study investigated two alternative mechanisms by which a mixed odour in artificial groups ofFormica selysi/Manica rubida can be created. Using [1-¹⁴C] sodium acetate as a precursor,de novo biosynthesis of hydrocarbons was demonstrated for both species whether reared in homospecific or mixed-species groups. The newly synthesized hydrocarbons occurred on the epicuticle, internally, and in particularly large amounts in the PPG. As expected from their PPG and epicuticular hydrocarbons composition, workersF. selysi synthesized alkanes and alkenes in comparable amounts irrespective of their rearing scheme. Likewise,M. rubida reared in bispecific groups synthesized mostly alkanes with only negligible amounts of alkenes, according to a ratio characteristic toM. rubida workers from homospecific groups and not toF. selysi workers. During dyadic encounters, a transfer of labeled hydrocarbons between nestmates (conspecific in homospecific groups and allospecific in mixed groups) was observed. These results suggest that the formation of the mixed hydrocarbon profile in artificial groups of ants is the result of a transfer of these chemicals between nestmates rather thande novo biosynthesis of the allospecific hydrocarbons. Behaviours like trophallaxis, grooming and body contact that occurred during the encounters mediated such a transfer.