Infiltration of a facultative ant–plant mutualism by the introduced Argentine ant: effects on mutualist diversity and mutualism benefits

1. Ant–plant mutualisms have been the focus of considerable empirical research, but few studies have investigated how introduced ants affect these interactions. Using 2 years of survey data, this study examines how the introduced Argentine ant [Linepithema humile (Mayr)] differs from native ants with respect to its ability to protect the extrafloral nectary‐bearing coast barrel cactus (Ferocactus viridescens) in Southern California. 2. Eighteen native ant species visited cacti in uninvaded areas, but cacti in invaded areas were primarily visited by the Argentine ant. The main herbivore of the coast barrel cactus present at the study sites is a leaf‐footed bug (Narnia wilsoni). 3. Herbivore presence (the fraction of surveys in which leaf‐footed bugs were present on individual cacti) was negatively related to ant presence (the fraction of surveys in which ants were present on individual cacti). Compared with cacti in uninvaded areas, those in invaded areas were less likely to have herbivores and when they did had them less often. 4. Seed mass was negatively related to herbivore presence, and this relationship did not differ for cacti in invaded areas versus those in uninvaded areas. 5. Although the Argentine ant might provide superior protection from herbivores, invasion‐induced reductions in ant mutualist diversity could potentially compromise plant reproduction. The cumulative number of ant species on individual cacti over time was lower in invaded areas and was associated with a shortened seasonal duration of ant protection and reduced seed mass. These results support the hypothesis that multiple partners may enhance mutualism benefits.     

Costs and benefits of ant attendance to the drepanosiphid aphid Tuberculatus quercicola

The defensive effects of ants against aphid predators have been well documented in the mutualistic relationship of aphids and their attending ants. However, it is not clear whether ant attendance has any direct effect on the aphids' growth and reproduction. Through field experiments, this study evaluates the benefits and, in particular, the costs of ant attendance to aphid colonies, focusing on the drepanosiphid aphid Tuberculatus quercicola which is associated with the Daimyo oak, Quercus dentata , and which is always attended by the red wood ant Formica yessensis . Ant attendance was clearly beneficial to the aphid; the exclusion of ants led to a significant increase in the extinction rate of aphid colonies. However, MANOVA and randomized block ANOVA indicated that in colonies continuously attended by ants, aphids had significantly smaller body size and produced a smaller number of embryos than in colonies isolated from ants when they were reared under homogeneous host conditions free from natural enemies. Thus, ant attendance had a negative influence on the growth and reproduction of the aphids, even though it contributed to the greater longevity of the aphid colonies. We hypothesize that ant-attended aphids are under intense selective pressures that act against aphid clones which fail to attract many ants, so that aphids have developed an adaptive mechanism to allocate a larger fraction of resources to the honeydew when they are requested to do so by the ants in order to ensure the ants' consistent visitation.     

Costs and benefits of a cleaning mutualism

Juvenile Atlantic salmon or parr ( Salmo salar L.) maintain station at certain locations in flowing stream water. This position choice is assumed to involve the maximization of energy intake, based upon food availability which is usually directly related to water flow rate. Conversely energy expenditure, including station holding behaviour, foraging and defending preferred sites, is inversely related to water flow rate. Adaptations of parr to life in fast flows implies that station holding is energetically inexpensive at water speeds up to the maximum sustained holding speed, which is fish specific, thus the most important energetic consideration for parr is the ability to maximize food intake. Ten groups of three parr were each observed for 60 min within an artificial stream tank over a heterogeneous substratum. Individual position choice and behaviour were recorded continuously. For each location chosen by the parr the potential upstream line‐of‐sight (LOS), defined as the maximum distance upstream that the water surface would be visible, was calculated. At those sites where foraging behaviour was observed, the mean upstream potential LOS was significantly greater than at sites where other behaviours were observed and at 400 randomly generated sites within the tank. When foraging, parr usually take food from the stream drift and there is a significant time expenditure on food location, identification and catching. Results presented here would seem to confirm that to maximize time available to make these decisions, a fish would be expected to maximize the distance over which it can observe potential food particles.     

New mutualism for old: indirect disruption and direct facilitation of seed dispersal following Argentine ant invasion

The indirect effects of biological invasions on native communities are poorly understood. Disruption of native ant communities following invasion by the Argentine ant (Linepithema humile) is widely reported to lead indirectly to the near complete collapse of seed dispersal services. In coastal scrub in southeastern Australia, we examined seed dispersal and handling of two native and two invasive alien plant species at Argentine ant-invaded or -uninvaded sites. The Argentine ant virtually eliminates the native keystone disperser Rhytidoponera victoriae, but seed dispersal did not collapse following invasion. Indeed, Argentine ants directly accounted for 92% of all ant-seed interactions and sustained overall seed dispersal rates. Nevertheless, dispersal quantity and quality among seed species differed between Argentine ant-invaded and -uninvaded sites. Argentine ants removed significantly fewer native Acacia retinodes seeds, but significantly more small seeds of invasive Polygala myrtifolia than did native ants at uninvaded sites. They also handled significantly more large seeds of A. sophorae, but rarely moved them >5 cm, instead recruiting en masse, consuming elaiosomes piecemeal and burying seeds in situ. In contrast, Argentine ants transported and interred P. myrtifolia seeds in their shallow nests. Experiments with artificial diaspores that varied in diaspore and elaiosome masses, but kept seed morphology and elaiosome quality constant, showed that removal by L. humile depended on the interaction of seed size and percentage elaiosome reward. Small diaspores were frequently taken, independent of high or low elaiosome reward, but large artificial diaspores with high reward instead elicited mass recruitment by Argentine ants and were rarely moved. Thus, Argentine ants appear to favour some diaspore types and reject others based largely on diaspore size and percentage reward. Such variability in response indirectly reduces native seed dispersal and can directly facilitate the spread of an invasive alien shrub.     

Ant benefits in a seed dispersal mutualism

Myrmecochorous plant seeds have nutrient rich appendages, elaiosomes, which induce some ant species to carry the seeds back to their nest where the elaiosome is consumed and the seed is discarded unharmed. The benefits to plants of dispersal of their seeds in this way have been well documented, but the benefits to the ants from consuming the elaiosomes have rarely been measured and are less clear. Ant benefits from myrmecochory were investigated in a laboratory experiment using the ant Myrmica ruginodis and seeds of Ulex species. To separate the effects of elaiosome consumption on the development of newly produced larvae versus existing larvae, ten ‘Queenright’ colonies containing a queen were compared to ten ‘Queenless’ colonies. Six measures of colony fitness over a complete annual cycle were taken: sexual production, larval weight and number, pupal weight and number, and worker survival. Queenless colonies fed with elaiosomes produced 100.0±29.3 (mean ± SE) of larvae compared to non-elaiosome fed colonies which produced 49.6±19.0; an increase of 102%. Larval weight increased in both Queenright and Queenless colonies. In colonies fed with elaiosomes, larvae weighed 1.02±0.1 mg, but in non-elaiosome fed colonies larvae weighed 0.69±0.1 mg; an increase of 48%. The food supplement provided by Ulex elaiosomes was trivial in energetic terms, under the conditions of an ample diet, suggesting that these effects might be due to the presence of essential nutrients. Chemical analysis of Ulex elaiosomes showed the presence of four essential fatty acids and four essential sterols for ants.     

Exploiting a mutualism: parasite specialization on cultivars within the fungus-growing ant symbiosis

Fungus-growing ants, their cultivated fungi and the cultivar-attacking parasite Escovopsis coevolve as a complex community. Higher-level phylogenetic congruence of the symbionts suggests specialized long-term associations of host-parasite clades but reveals little about parasite specificity at finer scales of species-species and genotype-genotype interactions. By coupling sequence and amplified fragment length polymorphism genotyping analyses with experimental evidence, we examine (i) the host specificity of Escovopsis strains infecting colonies of three closely related ant species in the genus Cyphomyrmex, and (ii) potential mechanisms constraining the Escovopsis host range. Incongruence of cultivar and ant relationships across the three focal Cyphomyrmex spp. allows us to test whether Escovopsis strains track their cultivar or the ant hosts. Phylogenetic analyses demonstrate that the Escovopsis phylogeny matches the cultivar phylogeny but not the ant phylogeny, indicating that the parasites are cultivar specific. Cross-infection experiments establish that ant gardens can be infected by parasite strains with which they are not typically associated in the field, but that infection is more likely when gardens are inoculated with their typical parasite strains. Thus, Escovopsis specialization is shaped by the parasite's ability to overcome only a narrow range of garden-specific defences, but specialization is probably additionally constrained by ecological factors, including the other symbionts (i.e. ants and their antibiotic-producing bacteria) within the coevolved fungus-growing ant symbiosis.     

Cheating in mutualism: defection of Yucca baccata against its yucca moths

Yucca baccata cheats in its obligate pollination/seed predation mutualism with yucca moths. Although all individuals use the pollination services of yucca moths, many individuals do not reciprocate in sustaining yucca moth larvae. Cheating is associated with the morphology of Y. baccata pistils. In Y. baccata , the apex of the ovary contains only inviable ovules, and there are two distinct flower types, one of which has twice as many potentially viable ovules as the other. Because yucca moths oviposit at the apex of Y. baccata ovaries, larvae in flowers with few viable ovules fail to encounter viable ovules and therefore perish. Inflorescences generally have just one flower type, implying that some individuals cheat whereas others maintain the yucca moth population. Our most surprising observation, however, is that although the proportion of cheaters should be low, over 70% of Y. baccata individuals cheat. We hypothesize that both density- and frequency-dependent processes maintain a balance of cheaters and noncheaters in this system.     

Costs of ant attendance for aphids

1. Interactions between aphids and ants are considered to be mutualistic, with both partners benefiting. Costs associated with such interactions are likely to be less obvious, although they can be expected, especially if these associations are facultative.
2. It is demonstrated here that there are costs in several life-history parameters to individual aphids resulting from ant attendance. Over several generations Aphis fabae cirsiiacanthoides feeding on Cirsium arvense , at a range of developmental stages, suffered significant costs when tended by Lasius niger , e.g. in terms of a prolonged developmental time, delayed offspring production, proportionally smaller gonads, fewer well developed embryos and a reduced mean relative growth rate. These effects are similar to those observed when aphids feed on poor quality plants.
3. This is the first indication that there is a cost for aphids associated with ant attendance. The significance of this for the evolution of ant attendance in aphids is discussed.     

Ants andPolyommatus icarus immatures (Lycaenidae) —sex-related developmental benefits and costs of ant attendance

Summary Third and fourth instar larvae and pupae of the facultatively myrmecophilous Palaearctic blue butterflyPolyommatus icarus showed no alteration in developmental time when reared in the presence of two species ofLasius ants. Sex differences were observed in larval growth and pupal weight, with males growing larger and faster. Sex-related differences also occurred in the costs and benefits of ant-attendance. Male pupal masses tended to be larger in individuals associated with ants, and their pupal weight loss was not enhanced by ant attendance. This positive developmental effect of myrmecophily is tentatively attributed to a stimulating influence of ants on caterpillar feeding behavior. In contrast, females associated with ants tended to lose more weight during the pupal stage. Hence there is evidence for developmental benefits, rather than costs, of myrmecophily in maleP. icarus immatures, whereas ant attendance appears to be more costly for females during the pupal stage. These findings are discussed in relation to data on other myrmecophilous lycaenid species. It is suggested that maintaining low-level myrmecophily and its related organs is a comparatively inexpensive evolutionary stable strategy among this butterfly group.     

Aphid–ant mutualism: how honeydew sugars influence the behaviour of ant scouts

Honeydew is the keystone on which ant–aphid mutualism is built. The present study investigates how each sugar identified in Aphis fabae Scopoli honeydew acts upon the feeding and the laying of a recruitment trail by scouts of the aphid‐tending ant Lasius niger Linnaeus, and thus may enhance collective exploitation by the ant mutualists. The feeding preferences shown by L. niger for honeydew sugars are: melezitose = sucrose = raffinose > glucose = fructose > maltose = trehalose = melibiose = xylose. Although feeding is a prerequisite to the launching of trail recruitment, the reverse is not necessarily true: not all ingested sugar solutions elicit a trail‐laying behaviour among fed scouts. Trail mark laying is only triggered by raffinose, sucrose or melezitose, with the latter sugar being specific to honeydew. By comparing gustatory and recruitment responses of ant foragers to sugar food sources, the present study clarifies the role of honeydew composition both as a source of energy and as a mediator in ant–aphid interactions. Lasius niger feeding preferences can be related to the physiological suitability of each sugar (i.e. their detection by gustatory receptors as well as their ability to be digested and converted into energy). Regarding recruitment, the aphid‐synthesized oligosaccharide (melezitose) could be used by ant scouts as a cue indicative of a long‐lasting productive resource that is worthy of collective exploitation and defence against competitors or aphid predators.     

Assembling a mutualism: ant symbionts locate their host plants by detecting volatile chemicals

In most mutualisms, partners disperse independently of each other. For instance, in ant-plant symbioses, plants disperse as seeds, and ants disperse as winged queens. For an ant-plant mutualism to persist, therefore, queens must be able to locate and colonise host plant saplings. It has been suggested that host plants emit volatile chemical cues that attract dispersing queens, but this has never been demonstrated experimentally. We used a Y-tube olfactometry protocol to test this hypothesis in the tropical understorey antplant Cordia nodosa Lam. (Boraginaceae), which associates with two genera of ants, Azteca (Dolichoderinae) and Allomerus (Myrmicinae). Both genera show significant attraction to the volatiles of C. nodosa over control understorey plant species that do not associate with ants. These results support the hypothesis that ants are attracted to volatiles emitted by their host plant and suggest a key preadaptation that promoted the evolution of ant-plant symbioses. Received 1 July 2005; revised 2 November 2005; accepted 8 November 2005.     

A carnivorous plant fed by its ant symbiont: a unique multi-faceted nutritional mutualism

Scarcity of essential nutrients has led plants to evolve alternative nutritional strategies, such as myrmecotrophy (ant-waste-derived nutrition) and carnivory (invertebrate predation). The carnivorous plant Nepenthes bicalcarata grows in the Bornean peatswamp forests and is believed to have a mutualistic relationship with its symbiotic ant Camponotus schmitzi. However, the benefits provided by the ant have not been quantified. We tested the hypothesis of a nutritional mutualism, using foliar isotopic and reflectance analyses and by comparing fitness-related traits between ant-inhabited and uninhabited plants. Plants inhabited by C. schmitzi produced more leaves of greater area and nitrogen content than unoccupied plants. The ants were estimated to provide a 200% increase in foliar nitrogen to adult plants. Inhabited plants also produced more and larger pitchers containing higher prey biomass. C. schmitzi-occupied pitchers differed qualitatively in containing C. schmitzi wastes and captured large ants and flying insects. Pitcher abortion rates were lower in inhabited plants partly because of herbivore deterrence as herbivory-aborted buds decreased with ant occupation rate. Lower abortion was also attributed to ant nutritional service. The ants had higher δ(15)N values than any tested prey, and foliar δ(15)N increased with ant occupation rate, confirming their predatory behaviour and demonstrating their direct contribution to the plant-recycled N. We estimated that N. bicalcarata derives on average 42% of its foliar N from C. schmitzi wastes, (76% in highly-occupied plants). According to the Structure Independent Pigment Index, plants without C. schmitzi were nutrient stressed compared to both occupied plants, and pitcher-lacking plants. This attests to the physiological cost of pitcher production and poor nutrient assimilation in the absence of the symbiont. Hence C. schmitzi contributes crucially to the nutrition of N. bicalcarata, via protection of assimilatory organs, enhancement of prey capture, and myrmecotrophy. This combination of carnivory and myrmecotrophy represents an outstanding strategy of nutrient sequestration.     

Induced indirect defence in a lycaenid-ant association: the regulation of a resource in a mutualism

Indirect defences involve the protection of a host organism by a mutualistic partner. Threat of predation to the host organism may induce the production of rewards and/or signals that attract the mutualistic partner. In laboratory and field experiments we show that threatened lycaenid butterfly larvae (Plebejus acmon) produce more nectar rewards from their gland and were tended by protective ants twice as much as controls. Ant attendance did not affect the leaf consumption or feeding behaviour of larvae in the absence of predators. Inducible nectar production and indirect defence in this system may be a mechanism by which larvae provide rewards for services when they are needed the most. Such a system may stabilize the mutualistic association between lycaenid larvae and ants by preventing exploitation by either partner.     

Experimental evidence of a tripartite mutualism: bacteria protect ant fungus gardens from specialized parasites

Symbioses shape all levels of biological organization. Although symbiotic interactions are typically viewed as bipartite associations, with two organisms interacting largely in isolation from other organisms, the presence and importance of additional symbionts is becoming increasingly more apparent. This study examines the importance of a third mutualist within the ancient symbiosis between leaf-cutting ants and their fungal cultivars. Specifically, we experimentally examine the role of a filamentous bacterium (actinomycete), which is typically carried on the cuticle of fungus-growing ants, in suppressing the growth of a specialized microfungal parasite ( Escovopsis ) of the fungus garden. We conducted two-by-two factorial design experiments crossing the presence/absence of actinomycete with the presence/absence of Escovopsis within small sub-colonies of Acromyrmex octospinosus . In these experiments, infection by Escovopsis became much more extensive within fungus gardens and had a greater impact on the health of gardens in those sub-colonies with the bacterium removed from workers as compared to gardens with the bacterium still present on the ants. We establish that the actinomycete bacterium is most abundant on those major workers tending the garden, providing further support that the bacterium is involved in garden hygiene. We also found a significantly higher abundance of actinomycete on workers in colonies experimentally infected with Escovopsis as compared to uninfected control colonies. We suggest that mutualisms between antibiotic-producing microbes and higher organisms may be common associations that are mostly overlooked and that the role of symbionts in reducing the impact of parasites is likely an important aspect in the cost-benefit assessment of mutualisms.     

Population dynamics and mutualism: functional responses of benefits and costs

We develop an approach for studying population dynamics resulting from mutualism by employing functional responses based on density-dependent benefits and costs. These functional responses express how the population growth rate of a mutualist is modified by the density of its partner. We present several possible dependencies of gross benefits and costs, and hence net effects, to a mutualist as functions of the density of its partner. Net effects to mutualists are likely a monotonically saturating or unimodal function of the density of their partner. We show that fundamental differences in the growth, limitation, and dynamics of a population can occur when net effects to that population change linearly, unimodally, or in a saturating fashion. We use the mutualism between senita cactus and its pollinating seed-eating moth as an example to show the influence of different benefit and cost functional responses on population dynamics and stability of mutualisms. We investigated two mechanisms that may alter this mutualism's functional responses: distribution of eggs among flowers and fruit abortion. Differences in how benefits and costs vary with density can alter the stability of this mutualism. In particular, fruit abortion may allow for a stable equilibrium where none could otherwise exist.     

Indirect interactions between two aphid species in relation to ant attendance

The influence of foraging by the ant, Lasius niger, on the population growth of two aphid species, Lachnus tropicalis and Myzocallis kuricola, on chestnut trees, Castanea crenata, was examined. The ant-tending effect was divergent depending on the aphid density per ant: it was positive when there were few aphids per ant, but negative when there were many aphids per ant. In addition, the density of one aphid species also influenced the ant-tending effect on the other aphid. Furthermore, the influences were asymmetrical: an increase in L. tropicalis density per ant reversed the ants effect on this species and on M. kuricola, while an increase in M. kuricola per ant did not significantly influence the ants effect on L. tropicalis. Thus, the ant seems to stabilize the L. tropicalis population density and keep this species from extinction, while the ants effect on M. kuricola depends on the density of L. tropicalis and may lead M. kuricola to extermination. This change in the ant-tending effect corresponds to the previously detected density-dependent change in predation activity of the ants on aphids. In contrast, the density-dependent change in the protection effect of the ants against natural enemies does not explain the results.     

Adding biotic complexity alters the metabolic benefits of mutualism

Mutualism is ubiquitous in nature and plays an integral role in most communities. To predict the eco‐evolutionary dynamics of mutualism it is critical to extend classic pair‐wise analysis to include additional species. We investigated the effect of adding a third species to a pair‐wise mutualism in a spatially structured environment. We tested the hypotheses that selection for costly excretions in a focal population (i) decreases when an exploiter is added (ii) increases when a third mutualist is added relative to the pair‐wise scenario. We assayed the selection acting on Salmonella enterica when it exchanges methionine for carbon in an obligate mutualism with an auxotrophic Escherichia coli. A third bacterium, Methylobacterium extorquens, was then added and acted either as an exploiter of the carbon or third obligate mutualist depending on the nitrogen source. In the tripartite mutualism M. extorquens provided nitrogen to the other species. Contrary to our expectations, adding an exploiter increased selection for methionine excretion in S. enterica. Conversely, selection for cooperation was lower in the tripartite mutualism relative to the pair‐wise system. Genome‐scale metabolic models helped identify the mechanisms underlying these changes in selection. Our results highlight the utility of connecting metabolic mechanisms and eco‐evolutionary dynamics.     

Overcompensation of plants in response to herbivory and the by-product benefits of mutualism

Plants that overcompensate for herbivory are relatively healthier when damaged. In this mutualistic association, the herbivore benefits from the plant, and the plant benefits from the herbivore's actions. As long as the benefit to the plant outweighs the costs imposed by browsing herbivores, this interaction should remain stable. Many apparently parasitic associations can be mutually beneficial under some environmental conditions.     

Effects of a belowground mutualism on an aboveground mutualism

Studies of multitrophic interactions between below‐ and aboveground communities have generally focused on soil organisms and antagonists of plant shoots and leaves (herbivores). Despite the widespread occurrence of plant mutualists below‐ and aboveground which can occur on the same host plant, the potential for interactions between them has not been considered. Here we demonstrate that aboveground plant mutualists, insect pollinators, are strongly influenced by belowground plant mutualists, arbuscular mycorrhizal fungi. The presence of arbuscular mycorrhizal fungi in the roots of Chamerion angustifolium increased pollinator visitation and per cent seed set of this plant in the field by up to two times compared with non‐mycorrhizal plants. We propose that interactions between belowground and aboveground mutualisms are widespread and may play important functional roles in populations and communities.     

Lack of ant attendance may induce compensatory plant growth

Three levels in ant–plant protection systems need to be considered to fully understand how these symbiotic systems work. Here we present the effect of Oecophylla smaragdina ants on (1) the arthropod community, (2) herbivory, and (3) plant performance, within a studied mangrove ant–plant protection system. On Rhizophora mucronata trees in Thailand ants successfully colonised ant trees attached with a string to a natural ant tree, whereas they were unable to colonise control trees without this connection. Trees were monitored and arthropods (numbers and composition), leaf damage, leaf turnover and growth rates (stem diameter, tree height and total leaf area) were recorded in two surveys covering a period of 12 months. The number of herbivorous arthropods, but not the number of predators, was significantly lower on ant trees compared to control trees. Likewise, the amount of leaf damage inflicted by the four major groups of herbivores (Chrysomelidae, Tortricidae, Geometridae and Sesarminae) was significantly lower on ant trees compared to control trees and so was the leaf turnover rate. In spite of this, the released herbivore pressure on ant trees did not translate into higher growth rates. In contrast, all growth responses increased more on control trees compared to ant trees. Differences between the two groups were insignificant but leaf area increase was only marginally nonsignificant (P=0.062). The results show that ants remove herbivorous arthropods more efficiently than predators but ant-colonised mangroves do not necessarily benefit from this despite the resulting decrease in herbivory.