Influence of elaiosome on postdispersal dynamics of an ant-dispersed plant

Specialized myrmecochorous plants bear an elaiosome attached to the seed. Ants benefit from food provided by this appendage and plants get their seeds eventually dispersed. Aside from the benefits conferred by seed dispersal, we hypothesize that the influence of the elaiosome might extend to later life history stages such as emergence or survival, but few studies have examined rigorously such delayed effects on seed and seedling dynamics. This paper tests this hypothesis for the ant-dispersed herb Helleborus foetidus. Presence or lack of the elaiosome had some interesting delayed influences. Specifically, our results show that elaiosome removal not only enhanced seedling emergence, but also reduced cumulative long-term seed predation. The combination of these two effects, together with the inherent positive effect of the elaiosome on seed dispersal, highlights the ecological and evolutionary importance of myrmecochorous systems.     

The effect of ants on the seed dispersal cycle of the typical myrmecochorous <Emphasis Type="Italic">Ricinus communis</Emphasis>

The directed dispersal hypothesis has two components: (1) non-random seed deposition by a predicted vector, which removes greater amounts of seeds to specific sites, and (2) higher seed survival and seedling establishment in these specific sites. Several studies suggest that ants perform both tasks. This study was designed to cover the processes from post-dispersed seeds to established juvenile plants of a typical ant-dispersed species. Our main objective was to determine whether Ricinus communis benefits from directed dispersal by ants to maintain its populations in previously colonized habitats. We examined whether there were differences between ant nest pile mounds and their vicinities in the: (1) densities of seeds with and without elaiosome, seedlings and juveniles; (2) performance of post-dispersed seeds (without elaiosome), which may be affected by seed density, a key feature attracting seed predators; (3) nutrient quantities; (4) number of germinated seeds and juvenile biomass measurements; and (5) ant protection of seedlings from herbivores. There were more seeds without elaiosome, seedlings and juveniles in pile mounds, and seeds with elaiosome were equally distributed. There was no difference in the number of non-removed seeds in pile mounds and in their vicinities, and there was no tendency for this difference to increase or decrease with time or with initial seed density. Apparently, there was no difference in nutrient contents in soils of pile mounds and their vicinities. Likewise, there was no difference in the number of seeds germinated and in the biomass measurements of juveniles in both soils. Ants did not provide differential protection for seedlings in pile mounds against potential herbivores. The dispersal of Ricinus seeds by ants had a marked effect on the distribution pattern of the seeds, seedlings and juveniles of this species. However, there were no additional advantages for the seeds, seedlings and juveniles in pile mounds and, therefore, Ricinus does not benefit from directed dispersal by ants to maintain its populations in the study sites.     

Development and Function of the Elaiosome in Knautia (Dipsacaceae)

The fruits of the genus Knautia differ from those of all other Dipsacaceae by the possession of a white, ± hemispherical basal appendage (elaiosome). In a series of field experiments Sernander (1906) demonstrated that, due to this elaiosome, the fruits of Knautia are dispersed by ants. Sernander (1906) assumed that the elaiosome is built from fused bracts (forming the epicalyx) whereas Bresinsky (1963) interpreted it as part of the perianth. In the present study the elaiosome development was investigated in four species of all three subgenera of Knautia: subg. Trichera (K. drymeia, K. dipsacifolia), subg. Tricheranthes (K. integrifolia) and subg. Knautia (K. orientalis). A comparison with early stages of the fruit development in other Dipsacaceae (e.g., Lomelosia graminifolia) indicates that the elaiosome is of epicalyx origin. In all species of Knautia the development of the elaiosome begins prior to anthesis and independently of fertilization. The growth of the elaiosome tissue in K. orientalis is mainly caused by periclinal cell divisions in the subepidermal layers of the epicalyx base. In K. dipsacifolia and K. drymeia a meristem around the base of the central bundle, which supplies epicalyx and flower, becomes active and develops cells in cascade-like rows. The elaiosome cells of ripe fruits are filled with numerous oil droplets. Moreover the central bundle increases by the formation of additional phloem cells. In addition, other important morphological adaptations to ant dispersal are demonstrated: the hair-cover of the fruit and strongly lignified horizontal vascular bundles between elaiosome and seed prevent the seed from being eaten; a lignified cramp anchors the elaiosome to the fruit; and thickened and strongly cutinized walls of the epidermis protect the elaiosome during dispersal. In regard to germination, a correlation between the removal of the elaiosome and germination speed and rate was found; fruits with dissected elaiosomes germinate faster and with a higher germination rate than fruits with elaiosomes.     

Effects of seed morphology and elaiosome chemical composition on attractiveness of five Trillium species to seed‐dispersing ants

Morphological and chemical attributes of diaspores in myrmecochorous plants have been shown to affect seed dispersal by ants, but the relative importance of these attributes in determining seed attractiveness and dispersal success is poorly understood. We explored whether differences in diaspore morphology, elaiosome fatty acids, or elaiosome phytochemical profiles explain the differential attractiveness of five species in the genus Trillium to eastern North American forest ants. Species were ranked from least to most attractive based on empirically‐derived seed dispersal probabilities in our study system, and we compared diaspore traits to test our hypotheses that more attractive species will have larger diaspores, greater concentrations of elaiosome fatty acids, and distinct elaiosome phytochemistry compared to the less attractive species. Diaspore length, width, mass, and elaiosome length were significantly greater in the more attractive species. Using gas chromatography–mass spectrometry, we found significantly higher concentrations of oleic, linoleic, hexadecenoic, stearic, palmitoleic, and total fatty acids in elaiosomes of the more attractive species. Multivariate assessments revealed that elaiosome phytochemical profiles, identified through liquid chromatography–mass spectrometry, were more homogeneous for the more attractive species. Random forest classification models (RFCM) identified several elaiosome phytochemicals that differed significantly among species. Random forest regression models revealed that some of the compounds identified by RFCM, including methylhistidine (α‐amino acid) and d‐glucarate (carbohydrate), were positively related to seed dispersal probabilities, while others, including salicylate (salicylic acid) and citrulline (L‐α‐amino acid), were negatively related. These results supported our hypotheses that the more attractive species of Trillium—which are geographically widespread compared to their less attractive, endemic congeners—are characterized by larger diaspores, greater concentrations of fatty acids, and distinct elaiosome phytochemistry. Further advances in our understanding of seed dispersal effectiveness in myrmecochorous systems will benefit from a portrayal of dispersal unit chemical and physical traits, and their combined responses to selection pressures.     

Odor of achlorophyllous plants’ seeds drives seed‐dispersing ants

Seed dispersal by ants is an important means of migration for plants. Many myrmecochorous plants have specialized appendages in their seeds called elaiosome, which provides nutritional rewards for ants, and enable effective seed dispersal. However, some nonmyrmecochorous seeds without elaiosomes are also dispersed by ant species, suggesting the additional mechanisms other than elaiosomes for seed dispersal by ants. The seeds of the achlorophyllous and myco‐heterotrophic herbaceous plant Monotropastrum humile are very small without elaiosomes; we investigated whether odor of the seeds could mediate seed dispersal by ants. We performed a bioassay using seeds of M. humile and the ant Nylanderia flavipes to demonstrate ant‐mediated seed dispersal. We also analyzed the volatile odors emitted from M. humile seeds and conducted bioassays using dummy seeds coated with seed volatiles. Although elaiosomes were absent from the M. humile seeds, the ants carried the seeds to their nests. They also carried the dummy seeds coated with the seed volatile mixture to the nest and left some dummy seeds inside the nest and discarded the rest of the dummy seeds outside the nest with a bias toward specific locations, which might be conducive to germination. We concluded that, in M. humile seeds, volatile odor mixtures were sufficient to induce seed‐carrying behavior by the ants even without elaiosomes.     

The possible advantage of myrmecochory in diplochorous species: A test on two Mediterranean Euphorbia species

Myrmecochory commonly complements the advantages of ballistic dispersal in diplochorous species. We studied the role of the elaiosome in two populations of the two diplochorous Mediterranean spurges Euphorbia boetica and E. nicaeensis, which share an efficient ballistic dispersal mechanism followed by secondary removal by ants. They differ in elaiosome persistence, as most E. boetica seeds lose the elaiosome during explosive dispersal. Self-assessed dietary preferences with seeds with and without elaiosomes of each species showed differences in behaviour among and within ant species. In general, the absence of elaiosome entailed a decrease in the number of disperser ant species interacting with the seeds, whereas the number of predatory ants remains invariable. However, in one population of E. nicaeensis, experimental elimination of the elaiosome did not affect seed removal by mutualistic ants. On the other hand, analysis of refuse piles of the granivorous Messor marocanus and M. bouvieri suggests that they act as seed predators in E. boetica, whereas unintentional dispersal can be important in E. nicaeensis. We suggest, therefore, that the presence of the elaiosome in the seeds of the studied spurges increases the interaction with disperser ant species, but the possible dispersal advantage is not apparent and is spatially variable.     

Steep Decline and Cessation in Seed Dispersal by Myrmica rubra Ants

Myrmecochorous diaspores bear a nutrient-rich appendage, the elaiosome, attractive to ant workers that retrieve them into the nest, detach the elaiosome and reject the seed intact. While this interaction is beneficial for the plant partner by ensuring its seed dispersal, elaiosome consumption has various effects −positive, negative or none − on ants’ demography and survival, depending on both the ant/plant species involved. In this context, the contribution of ants to seed dispersal strongly varies according to the ant/plant pairs considered. In this paper, we investigate whether the dynamics of myrmecochory also vary on a temporal scale, for a given pair of partners: Myrmica rubra ants and Viola odorata seeds. During their first encounter with seeds, ants collect all the diaspores and eat the majority of elaiosomes. Both the harvesting effort and the elaiosome consumption decline when seeds are offered on the next week and completely cease for the following weeks. This is related to a decrease in the number of foragers reaching the food source, as well as to a reduced probability for an ant contacting a seed to retrieve it. Seed retrieval is not reactivated after seven weeks without any encounter with V. odorata seeds. By contrast, naive ant colonies only fed with fruit flies do not show a decline of prey harvesting of which the speed of retrieval even increases over the successive weeks. Myrmecochory may thus be labile at the scale of a fruiting season due to the ability of ants to steeply tune and cease for several months the harvesting of these seemingly poorly rewarding items and to maintain cessation of seed exploitation. The present study emphasizes the importance of a long-lasting follow up of the myrmecochory process, to assess the stability of this ant-plant partnership and to identify mechanisms of adaptive harvesting in ants.     

Multiphase myrmecochory: the roles of different ant species and effects of fire

Seed dispersal by ants (myrmecochory) can be influenced by changes to ant assemblages resulting from habitat disturbance as well as by differences in disperser behaviour. We investigated the effect of habitat disturbance by fire on the dispersal of seeds of a myrmecochorous shrub, Pultenaea daphnoides. We also investigated the consequence of the seed relocation behaviours of two common dispersers (Pheidole sp. A and Rhytidoponera metallica) for the redispersal of seeds. Pheidole sp. A colonies did not relocate seeds outside their nests. In contrast, R. metallica colonies relocated 43.6 % of seeds fed to them, of which 96.9 % had residual elaiosome that remained attached. On average, R. metallica relocated seeds 78.9 and 60.7 cm from the nest entrances in burned and unburned habitat, respectively. Seeds were removed faster in burned than in unburned habitat, and seeds previously relocated by R. metallica were removed at similar rates to seeds with intact elaiosomes, but faster than seeds with detached elaiosomes. Dispersal distances were not significantly different between burned (51.3 cm) and unburned (70.9 cm) habitat or between seeds with different elaiosome conditions. Differences between habitat types in the frequency of seed removal, the shape of the seed dispersal curve, and the relative contribution of R. metallica and Pheidole sp. A to seed dispersal were largely due to the effect of recent fire on the abundance of Pheidole sp. A. Across habitat types, the number of seeds removed from depots and during dispersal trials most strongly related to the combined abundances of R. metallica and Pheidole. Our findings show that myrmecochory can involve more than one dispersal phase and that fire indirectly influences myrmecochory by altering the abundances of seed-dispersing ants.     

Geographic variation for elaiosome–seed size ratio and its allometric relationship in two closely related Corydalis species

Abstract

Background: Elaiosomes serve as a reward for seed-dispersing ants. The size of the elaiosome and the elaiosome–seed mass (ESM) ratio are expected to affect seed dispersal, whilst seed size affects the establishment of seedlings. Elaiosomes and seeds can undergo independent change, thus variation in these traits may arise through heterogeneous selection for seedling establishment and for dispersal. Only a few studies have examined intraspecific variation in these traits.

Aims: The aim of this study was to determine if the ratio of elaiosome–seed mass was different in two co-occurring congeneric plant species (Corydalis intermedia and C. solida). Under the hypothesis that the ESM ratio affected diaspore attractiveness to ants, the allometric relationship between elaiosome and seed mass was used to infer how selection by ants may shape the diaspore.

Methods: Seed and elaiosome mass were measured in fruits collected from central and marginal populations of the two plant species. The allometric relationship between seed and elaiosome mass was obtained using major axis regression. In situ cafeteria experiments estimated the removal rate of diaspores by ants.

Results: Intra- and interspecific variation in diaspore traits were found. The selfing C. intermedia produced heavier seeds and elaiosomes than the outcrossing C. solida. Ants removed more diaspores from C. intermedia where both species were present. A slope larger than one characterised the allometric relationship between elaiosome and seed mass in both species. This slope was twice as steep in the central populations of C. intermedia compared to marginal ones.

Conclusions: Results indicate that in C. intermedia elaiosome mass must increase more than proportionally with increasing seed mass for the diaspore to remain attractive to ants. The direction of interspecific differences suggests that a plant-mating system may affect selection for dispersal.     

SEED DISPERSAL OF DENDROMECON: ITS ECOLOGIC,EVOLUTIONARY, AND TAXONOMIC SIGNIFICANCE

Both species of Dendromecon are diplochorous, that is, the seeds are first scattered from the mother plant by violently dehiscing capsules and then transported further away by ants, which are strongly attracted to the elaiosome at the micropylar end of the seed. The tensions that cause the capsule to explode develop between the two carpels and also between lateral and central fiber strands in the capsular valves. The elaiosome is unusual in that it contains large quntities of starch in addition to the commonly present fatty oils and that it is more drought resistant than most known elaiosomes. Seed dispersal is very efficient over short distances, but migration barriers a few hundred meters or more cannot be crossed except by chance dispersal. Dendromecon differs strikingly from most ant-dispersed plants in being woody and xerophytic; so far, D. rigida is the only chaparral plant known to be habitually dispersed by ants. The presence of an auto-chorous-myrmecochorous type of seed dispersal in D. rigida may partly explain the polymorphism of this species, since this type of seed dispersal tends to keep local populations isolated. The nearly complete correspondence, anatomically, morphologically, and functionally, between the fruit of Dendromecon and that of Eschscholzia strongly supports the idea of a close taxonomic relationship between these two genera. The seed appendage is believed to be a “recent” specialization in Dendromecon, phylogenetically independent from seed appendages facilitating dispersal by ants in other genera of the Papaveraceae.     

Species,Diaspore Volume and Body Mass Matter in Gastropod Seed Feeding Behavior

Background

Seed dispersal of ant-dispersed plants (myrmecochores) is a well studied ecosystem function. Recently, slugs have been found to act as seed dispersers of myrmecochores. The aim of our study was to (1) further generalize the finding that gastropods feed on seeds of myrmecochores and hence may act as seed dispersers, (2) to test whether gastropod body mass and the volume of diaspores have an influence on the seed dispersal potential.

Methodology and Principal Findings

We assessed the seed dispersal potential of four slug and snail species with a set of seven myrmecochorous plant species from seven different plant families common to Central European beech forests. Diaspores differed in shape and size. Gastropods differed in their readiness to feed on diaspores and in the proportion of seeds that were swallowed as a whole, and this readiness generally decreased with increasing diaspore size. Smaller Arionid slugs (58 mm body length; mean) mostly fed on the elaiosome but also swallowed small diaspores and therefore not only act as elaiosome consumers, a nutrient rich appendage on myrmecochorous diaspores, but may also disperse seeds. Large Arionid slugs (>100 mm body length) swallowed diaspores of all sizes. Diaspores swallowed by gastropods were defecated without damage. Within-species variability in body size also affect seed dispersal potential, as larger individuals of the red slug (Arion rufus) swallowed more diaspores of wood anemone (Anemone nemorosa) than smaller ones.

Conclusions and Significance

Our results help to generalize the finding that gastropods consume and potentially disperse seeds of myrmecochores. The dispersal potential of gastropods is strongly influenced by diaspore size in relation to gastropod size.     

Mutualism vs. antagonism in introduced and native ranges: Can seed dispersal and predation determine Acacia invasion success?

Plant species introduced to new regions can escape their natural enemies but may also lose important mutualists. While mutualistic interactions are often considered too diffuse to limit plant invasion, few studies have quantified the strength of interactions in both the native and introduced ranges, and assessed whether any differences are linked to invasion outcomes. For three Acacia species adapted for ant dispersal (myrmecochory), we quantified seed removal probabilities associated with dispersal and predation in both the native (Australian) and introduced (New Zealand) ranges, predicting lower removal attributable to dispersal in New Zealand due to a relatively depauperate ant fauna. We used the role of the elaiosome to infer myrmecochory, and included treatments to measure vertebrate seed removal, since this may become an important determinant of seed fate in the face of reduced dispersal. We then tested whether differences in seed removal patterns could explain differences in the invasion success of the three Acacia species in New Zealand.Overall seed removal by invertebrates was lower in New Zealand relative to Australia, but the difference in removal between seeds with an elaiosome compared to those without was similar in both countries. This implies that the probability of a removed seed being dispersed by invertebrates was comparable in New Zealand to Australia. The probability of seed removal by vertebrates was similar and low in both countries. Differences in the invasive success of the three Acacia species in New Zealand were not explained by differences in levels of seed predation or the strength of myrmecochorous interactions. These findings suggest that interactions with ground foraging seed predators and dispersers are unlikely to limit the ability of Acacia species to spread in New Zealand, and could not explain their variable invasion success.     

Fragmentation, habitat composition and the dispersal/predation balance in interactions between the Mediterranean myrtle and avian frugivores

Human‐induced fragmentation and disturbance of natural habitats can shift abundance and composition of frugivore assemblages, which may alter patterns of frugivory and seed dispersal. However, despite their relevance to the functioning of ecosystems, plant‐frugivore interactions in fragmented areas have been to date poorly studied. I investigated spatial variation of avian frugivore assemblages and fruit removal by dispersers and predators from Mediterranean myrtle shrubs (Myrtus communis) in relation to the degree of fragmentation and habitat features of nine woodland patches (72 plants). The study was conducted within the chronically fragmented landscape of the Guadalquivir Valley (SW Spain), characterized by ~1% of woodland cover. Results showed that the abundance and composition of the disperser guild was not affected by fragmentation, habitat features or geographical location. However, individual species and groups of resident/migrant birds responded differently: whereas resident dispersers were more abundant in large patches, wintering dispersers were more abundant in fruit‐rich patches. Predator abundances were similar between patches, although the guild composition shifted with fragmentation. The proportion of myrtle fruits consumed by dispersers and predators varied greatly between patches, but did not depend on bird abundances. The geographical location of patches determined the presence or absence of interactions between myrtles and seed predators (six predated and three non‐predated patches), a fact that greatly influenced fruit dispersal success. Moreover, predation rates were lower (and dispersal rates higher) in large patches with fruit‐poor heterospecific environments (i.e. dominated by myrtle). Predator satiation and a higher preference for heterospecific fruits by dispersers may explain these patterns. These results show that 1) the frugivore assemblage in warm Mediterranean lowlands is mostly composed of fragmentation‐tolerant species that respond differently to landscape changes; and 2) that the feeding behaviour of both dispersers and predators influenced by local fruit availability may be of great importance for interpreting patterns of frugivory throughout the study area.     

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.     

Effects of the Argentine ant Linepithema humile on seed dispersal and seedling emergence of Rhamnus alaternus

We studied influence of the Argentine ant's Linepithema humile occurrence on seed dispersal of Rhamnus alaternus (Rhamnaceae). Rhamnus alaternus is a fleshy fruit plant dispersed primarily by birds; the seeds have an elaiosome attractive to ants. The observations were made in two study plots of Mediterranean cork-oak forests (one invaded and the other not invaded by L. humile ) over two years. For R. alaternus , presence of L. humile was associated with of the following: reduction of seed transport to ant nests (80.2% of seed offered were removed in non-invaded vs 7.1% in invaded area), a shorter seed dispersal distance (mean=90.2 cm in non-invaded vs 1.1 cm in invaded area), increase in seed retention time on the soil surface (median time in non-invaded was 50 vs 209.2 min in invaded area) and increased vulnerability to predation. In addition there was lower probability of seedling emergence, due to little elaiosome removal from the seeds (82.0% of emergence for seeds without elaiosome and 57.3% for seeds with elaiosome). These results, similar to findings obtained in South African fynbos, confirm that the Argentine ant invasion can strongly affect ecosystem processes in the Mediterranean biome.     

Ant behaviour and seed morphology: a missing link of myrmecochory

Seed dispersal by ants (myrmecochory) is mediated by the presence of a lipid-rich appendage (elaiosome) on the seed that induces a variety of ants to collect the diaspores. When seeds mature or fall onto the ground, these ant species transport them to their nest. After eating the elaiosome, the seed is discarded in nest galleries or outside, in the midden or farther away, where seeds can potentially germinate. The final location of seeds with their elaiosomes removed was evaluated to assess the importance of possible handles (structures that ants can grasp to carry) in transporting ants during re-dispersal experiments of seeds from nests of six species of ants. The results indicate that seeds remained within the nest because the ants were not able to transport them out of the nest. As a consequence of the elaiosome being removed, small ant species could not take Euphorbia characias seeds out of their nests. Only large ant species could remove E. characias seeds from their nests. Attaching an artificial handle to E. characias seeds allowed small ant species to redistribute the seeds from their nests. On the other hand, Rhamnus alaternus seeds that have a natural handle after the elaiosome removal were removed from the nests by both groups of ant species. If a seed has an element that acts as a handle, it will eventually get taken out of the nest. The ants’ size and their mandible gap can determine the outcome of the interaction (i.e. the pattern of the final seed shadow) and as a consequence, could influence the events that take place after the dispersal process.     

Impact of seed abundance on seed processing and dispersal by the red ant Myrmica rubra

1. Myrmecochory sensu stricto is an ant–plant mutualism in which non‐granivorous ants disperse plant diaspores after feeding on their nutrient‐rich seed appendage, the elaiosome. Phenological traits associated with the diaspore can influence the behaviour of ants and thus their ultimate efficiency as seed dispersers. 2. This study investigated how a contrasting availability of seeds (20 vs. 200 seeds) from the diplochorous Chelidonium majus (Papaveraceae, Linnaeus) plant species influences the behaviour of Myrmica rubra (Formicidae, Linnaeus) ants, from the retrieval of seeds until their dispersal outside the ant nest. 3. Regardless of seed abundance, the ants collected the first diaspores at similar rates. Then, seed retrieval sped up over time for large seed sources until satiation took place with only one‐third of the tested colonies wholly depleting abundant seed sources. 4. No active recruitment by trail‐laying ants was triggered, even to an abundant seed source 5. In both conditions of seed abundance, the majority of the diaspores retrieved inside the nest were discarded with the elaiosome removed and were dispersed at similar distances from the nest. 6. The paper concludes with a discussion of how the quantity of seeds released by a plant with a dual mode of dispersal can potentially influence the behaviour of ant dispersers and hence the dispersal efficiency derived from myrmecochory.     

Opening myrmecochory’s black box: what happens inside the ant nest?

In the process of seed dispersal by ants (myrmecochory), foragers bring diaspores back to their nest, then eat the elaiosome and usually reject viable seeds outside the nest. Here, we investigate what happens inside the nest, a barely known stage of the myrmecochory process, for two seed species (Viola odorata, Chelidonium majus) dispersed either by the insectivorous ant Myrmica rubra or by the aphid-tending ant Lasius niger. Globally, elaiosome detachment decreased ants’ interest towards seeds and increased their probability of rejecting them. However, we found marked differences in seed management by ants inside the nest. The dynamics of elaiosome detachment were ant- and plant-specific whereas the dynamic of seed rejection were mainly ant-specific. Seeds remained for a shorter period of time inside the nest of the carnivorous ant Myrmica rubra than in Lasius niger nest. Thus, elaiosome detachment and seed rejection were two competing dynamics whose relative efficiency leads to variable outcomes in terms of types of dispersed items and of nutrient benefit to the ants. This is why some seeds remained inside the nest even without an elaiosome, and conversely, some seeds were rejected with an elaiosome still attached. Fresh seeds may be deposited directly in contact with the larvae. However, the dynamics of larvae-seeds contacts were also highly variable among species. This study illustrates the complexity and variability of the ecological network of ant–seed interactions.     

Dispersal mechanisms of the narrow endemic <Emphasis Type="Italic">Polygala vayredae</Emphasis>: dispersal syndromes and spatio-temporal variations in ant dispersal assemblages

This study assesses the dispersal mechanisms of the narrow endemic Polygala vayredae, analysing the functioning of its dispersal syndromes (anemochory and myrmecochory), the spatio-temporal variability of the disperser assemblage, foraging behaviour and dispersal ability, and the role of the elaiosome in ant attraction and seed germination. The dispersion of diaspores begins when either (1) capsules or seeds fall beneath the mother plant (barochory) or (2) the seeds are directly collected in the suspended capsules by ants (myrmecochory). As capsules frequently open and expose/disseminate seeds before leaving the mother plant, the adaptation for anemochory appears to be reduced and rarely functional, possibly with only occasional events of long-distance dispersal (e.g. under extreme weather conditions). P. vayredae is essentially myrmecochorous and a diverse array of ant species are involved in seed manipulation, with the elaiosome playing a major role in ant attraction. From the plant’s perspective for dispersal, the majority of ant species had a positive interaction with the seeds, but negative and potential neutral interactions were also observed. Overall, dispersal distances were limited and were mainly determined by ant body size. The frequency of interactions and the ant assemblage varied significantly both spatially and temporally, and these factors may have an effect on directing or disrupting the selection of plant traits. Low seed predation and similar germination rates of intact seeds compared with seeds without elaiosome indicate that seed predator avoidance and seed germination improvement after ant manipulation are not among the selective advantages of myrmecochory operating at present. Dispersal mechanisms that enhance seed dispersal within the population and only occasionally lead to long-distance dispersal events, along with the rarity and patchiness of suitable habitats, may be the main factors explaining the actual density and narrow distribution of this species.     

Myrmecochory and short-term seed fate in Rhamnus alaternus: Ant species and seed characteristics

Benefits conferred on plants in ant-mediated seed dispersal mutualisms (myrmecochory) depend on the fate of transported seeds. We studied the effects of elaiosome presence, seed size and seed treatment (with and without passage through a bird's digestive tract) on short-term seed fate in Rhamnus alaternus. In our study, we define short-term seed, or initial, seed fate, as the location where ants release the seeds after ant contact with it. The elaiosomes had the most influence on short-term fate, i.e. whether or not seeds were transported to the nest. The workers usually transported big seeds more often than small ones, but small ants did not transport large seeds. Effect of seed size on transport depended on the ant species and on the treatment of the seed (manual extraction simulating a direct fall from the parent plant vs. bird deposition corresponding to preliminary primary dispersal). Probability of removal of elaiosome-bearing seeds to the nest by Aphaenogaster senilis increased with increasing seed weight.