Timing and spatial patterning of seed dispersal and redistribution in a South American warm desert

We measured newly-produced seeds entering the soil (Potential Seed Bank) to assess the timing and spatial patterning of Phase I dispersal in the central Monte desert, Argentina. Rates of forb- (6.13 mg m^-2 d^-1) and shrub-seed input (48.9 mg m^-2 d^-1) were maximum in early summer. The rate of grass-seed input, instead, was similar in early and late summer (7 to 8 mg m^-2 d^-1). About 90% of forb- and shrub-seed mass entered the habitat through protected (i.e., under canopy) areas, whereas 70% of grass-seed mass did so through exposed areas. Adult plant location and the uneven impact of wind on shrub, forb and grass seeds may explain such patterns.We also compared the Potential Seed Bank with the soil seed bank in the following spring (Realized Seed Bank). Seeds that form transient banks in other ecosystems (e.g., shrub seeds of the genus Larrea, or perennial grass seeds like those of Pappophorum and Trichloris) prevailed in the Potential Seed Bank. Some annual forb seeds, instead, appeared to form a more persistent seed bank, and prevailed in the Realized Seed Bank (e.g., Chenopodium). Horizontal redistribution did not affect the spatial patterning of forb and shrub seeds, but produced a more homogeneous distribution of grass seeds in the habitat. The impact of wind could explain the redistribution pattern of grass seeds. Finally, we found almost 80% of total seeds in the top 2 cm of soil. The smallest grass and forb seeds (Sporobolus and Descurainia) as well as some medium-sized and large forb seeds (e.g., Glandularia, Sphaeralcea, Phacelia) were able to reach deeper soil layers in the central Monte desert.     

Seed cleaning behavior by tropical ants and its anti-fungal effect

In tropical forests, ants frequently consume fruit pulp around seeds of vertebrate-dispersed plants, which protects the seeds from infection by fungus and pathogens. Seed cleaning behavior by omnivorous ants was observed in the secondary forests of Bogor botanical garden in West Java. Fruit pulp around the seeds of the rambutan Nephelium lappaceum was completely removed by Pheidole plagiaria, Anoplolepis gracilipes, and other ants. When seed cleaners were excluded, however, many seeds were attacked by fungus and died. Seeds that were cleaned by hand, but not by ants, were similarly attacked. Field experiments on seed cleaning by P. plagiaria revealed that the anti-fungal effect was not merely caused by removal of fruit pulp, and that seed cleaning reduced the development of spores and hypha of fungi. We suggest that the workers apply anti-fungal substances to the fruit pulp and seed surface.     

Seed dispersal by ants: behaviour-releasing compounds in elaiosomes

Summary In a study of the biochemical basis of seed dispersal by ants, elaiosomes of Acacia myrtifolia and Tetratheca stenocarpa induced seed collection: intact diaspores and elaiosomes were taken rapidly by ants while most seeds remained on the forest floor. Extracts of elaiosomes (non-polar lipids, polar lipids, and aqueous fractions) were differentially collected by ants. Small pieces of pith impregnated with the polar lipid fraction from elaiosomes of either species elicited a removal rate by ants equivalent to that of intact elaiosomes and significantly higher than that of untreated pith. The non-polar lipid fraction, highest in concentration in elaiosomes of both species, elicited removal that did not differ from that of untreated pith. In T. stenocarpa, however, the aqueous fraction also induced removal equivalent to the polar lipid fraction. 1,2-Diglycerides with unsaturated groups are present in the active polar fractions of both species and unsaturated oleate is the major acid group of the glycerides in elaiosomes. Most oleate-containing compounds tested were taken more rapidly by ants than saturated compounds, and oleic acid, associated with corpse-carrying in ants, induced rapid removal. 1,2-Diolein, but not 1,3-diolein, was taken suggesting that the specific configuration of fatty acid moieties influences collection by ants. We hypothesize that a small suite of oleyl-containing compounds in elaiosomes elicit a stereotyped carrying response by a variety of ants. While the nutrient composition of elaiosomes may provide the underlying selective advantage for ants in seed dispersal, specific compounds may manipulate their behaviour and maximize seed dispersal.     

Foliar fungi alter reproductive timing and allocation in Arabidopsis under normal and water-stressed conditions

Microbes influence plant phenotypes but most known examples of this are from the study of below-ground microbes and plant disease modification. To examine the potential importance of phyllosphere microbes on non-disease related plant traits, we used sterile Arabidopsis clones to test the effects of foliar fungi on flowering phenology and reproductive allocation under conditions of varying water stress. We inoculated the sterile plants with fully-factorial combinations of four fungal isolates, then measured flowering time and reproductive allocation for each treatment group under normal and water-stressed conditions. All plants inoculated with foliar fungi had significantly later flowering and greater seed mass than the sterile control groups. The magnitude of this effect depended on the specific fungi present, but individual fungal effects diminished as inoculum richness increased. Above-ground microbes likely influence other plant traits as well and should be considered in any study measuring plant phenotypes.     

Savanna fires increase rates and distances of seed dispersal by ants

Myrmecochory (seed dispersal by ants) is a prominent dispersal mechanism in many environments, and can play a key role in local vegetation dynamics. Here we investigate its interaction with another key process in vegetation dynamics—fire. We examine ant dispersal of seeds immediately before and after experimental burning in an Australian tropical savanna, one of the world’s most fire-prone ecosystems. Specifically, our study addressed the effects of burning on: (1) the composition of ants removing seeds, (2) number of seed removals, and (3) distance of seed dispersal. Fire led to higher rates of seed removal post-fire when compared with unburnt habitat, and markedly altered dispersal distance, with mean dispersal distance increasing more than twofold (from 1.6 to 3.8 m), and many distance dispersal events greater than the pre-fire maximum (7.55 m) being recorded. These changes were due primarily to longer foraging ranges of species of Iridomyrmex, most likely in response to the simplification of their foraging landscape. The significance of enhanced seed-removal rates and distance dispersal for seedling establishment is unclear because the benefits to plants in having their seeds dispersed by ants in northern Australia are poorly known. However, an enhanced removal rate would enhance any benefit of reduced predation by rodents. Similarly, the broader range of dispersal distances would appear to benefit plants in terms of reduced parent–offspring conflict and sibling competition, and the location of favourable seedling microsites. Given the high frequency of fire in Australian tropical savannas, enhanced benefits of seed dispersal by ants would apply for much of the year.     

Density-dependent foraging in the harvester ant Messor ebeninus: two experiments

Harvester ants are important seed predators in many xeric environments, and their foraging choices can influence the composition of plant communities. Seed abundance has been cited as an important factor in determining such foraging preferences. Three seed types (sesame, millet, and flax) were experimentally introduced in differing proportions near nests of the ant Messor ebeninus near the Dead Sea, in territory administered by the state of Israel. Two experiments were designed to investigate the effects of this density conditioning on two stages of the ants' subsequent foraging behavior: recruitment to seed patches and selection of seeds from within a patch. When seeds were presented in small, single-species patches, experimentally common seeds were discovered and exploited significantly faster than rare seeds, especially among less preferred seed types. When seeds were presented in large, mixed patches, however, no consistent effect of density was observed.     

Two-phase seed dispersal: linking the effects of frugivorous birds and seed-caching rodents

Frugivorous birds disperse the seeds of many fruit-bearing plants, but the fate of seeds after defecation or regurgitation is often unknown. Some rodents gather and scatter hoard seeds, and some of these may be overlooked, germinate, and establish plants. We show that these two disparate modes of seed dispersal are linked in some plants. Rodents removed large (>25 mg) seeds from simulated bird feces (pseudofeces) at rates of 8–50%/day and scatter hoarded them in soil. Ants (Formica sibylla) also harvested some seeds and carried them to their nests. Rodents carried seeds 2.5±3.2 m to cache sites (maximum 12 m) and buried seeds at 8±7 mm depth. Enclosure studies suggest that yellow pine chipmunks (Tamias amoenus) and deer mice (Peromyscus maniculatus) made the caches. In spring, some seeds germinated from rodent caches and established seedlings, but no seedlings established directly from pseudofeces. This form of two-phase seed dispersal is important because each phase offers different benefits to plants. Frugivory by birds permits relatively long-range dispersal and potential colonization of new sites, whereas rodent caching moves seeds from exposed, low-quality sites (bird feces on the ground surface) to a soil environment that may help maintain seed viability and promote successful seedling establishment.     

Dietary similarity and foraging range of two seed-harvester ants during resource fluctuations

Summary Diets of desert seed-harvester ants Veromessor pergandei and Pogonomyrmex rugosus were studied for 3 years at two habitats where they are common and sympatric. Diets of the two species were similar, consisting mainly (87% of 23,913 seeds) of three annual plant species (Schismus arabicus, Plantago insularis, and Pectocarya recurvata). Diets converged following a drought in Winter/Spring 1984 which reduced seed production during this time. Foraging range of P. rugosus almost doubled following the drought while foraging range of V. pergandei remained constant. Neither ant species move their nests once established rendering them effectively sessile granivores. This limits the dietary options of a given colony and may explain interspecific dietary convergence.     

Pygidial gland chemistry and potential alarm-recruitment function in column foraging,but not solitary,Nearctic Messor harvesting ants (Hymenoptera: Formicidae: Myrmicinae)

We investigated the role of the pygidial gland on foraging behavior in two ecologically dominant column foraging Nearctic harvesting ants (Messor pergandei and Messor andrei). Using chemical analyses and behavioral tests, we show that n-tridecane is the major biologically active compound of pygidial gland secretions in both species, and that this chemical functions as a powerful alarm-recruitment pheromone. Another major compound of pygidial gland contents is benzaldehyde; this substance does not release behavioral reactions in M. pergandei workers but might function as a defensive secretion. Six solitary foraging Nearctic Messor and two column foraging Palearctic Messor species, did not have large pygidial gland reservoirs.     

Effects of seed predation by ants on Mediterranean grassland related to seed size

Questions: 1. Do harvester ants (Messor barbarus) promote seed mortality in Mediterranean grassland?; 2. Is this effect greater in large‐seeded species? Location: Central Spain. Methods: We established an ant‐exclusion experiment of five circular (1.5 m diameter) plots from where ants were excluded during one year, along with ten control plots. We recorded the seed bank of all species in the plots both before and after the treatment. The effect of seed length and weight was analysed after transforming data into phylogenetically independent contrasts, and alternatively by dividing the species data set into morphological groups. Results: Longer and heavier seeded species significantly increased in the seed banks under the exclusion treatment, although ants did not significantly modify overall seed densities. Conclusions Although the ants do not collect large numbers of seeds, they differentially affect the composition of the seed banks by selecting the longest or heaviest seeds, or both. The persistence of this short‐term effect in the seed bank may result, over a number of years, in the system evolving towards a predominance of small‐seeded annuals, congruent with the species composition actually observed in Mediterranean grasslands.     

Quantity,quality and the effectiveness of seed dispersal by animals

Disperser effectiveness is the contribution a disperser makes to the future reproduction of a plant. Although it is a key notion in studies of seed dispersal by animals, we know little about what determines the effectiveness of a disperser. The role of the present paper is to review the available information and construct a hierarchical framework for viewing the components of disperser effectiveness.Effectiveness has both quantitative and qualitative components. The quantity of seed dispersal depends on (A) the number of visits made to the plant by a disperser and (B) the number of seeds dispersed per visit. The quality of seed dispersal depends on (A) the quality of treatment given a seed in the mouth and in the gut and (B) the quality of seed deposition as determined by the probability that a deposited seed will survive and become an adult. In this paper I review the ways disperser behavior, morphology and physiology can influence these major components of disperser effectiveness, and when data permit present preliminary analyses of relationships among components.     

Conflicting selection on diaspore traits limits the evolutionary potential of seed dispersal by ants

Conflicts of selection on diaspore traits throughout the dispersal cycle can limit the evolutionary consequences of seed dispersal. However, these conflicts have never been investigated in directed dispersal systems. We explored conflicts of selection through life stages of dispersal in the myrmecochorous herb Helleborus foetidus. Seeds are subject to two contrasting partial selective scenarios. Undispersed seeds are subject to positive directional selection on seed size characters, whereas seeds dispersed are subject to stabilizing selection for size. In both scenarios, seedling establishment determined the magnitude and direction of selection. This does not reflect ant preferences for seed size. However, total selection still depends largely on ant activity, as ants control the relative importance of each selective scenario. We advocate the use of analytical approaches combining multiplicative fitness and microenvironment‐specific selection to more realistically estimate the realized selection on traits functional during several life stages. This approach may be extended to any organism dispersing offspring to different environments.     

Endophytic fungi increase the processing rate of leaves by leaf‐cutting ants (Atta)

1. Fungal endophytes are microfungi that reside asymptomatically inside of leaf tissues, increasing in density and diversity through time after leaves flush. Previous studies have suggested that the presence of fungal endophytes in the harvest material of leaf‐cutting ants (Atta colombica, Guérin‐Méneville) may negatively affect the ants and their fungal cultivar. 2. In the present study, it was tested whether the presence and diversity of fungal endophytes affected the amount of time necessary for leaf‐cutter ants to cut, process, and plant leaf material in their fungal garden. It was found that ants took 30–43% longer to cut, carry, clean, and plant leaf tissue with high relative to low endophyte abundance, and that the ants responded similarly to leaf tissue with high or low endophyte diversity. 3. It was further investigated whether the fungal cultivars' colonisation rate was greater on leaf material without fungal endophytes. No difference in the ants' cultivar colonisation rate on leaf tissue with high or low endophyte abundance was observed.     

舞草种子的蚂蚁传播

舞草(Codariocalyx motorius)由于其小叶具有自身"摆动"的功能,从而具有较高的观赏价值.在长期的演化过程中,舞草与蚂蚁形成了互惠共生的关系舞草种子生成了附生其上的能吸引蚂蚁的油质体,蚂蚁在搬运取食中,使舞草种子得以传播.舞草种子最重要的传播者是圆叶铺道蚁(Tetramorium cyclolobium Xu et Zheng)和布立毛蚁(Paratrechina bourbonica Forel).另外长足光结蚁(Anoplolepis gracilipes Smith)和两种大头蚁(Pheidole sp.1和sp.2)也搬运其种子.野外试验表明,圆叶铺道蚁日搬运活动与气温呈显著正相关,即y(搬运种子数)=-9.5038+0%5608X(气温)(r=0.7196**,n=33,P＜0.01),中午搬运效率达到高峰.布立毛蚁日搬运活动在上、下午各有一个高峰,上午的高峰出现时间不稳定,下午的高峰出现在1 600～1800.舞草种子上附生的油质体是吸引蚂蚁并产生搬运行为的主要物质.化学分析表明,油质体富含蚂蚁生长发育所必需的10种氨基酸和多种无机元素.样地采用陷阱诱捕蚂蚁的调查显示,5种搬运者中,圆叶铺道蚁数量最大,分别占蚂蚁总量的8.26%和搬运蚂蚁总量的48%.这说明圆叶铺道蚁在舞草种子的搬运中起着主要作用.     

Genotypic diversity in root‐endophytic fungi reflects efficient dispersal and environmental adaptation

Studying community structure and dynamics of plant‐associated fungi is the basis for unravelling their interactions with hosts and ecosystem functions. A recent sampling revealed that only a few fungal groups, as defined by internal transcribed spacer region (ITS) sequence similarity, dominate culturable root endophytic communities of nonmycorrhizal Microthlaspi spp. plants across Europe. Strains of these fungi display a broad phenotypic and functional diversity, which suggests a genetic variability masked by ITS clustering into operational taxonomic units (OTUs). The aims of this study were to identify how genetic similarity patterns of these fungi change across environments and to evaluate their ability to disperse and adapt to ecological conditions. A first ITS‐based haplotype analysis of ten widespread OTUs mostly showed a low to moderate genotypic differentiation, with the exception of a group identified as Cadophora sp. that was highly diverse. A multilocus phylogeny based on additional genetic loci (partial translation elongation factor 1α, beta‐tubulin and actin) and amplified fragment length polymorphism profiling of 185 strains representative of the five dominant OTUs revealed a weak association of genetic differences with geography and environmental conditions, including bioclimatic and soil factors. Our findings suggest that dominant culturable root endophytic fungi have efficient dispersal capabilities, and that their distribution is little affected by environmental filtering. Other processes, such as inter‐ and intraspecific biotic interactions, may be more important for the local assembly of their communities.     

Asymmetric interactions between plants and seed-harvesting ants in a Mediterranean pasture

A four-year study on the interactions between plants and seed-harvesting ants in a Mediterranean pasture is reviewed in this paper. As previously reported in many studies on plant–herbivore interactions, ant–plant relationships are also asymmetric; plants had a larger impact on herbivore dynamics than vice versa. However, the asymmetry did not refer to population dynamics but rather to animal foraging strategies. Ants did not exert a significant influence on vegetation dynamics in terms of plant abundance. The main constraints underlying vegetation change were self-regulation and rainfall. In contrast, the structural characteristics and abundance of vegetation had a significant impact on several important features of food harvesting by ants. This influence was not only associated with their feeding requirements but also with their foraging activities.     

Nest-plugging: interference competition in desert ants (Novomessor cockerelli and Pogonomyrmex barbatus)

Summary Pogonomyrmex barbatus and Novomessor cockerelli, sympatric species of harvester ants in the Lower Sonoran desert, compete for seed resources. This study reports on a method of interference competition. Early in the morning, before P. barbatus' activity period, N. cockerelli fills the nest entrances of P. barbatus with sand. This delays the beginning of the P. barbatus activity period for 1–3 h. P. barbatus colonies near N. cockerelli nests were more likely to be plugged. Nest-plugging shifts the typical daily sequence of P. barbatus activities, including the onset of foraging, forward towards midday, when high temperatures force the colony back inside the nest. P. barbatus colonies do not compensate for late emergence or events impeding foraging by increasing foraging rate. Thus nest-plugging by N. cockerelli decreases the foraging capacity of P. barbatus colonies.     

Limited female dispersal predicts the incidence of Wolbachia across ants (Hymenoptera: Formicidae)

The endosymbiotic bacterium Wolbachia is perhaps the greatest panzootic in the history of life on Earth, yet remarkably little is known regarding the factors that determine its incidence across species. One possibility is that Wolbachia more easily invades species with structured populations, due to the increased strength of genetic drift and higher initial frequency of infection. This should enable strains that induce mating incompatibilities to more easily cross the threshold prevalence above which they spread to either fixation or a stable equilibrium infection prevalence. Here, we provide empirical support for this hypothesis by analysing the relationship between female dispersal (as a proxy for population structure) and the incidence of Wolbachia across 250 species of ants. We show that species in which the dispersal of reproductive females is limited are significantly more likely to be infected with Wolbachia than species whose reproductive ecology is consistent with significant dispersal of females, and that this relationship remains after controlling for host phylogeny. We suggest that structured host populations, in this case resulting from limited female dispersal, may be an important feature determining how easily Wolbachia becomes successfully established in a novel host, and thus its occurrence across a wide diversity of invertebrate hosts.     

Seed removal by small mammals, birds and ants in semi-arid Chile, and comparison with other systems

Aim This study aims to evaluate the relative importance of birds, small mammals, and ants as seed predators at a semi‐arid site in northern Chile. Location Northern Chile, in Parque Nacional Bosque Fray Jorge (30°41′ N, 71°40′ W, c. 80 m elevation). Methods We studied the relative abilities of birds, small mammals, and ants to find and remove millet seeds either singly (i.e. background seed removal) or in bulk. Single seeds were set in shallow depressions in Plexiglas trays which were established in long and arbitrary transects, and were available either to birds (diurnally) or small mammals (nocturnally) or were covered by hardware cloth and therefore available only to ants. Bulk removal was evaluated with seeds in Petri dishes that also were established in long and arbitrary transects, and trays were either open diurnally (birds) or nocturnally (small mammals); a third set of trays was covered with hardware cloth cages to excluded vertebrates, and ants were given access to Petri dishes with twigs that were arranged across the edge of the dishes. All experiments lasted four days and nights, and trays and dishes were checked and replenished as needed in the morning and evening. In the former study vertebrate consumption was determined as the mean number of seeds removed from trays (within a given transect) minus the number removed from ant‐only trays. Because ants were rarely seen in vertebrate access Petri dishes, however, we did not correct consumption there. Treatments were compared using repeated measures mixed model analysis of variance. In addition to evaluating patterns within this community, we compared our results against those obtained in similar studies in various arid regions. Results Diurnal seed consumption was significantly greater than nocturnal seed consumption, which in turn was significantly greater than consumption by ants. Diurnal consumption was highly seasonal, evidently corresponding to the seasonal arrival and departure of migratory birds. In general, South American sites exhibit much lower levels of seed predation than sites in the northern hemisphere, but removal at our site appears to be much greater and more strongly avian‐dominated than at other sites in South America. Our results are consistent with predictions based on a hypothesis relating precipitation to seed predictability. Main conclusions Both birds and small mammals were much more important seed consumers at our site than elsewhere in South America, whereas ants have been relatively unimportant at all South American sites studied to date. Although the dominant seed consumers differ across sites, overall levels of seed removal appear similar in South America and Australia, and substantially lower than reported from sites in the northern hemisphere and Africa.     

Colony age, neighborhood density and reproductive potential in harvester ants

At about age 5 years, colonies of the harvester ant, Pogonomyrmex barbatus, begin to produce winged, sexual forms (alates) that mate in large annual aggregations. We examined how colony age and neighborhood density affect the numbers, body mass, and body fat of alates produced by 172 colonies ranging in age from 4 to 17 years. Over one-third (36%) of all colonies produced no alates. Failure to reproduce was independent of colony age. Of those colonies that did produce alates, older colonies produced more alates than younger colonies. Older colonies produced lighter female alates (in dry mass), but the total biomass of additional alates produced by older colonies far outweighed the reduced allocation to female alate body mass. Body fat content was much higher in female alates (36.0% on average) than in males (3.7% on average). Alate body fat content was not related to colony age. The fitness of female alates may be related to their fresh body mass; that of females captured after mating and reared in the laboratory was positively correlated with egg-laying rate, although not with the total number of eggs in the first brood. Neighborhood density was not related to alate number, mass, or fat content, in contrast to the results of a 1995 study at the site, in which alate numbers were negatively related to neighborhood density. Thus the influence of crowding on reproductive output appears to vary from year to year, perhaps in response to variation in rainfall and food supply. Alate output by individual colonies was correlated among years. These results suggest that a few, older colonies dominate the pool of reproductives year after year. Received: 11 May 1998 / Accepted: 19 December 1998