Acorn dispersal estimated by radio-tracking

Bird-dispersed seeds are difficult to track, especially in the case of long-distance dispersal events. To estimate the oak dispersal distance and the seed shadow generated by the European jay (Garrulus glandarius), we inserted radio-transmitters in 239 acorns, placed them in bird-feeders and then located them by radio-tracking. Using this methodology we located the exact caching site of 94 Quercus ilex and 54 Q. suber acorns and determined the caching habitat characteristics (vegetation type, distance, spatial distribution). The results show that: (1) there is no differences in the dispersal distance distribution between the different acorn species or sizes, (2) dispersal distances range from approximately 3 m up to approximately 550 m (mean = 68.6 m; median = 49.2 m), (3) recently abandoned fields and forest tracks were the sites preferred by jays to cache acorns, whereas fields and shrublands were avoided and (4) seed shadows showed acorn aggregation zones (i.e. clusters of caches) close to the feeder as well as isolated caches at longer distances. The results also suggest that radio-transmitters are a cheap and reliable way to determine seed shadows and quantify both seed dispersal and post-dispersal seed predation for medium to large seeds.     

Masting,seed dispersal and seed predation in the cycad Macrozamia communis

Summary Little is known about the adaptive value of mast seeding, a common phenomenon in temperate trees and shrubs. Masting is likely to affect both seed dispersal and seed predation. In systems where similar taxa of animals are involved in these two processes, the consequences of mast seeding are likely to be particularly complicated. This study examined the effects of mast seeding in a cycad, Macrozamia communis, on the dispersal of seeds, the pattern of dispersion of seeds and post-dispersal predation on seeds. Dispersal of seeds by possums was poorer from source plants in a masting population than from source plants in an adjacent, non-masting population. This resulted in fewer seeds per seeding female plant in the masting plot being dispersed to favourable sites. We conclude that this is caused by the feeding behaviour and movements of possums in the masting site. The abundance of seeds in this site did not satiate the post-dispersal predators, native rats. In fact, more seeds in this site were eaten than in the nonmasting site. We suggest that the mast seeding observed in M. communis may not be adaptive, but is more likely a consequence of other factors which synchromize flowering within local populations.     

Small-scale spatial structure within patterns of seed dispersal

A large proportion of dispersing propagules land near their maternal plant, even in species that have evolved structures which enhance dispersal. For these propagules, their post-dispersal spatial pattern is likely to reflect the overall shape and scale of the parental plant canopy and, especially in poorly dispersing species, aggregation of propagules on the plant prior to dispersal. Localised patterns within seed shadows are also likely to be affected by secondary movement after dispersal, leading to either more or less small-scale aggregation, depending on the mechanism. Our general aim was to study the small-scale spatial structure within patterns of seed dispersal of Raphanus raphanistrum L. to generate hypotheses about the sequence of processes and events leading to the spatial pattern of dispersal in this species. More specifically, we determined the sizes of small-scale structures within the seed shadows on the ground after dispersal and the extent to which these match the sizes of pre-dispersal aggregations within the parental canopy. Variation in plant size and shape was provided by four levels of inter-specific competition resulting from differing wheat crop densities. Positions of propagules were determined using a three-dimensional digitizer, and the data for each plant were analysed using point pattern analysis. Not surprisingly, larger plants, growing at lower plant density, had larger seed shadows, showing an overall influence of maternal plant size. The pattern of propagules exhibited significant small-scale aggregates, with similar sizes on the plant and on the ground. There was no evidence that aggregation size was greater on the ground or increased with time, but the strength of the aggregation increased with length of time on the ground.     

Frugivory and the effects of ingestion by bats on the seed germination of three pioneering plants

The dispersion and seedling establishment of pioneering plants can be favoured by the presence of frugivorous bats because the bats usually improve seed germination after ingestion. Although seed germinability is known to vary greatly after ingestion by different bats, the relative contribution of each bat species to seed germination within plant communities is poorly understood. In this study, we first determined the fauna of frugivorous bats in a semideciduous seasonal forest remnant in southern Brazil and subsequently identified the plant species of the seeds passed through their guts. Second, the germination performance (i.e., germination percentage and speed) of the seeds of three pioneering plants (Piper aduncum, Piper hispidinervum and Solanum granuloso-leprosum) ingested by the most abundant bats was compared with that of the non-ingested seeds (seeds collected from fruits). Additionally, the effects on seed germination of different bat species were compared. During one year, five species of frugivorous bats were caught, and the seeds of eleven identifiable plant species (not counting those of undetermined species) were found in their faeces. We found that the germination performance of the seeds of Piper species was significantly enhanced after ingestion by bats, whereas S. granuloso-leprosum seeds had neutral or reduced germinability when seeds in faeces were compared with pulp-removed seeds. Our results revealed that the bat species that were captured exerted different effects upon seed germination; such a disparity is expected to result in different rates of early establishment of these pioneer plants in tropical forests, most likely affecting forest composition and structure, particularly during the initial stages of succession.     

The effects of seed abundance on seed predation and dispersal by rodents in <Emphasis Type="Italic">Castanopsis fargesii</Emphasis> (Fagaceae)

Based on the animal dispersal hypothesis and the predator satiation hypothesis, we examined the effects of seed abundance at both population (i.e., mast seeding) and community levels on seed predation and dispersal of Castanopsis fargesii (Fagaceae), a rodent-dispersed mast species in Eastern Asia. In a subtropical evergreen broadleaved forest in the Dujiangyan region of Sichuan Province, China, individual seeds with coded tin tags were tracked in two contrasting stands (seed-poor and seed-rich) over two years (2000, a low-seed year; 2001, a high-seed year). Our results showed that: (1) small rodents did not harvest the tagged seeds of C. fargesii more rapid in the high-seed year than in the low-seed year in either stand. But, seed harvest was significantly faster in the seed-rich stand than in the seed-poor stand. (2) The removal proportion was significantly lower in the high-seed year than in the low-seed year for either stand, but the removal proportion was slightly higher in the seed-poor stand than in the seed-poor stand. This indicates that high seed abundance decreases seed removal (predator satiation hypothesis). (3) There were only small differences about seed caching, seed survival and seedling establishment of C. fargesii between years and stands. During the survey, no cached seeds survived to geminate in the spring for both stands and years. (4) Mean dispersal distances of the cached seeds are much shorter in the high-seed year (3.1 m) than in the low-seed year (8.1 m) in the seed-rich stand, though similar trend is not examined in the seed-poor stand. Our results indicate that seed predation and dispersal of C. fargesii are influenced by both mast seeding and community-level seed abundance, which is not completely consistent with either the animal dispersal hypothesis or the predator satiation hypothesis, but seems more related to the predator satiation hypothesis.     

Aggregated seed dispersal by wreathed hornbills at a roost site in a moist evergreen forest of Thailand

Hornbills (Bucerotidae) are widely regarded as important seed dispersers in tropical forests in Africa and Asia. We investigated how the roosting behavior of wreathed hornbills (Aceros undulatus) influences seed deposition and seedling survival at a roost site in a moist evergreen forest of Khao Yai National Park, Thailand. Fallen fruits and seeds were collected in traps that were placed around a roosting site for 14 months, and seedlings were monitored in adjacent quadrats for 3 years. Seedfall and seedlings of species represented in the hornbill diet occurred at significantly higher densities in the traps and quadrats located beneath the crown of the roosting tree than in those located beyond the crown. With the exception of Cinnamomum subavenium, the seeds and seedlings of most diet species rarely survived beyond the first year. The quality of hornbill dispersal to this roosting site may be poor due to the highly concentrated seedfall, which results in high seed and seedling mortality. However, the number of seeds deposited by each hornbill each day at roosting sites is relatively low. Wreathed hornbills are primarily scatter dispersers during the day and probably serve as agents of seed dispersal in the moist evergreen forest of Khao Yai.     

Early postfire seed dispersal, seedling establishment and seedling mortality of Pinus coulteri (D. Don) in central coastal California, USA

Seedling recruitment in many highly serotinous populations of Pinus coulteri on California's central coast depends almost entirely on periodic, stand-replacing fire. Compared to serotinous pines of the Mediterranean Basin, little detailed information is available on the postfire demography of California closed-cone pines, including P. coulteri. In September 1996 a wildfire burned the 760-ha American Canyon Research Natural Area (RNA). Using aerial photography, we mapped burn severity of P. coulteri-chaparral woodlands and forests within the RNA. From May to September of 1997, we also quantified seedling establishment and mortality in relation to biophysical site characteristics including fire severity. Seventy-six percent of P. coulteri forests and woodlands experienced high-severity burns, 9% moderate-severity burns, and 15% low-severity or unburned. Of the 53 plots used for seedling counts, 70% were high-severity, 26% moderate-severity, and 4% low-severity. Seedling densities 13 months postfire were low (0.21 m^–2), but seedling mortality also was low (8.4%). Aerial seed bank size increased from north-facing to south-facing slopes and from high-severity to low-severity burns. Seedling recruitment was unrelated to burn severity and increased with the size of the canopy seed bank (cone density). Many seedlings established from rodent seed caches; 23% of the seedlings established in clumps from seeds cached by Dipodomys agilis, Chaetodipus californicus and Peromyscus maniculatus. Pinus coulteri seeds have low potential for dispersal by wind, but secondary dispersal by rodents moves seeds away from source trees and into neighboring chaparral. We discuss the potential importance of rodent seed caching to postfire demography of California and Mediterranean serotinous pines.     

Evolution of a unique seed maturity pattern in Croton bonplandianum Baill strengthens ant-plant mutualism for seed dispersal

Summary The female flowers of Croton bonplandianum bear nectar glands which become active during fruit maturation and attain peak activity just prior to the splitting of fruits. This temporal specificity of nectar gland activity is shown to facilitate seed dispersal by ants, which are attracted to the plant only during the fruit maturation period. The nectar glands establish a nectar influence zone with a radius of 60 cm around the plant within which seed dispersal by ants is effective. Seed dispersal by ants is facilitated only if the seeds are placed within this nectar influence zone. This is accomplished by an intriguing evolutionary shift in the maturation pattern of the fruits. Unlike the usual acropetal development, fruit maturation in Croton is temporally asymmetrical, with the fruits nearer the parental axis maturing early. This unique pattern of fruit development together with the polychasial branching system leads to a concentration of seeds within the nectar influence zone and enhances seed dispersal by ants. The proximate factors responsible for this asynchronous fruit maturity were investigated.     

Fruit phenology of Prunus jamasakura and the feeding habit of the Asiatic black bear as a seed disperser

We investigated the relationship between the fruit phenology of Prunus jamasakura and the fruit-feeding period of the Asiatic black bear (Ursus thibetanus). The purposes of this study were to determine (1) when bears feed on the fruit of P. jamasakura in relation to phenology; (2) whether ingestion damages seeds; and (3) how ingestion influences seed germination. We assessed the relationship between the phenology of fruit maturation (size, sugar concentration, color, persistence, and germination percentage) and the feeding period of bears in the field, as judged from bear shelves and claw marks. We also compared the germination percentage of seeds ingested by captive bears with that of uningested. Bears fed on the fruit from days 50 to 66 after flowering, when most of the fruits were on the tree and became large, the germination percentage of seeds increased, and the sugar concentration became high. Bears fed only on mature fruits and so obtained high-quality nutrients. Germination tests showed that ingestion of fruits by the bears caused no physical damage to the seeds. Ingested seeds did not show a significant difference in germination percentage from seeds, whose pulp was artificially removed. These results indicate that bears are potentially effective dispersers from a qualitative perspective and, to some extent, from a quantitative perspective.     

Fruits foraging patterns and seed dispersal effect of frugivorous birds on Hippophae rhamnoides sinensis

Behaviors of 18 species of birds eating fruits of Hippophae rhamnoides spp. sinensis were observed from September 2003 to March 2004. Their foraging patterns were found to be very different and can be divided into five classes: (1) direct swallowing the fruits on crown of the shrubs and sometimes regurgitating seeds soon after; (2) carrying the fruits to their perching sites and swallowing; (3) pecking the fruits from the shrubs to the ground, eating pulp and seeds but leaving pericarp; (4) pecking through the pericarp, eating pulp and leaving pericarp and seeds; (5) pecking through the pericarp on the top of fruits, and only eating seeds. These foraging patterns have different effects on seed dispersal of H. rhamnoides spp. sinensis. The germination experiment of three groups of seeds (seeds from feces, dry fruits and extracted seeds from dry fruits) was carried out. Although ingestion processes of birds had some adverse effects on the seed germination of H. rhamnoides spp. sinensis, the seeds from feces still have a relatively higher germination ratio. H. rhamnoides spp. sinensis provides food to a variety of frugivorous birds, and the birds disperse its seeds. Thus, a mutually beneficial relationship between the bird and the seed is formed. __________ Translated from Chinese Journal of Ecology, 2005, 24(6): 635–638 [译自: 生态学杂志, 2005, 24(6): 635–638]     

The role of seed dispersal,pollination and historical effects on genetic patterns of an insular plant that has lost its only seed disperser

Aim The genetic structure of many plant species is heavily dependent on their pollinators and seed dispersers, and can thus be altered if either of the associated mutualistic interactions is disrupted. In this study we assess the genetic diversity and structure and infer the seed/pollen gene‐flow patterns among insular populations of Daphne rodriguezii, a shrub pollinated and dispersed by animals that has lost its only disperser (the lizard Podarcis lilfordi) in most of its populations. Location The island of Menorca and the islet of Colom (Balearic Islands, Western Mediterranean). Methods To assess the contribution of gene flow via pollen and seeds to the genetic structure of D. rodriguezii we used amplified fragment length polymorphisms (AFLPs; seeds and pollen) and plastid DNA sequences (cpDNA; seeds). We sampled individuals from all population nuclei of the species (12–19 adults per population): one population in Colom, where the plant–lizard interaction persists, and four in Menorca, where the seed dispersal mutualism disappeared with the extinction of the lizard. Results The highest heterozygosity values were found in Colom and in its closest population (Favàritx), whereas values were lower in the smallest Menorcan populations, which also had higher relatedness among individuals. We found distinct genetic signals between AFLP and cpDNA analyses. While AFLP markers showed low differentiation between populations, cpDNA showed a clear differentiation between them. Main conclusions Our results point to negative impacts of the disperser loss on genetic diversity and relatedness in the smaller and more isolated populations. They also suggest an old isolation by seeds, probably occurring well before the extinction of the lizard (c. 2000 years ago). Gene flow was maintained via pollination; however, the seed disperser loss may ultimately hinder pollinator‐mediated gene flow, as a result of reduced probabilities of effective pollination among increasingly distant and scarce individuals.     

Aspects of variation in a neotropical seed dispersal system

This paper explores the causes and consequences of seed and seedling mortality of the tree Virola nobilis (Myristicaceae) Central Panama in order to understand the advantage to local seed dispersal by birds and monkeys. Post-dispersal mortality due to insects (primarily Conotrachelus spp., Curculionidae) accounts for 30–35% of seed and seedling death during the first 12 weeks after seed fall. Because more seeds and seedlings are killed under and near fruiting trees than 15–45 m away, seed dispersal confers a 20–40 fold advantage on seeds carried 45 m from fruiting adults. In contrast, >60% of seed and seedling death during the first year is due to seed predation by mammals, with >90% due to mammals among the <2% seeds that survive until maternal endosperm is exhausted ±12 weeks after seed fall. Mortaliy due to mammals is independent of distance from parent trees, confering no advantage to seed dispersal. Insects account for variation in mortality attributable to distance effects, mammals to between site effects.Early weevil infestations put a premium on seed removal by large birds (Ramphastos swainsonii, R. sulfuratus, Penelope purpurascens), which carry >50% of the seeds that they eat >40 m, as compared with smaller birds (Baryphthengus martii, Tityra semifasciata, Trogon massena) and monkeys (Ateles geoffroyi), which leave most or all of the seeds that they eat under or near the tree crown.     

Ant-seed interactions: combined effects of ant and plant species on seed removal patterns

Seed dispersal by ants (i.e. myrmecochory) is usually considered as a mutualism: ants feed on nutritive bodies, called elaiosomes, before rejecting and dispersing seeds in their nest surroundings. While mechanisms of plant dispersal in the field are well documented, the behaviour of the ant partner was rarely investigated in details. Here, we compared in laboratory conditions the foraging behaviour of two ant species, the omnivorous Lasius niger and the insectivorous Myrmica rubra to which seeds of two European myrmecochorous plants (Chelidonium majus and Viola odorata) were given. Ant colonies were simultaneously presented three types of items: entire seeds with elaiosome (SE), seeds without elaiosome (S) and detached elaiosomes (E). The presence of elaiosomes on seeds did not attract workers from a distance since ants first contact equally each type of items. Although ants are mass-recruiting species, we never observed any recruitment nor trail-laying behaviour towards seeds. For ants having contacted seed items, their antennation, manipulation and seed retrieval behaviour strongly varied depending on the species of each partner. Antennation behaviour, followed by a loss of contact, was the most frequent ant-seed interaction and can be considered as a “hesitation” clue. For both plant species, insectivorous Myrmica ants removed items in larger number and at higher speed than Lasius. This fits with the hypothesis of a convergence between odours of elaiosomes and insect preys. For both ant species, the small Chelidonium seeds were retrieved in higher proportion than Viola ones, confirming the hypothesis that ants prefer the higher elaiosome/diaspore-ratio. Thus, in these crossed experiments, the ant-plant pair Myrmica/Chelidonium was the most effective as ants removed quickly almost all items after a few antennations. The presence of an elaiosome body increased the seed removal by ants excepting for Myrmica which retrieved all Chelidonium seeds, even those deprived of their elaiosome. After 24 h, all the retrieved seeds were rejected out of the nest to the refuse piles. In at least half of these rejected items, the elaiosome was discarded by ants. Species-specific patterns and behavioural differences in the dynamics of myrmecochory are discussed at the light of ant ecology. Received 10 September 2007; revised 5 February 2008; accepted 5 March 2008.     

Effects of harvester ants on seed availability and dispersal of Lavandula stoechas subsp. pedunculata in a Mediterranean grassland-scrubland mosaic

The role of harvester ants in Mediterranean grassland and scrubland has mostly focused on seed consumption. However, recent studies have reported their role as accidental dispersal agents of some of the collected seeds via refuse piles. The objective of this study is to examine the effect of the ant Messor barbarus on seed availability and dispersal of one of its major diet components, Lavandula stoechas subsp. pedunculata, in scrubland, grassland and the ecotones between them. After confirming and quantifying the Lavandula contribution to M. barbarus diet, we described the spatial and temporal patterns of pre- and post-dispersal seed predation, seed content and seedling occurrence in the refuse piles. Our results show that: (1) Lavandula propagules constitute a high proportion of the prey items collected by M. barbarus, with particularly intense collection activity in mid-summer, spring and autumn, in decreasing order. (2) Pre-dispersal predation rate was significantly higher in the ecotone than in the scrubland (76% and 13.5% of total seed production lost respectively). (3) Season and propagule type (seed vs. fruit) were the most significant variables explaining the post-dispersal predation probability, which approached 100% of seeds after 48 h in mid-summer. (4) Viable Lavandula seeds were found in refuse piles at densities of 0.06–0.2 per g of refuse pile material, or 58.8–207.2 per refuse pile. On the one hand, these results indicate that the ecotones are most affected by M. barbarus pre-dispersal consumption, which may locally limit Lavandula colonisation. On the other hand, the small proportion of consumed seeds that is dispersed to refuse piles may be relevant at the population level, as this dispersal implies arrival at potentially favourable sites for establishment.     

The dispersal behaviour of the phoretic mite Poecilochirus carabi (Mesostigmata,Parasitidae): adaptation to the breeding biology of its carrier Necrophorus vespilloides (Coleoptera,Silphidae)

Summary When the phoretic mite Poecilochirus carabi reproduces in the brood chamber of its carrier Necrophorus vespilloides, a beetle with biparental brood care, the first deuteronymphs of the new mite generation aggregate on the male beetle. They do not use sex-specific traits to discriminate between male and female beetles in the brood chamber, but traits that are related to the beetles' behaviour and may be displayed by both parent beetles. When the male beetle departs, it carries virtually all deuteronymphs then present in the brood chamber. Deuteronymphs that develop later congregate on the female, which leaves the crypt some days after the male. Only those deuteronymphs that miss the female's departure disperse on the beetle larvae, meaning they have to wait in their pupal chambers until the beetles have completed their development. On average, 86% of the deuteronymphs leave the brood chamber on the parent beetles, thereby gaining the advantage of an early departure. As soon as their carrier arrives at one of the beetles' meeting places, the deuteronymphs can transfer between the beetles present. Choice experiments revealed that the deuteronymphs tend to even out density differences between congregating carriers, and prefer sexually mature to immature beetles. Therefore, transferring between beetles results in a dispersion of deuteronymphs on the sexually mature beetles of the population.     

Landscape context outweighs local habitat quality in its effects on herbivore dispersal and distribution

Past studies with spatially structured herbivore populations have emphasized the primacy of intrinsic factors (e.g., patch quality), patch geometry (e.g., patch size and isolation), and more recently landscape context (e.g., matrix composition) in affecting local population abundance and dispersal rate. However, few studies have examined the relative importance of each factor, or how they might interact to affect herbivore abundance or dispersal. Here, we performed a factorial field experiment to examine the independent and interactive effects of patch quality (plant biomass, leaf protein, leaf phenolics) and matrix composition [mudflat or non-host grass (Bromus inermis)] on planthopper (Prokelisia crocea) emigration from host-plant patches (prairie cordgrass, Spartina pectinata). In addition, a field survey was conducted to examine the relative importance of patch quality, geography, and matrix composition on planthopper occupancy and density. In the experiment, we found that rates of emigration from low and intermediate quality patches were, on average, 21% percent higher for patches embedded in brome than mudflat. In contrast, the emigration rate was unaffected by matrix composition in nutrient-rich patches. Within matrix types, plant quality had little effect on emigration. In the survey, planthopper density and the patch occupancy rate of planthoppers increased nonadditively with increasing patch size and the percentage of the surrounding matrix composed of mudflat. This study suggests that landscape-level factors, such as the matrix, may be more important than factors intrinsic to the patches.     

The seed dimorphism of Spergularia marina in relation to dispersal by wind and water

Summary The seeds of the halophyte Spergularia marina differ both within and between individuals in that they either possess or lack a membranaceous border. This paper presents a morphological study of the length, weight and area of the seed types, and their dispersal characteristics under experimental conditions of wind and water dispersal. The winged seeds are shown to be larger both by length and by weight. Their rate of descent increases with wing loading. If the wing is lacking, however, the rate of descent increases with weight only. The distance of dispersal is equal for both seed types except at low wind speeds, when the winged seeds disperse farther. If the seed wing is removed, the excised seeds have shorter dispersal distances. When dispersed by water, a difference in the distance seeds are dispersed can only be detected in the presence of vegetation. The winged seeds are more frequently trapped in the vegetation as compared to the unwinged seeds. The hypothesis that the seed dimorphism is an adaptation for differential dispersal distances is discussed.     

The effect of bat (Rousettus aegyptiacus) dispersal on seed germination in eastern Mediterranean habitats

The fruit-bat Rousettus aegyptiacus (Pteropodidae) in Israel consumes a variety of cultivated and wild fruits. The aim of this study was to explore some of its qualities as a dispersal agent for six fruit-bearing plant species. The feeding roosts of the fruit-bat are located an average of 30 m from its feeding trees and thus the bats disperse the seeds away from the shade of the parent canopy. The bat spits out large seeds but may pass some (2%) of the small seeds (<4 mg) through its digestive tract. However, neither the deposited seeds nor the ejected seeds (except in one case) had a significantly higher percentage germinating than intact seeds. Although the fruit-bat did not increase the percentage germinating, seeds of three plant species subject to different feeding behaviors (deposited in feces or spat out as ejecta) had a different temporal pattern of germination from the intact seeds. The combined seed germination distribution generated by these different treatments is more even over time than for each treatment alone. It is sugested that this increases asynchronous germination and therefore enhances plant fitness by spreading the risks encountered during germination, especially in eastern Mediterranean habitats where the pattern of rainfall is unpredictable.