Inbreeding depression in Vriesea gigantea,a perennial bromeliad from southern Brazil

Inbreeding depression is a reduction of fitness in the progeny of closely related individuals and its effects are assigned to selfing or biparental inbreeding. Vriesea gigantea is a self‐compatible bromeliad species distributed in the Brazilian Atlantic rainforest and habitat destruction and fragmentation and collection have decreased the natural populations. We aim to describe the occurrence of inbreeding depression (δ) in three natural populations of V. gigantea and to correlate this phenomenon with previous studies of fertility, genetic diversity, population genetic structure, gene flow, mating system and seed dispersal in this species. Fifty‐four adult plants were sampled and 108 flowers were used for pollination treatments (selfing, outcrossing and control). For adult plants, we analysed plant and inflorescence height, flower numbers and seed set. In the progenies, evaluated parameters included seed germination and seedling survival rate. The results indicated low to moderate levels of inbreeding depression in V. gigantea (δ = 0.02 to 0.39), in agreement with molecular data from a previous study. Vriesea gigantea populations tolerate some degree of inbreeding, which is consistent with previous results on fertility, mating system, genetic diversity and gene flow. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 169 , 312–319.     

Seed germination of the carnivorous plant Byblis gigantea (Byblidaceae) is cued by warm stratification and karrikinolide

Byblidaceae is one of the most poorly studied carnivorous plant families, with seed dormancy and germination biology remaining unresolved. This knowledge deficit has significant conservation and management implications, particularly as the most southerly distributed species, the south‐west Western Australian endemic Byblis gigantea, is listed as critically endangered. This study examined the ecophysiology of seed dormancy and germination in B. gigantea in concert with a study of the population dynamics of a single plant community. Mass seedling emergence and plant establishment were observed after a wildfire in 2007 in the natural population, followed by a rapid decline in mature individuals (91%) over a 4‐year monitoring period, with almost no inter‐fire recruitment (1.4% of all emergence) observed. Seeds possessed a fully developed embryo, and the germination characteristics of fresh seeds classified them as showing physiological dormancy. Seed dormancy was partially alleviated by warm stratification (30 °C) for 8 weeks prior to incubation at 15 °C, with c. 40% germination observed. With the additional exposure of seeds to the germination‐active chemical in smoke, karrikinolide, the germination of warm‐stratified seeds increased to 89%. Seeds also displayed orthodox storage behaviour and appeared to be amenable to long‐term seed banking for conservation. These results present the first observation of the stimulation of the germination of a carnivorous plant by a smoke‐derived compound. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172 , 143–152.     

Fragmentation reduces regional‐scale spatial genetic structure in a wind‐pollinated tree because genetic barriers are removed

Gene flow strongly influences the regional genetic structuring of plant populations. Seed and pollen dispersal patterns can respond differently to the increased isolation resulting from habitat fragmentation, with unpredictable consequences for gene flow and population structuring. In a recently fragmented landscape we compared the pre‐ and post‐fragmentation genetic structure of populations of a tree species where pollen and seed dispersal respond differentially to forest fragmentation generated by flooding. Castanopsis sclerophylla is wind‐pollinated, with seeds that are dispersed by gravity and rodents. Using microsatellites, we found no significant difference in genetic diversity between pre‐ and post‐fragmentation cohorts. Significant genetic structure was observed in pre‐fragmentation cohorts, due to an unknown genetic barrier that had isolated one small population. Among post‐fragmentation cohorts this genetic barrier had disappeared and genetic structure was significantly weakened. The strengths of genetic structuring were at a similar level in both cohorts, suggesting that overall gene flow of C. sclerophylla has been unchanged by fragmentation at the regional scale. Fragmentation has blocked seed dispersal among habitats, but this appears to have been compensated for by enhanced pollen dispersal, as indicated by the disappearance of a genetic barrier, probably as a result of increased wind speeds and easier pollen movement over water. Extensive pollen flow can counteract some negative effects of fragmentation and assist the long‐term persistence of small remnant populations.     

荒漠孑遗植物裸果木种子时空扩散特性

裸果木(Gymnocarpos przewalskii)是亚洲中部荒漠区少有的第三纪孑遗物种,由于气候变化及人为干扰,其自然种群分布范围不断缩小。种子扩散作为植物生活史过程中的重要阶段,不仅对物种生存及其多样性至关重要,还影响物种分布范围和局部丰度。2015年和2016年分别在新疆哈密地区,采用布设种子收集器的方法,对其自然种群种子扩散的时空动态进行了定点连续观测。结果表明:该物种于当年6月上旬开始扩散,2015年略早于2016年。每年种子扩散持续时间约两个月,扩散趋势为单峰曲线,且呈集中大量扩散的模式,扩散高峰期与当年初次月降水高峰期吻合;在顺风的正南和东南方向上,种子扩散密度大且距离远;种子扩散主要集中在母株冠幅下,随着距母株距离的增加,种子扩散密度减少,二者间存在极显著的负相关性(P0.01),由于裸果木枝条繁多,对风力强度起到了一定的阻碍作用,可能是造成种子集中扩散在母株下的原因。裸果木种子扩散受外界环境(降水、风向)和自身因素等方面的影响,当种子在大量降水前完成扩散,将有利于种子在适宜的微生境萌发,是对多风、干旱的恶劣生境的一种长期适应。     

Spatiotemporal patterns of seed dispersal in a wind-dispersed Mediterranean tree (Acer opalus subsp. granatense): implications for regeneration

Seed dispersal can severely limit the quantity of plant recruits and their spatial distribution. However, our understanding of the role of dispersal in regeneration dynamics is limited by the lack of knowledge of seed deposition patterns in space and time. In this paper, we analyse the spatiotemporal variability of seed dispersal patterns in the Mediterranean maple, Acer opalus subsp. granatense, by monitoring seed rain along two years at a broad spatial scale (2 mountain ranges, 2 populations per range, 4 microhabitats per population). We quantified seed limitation and its components (source and dispersal limitation), and explored dispersal limitation in space by analysing dispersal distances, seed aggregation, and microhabitat seed distribution. Acer opalus subsp. granatense was strongly seed‐limited throughout the gradients explored, being always dispersal limitation much higher than source limitation. The distribution of seeds with distance from adult individuals was leptokurtic and right‐skewed in all populations, being both kurtosis and skewness higher the year of the highest seed production. Dispersal distances were shorter than expected by random in the four populations, which suggests distance‐limited dispersal. Dispersal patterns were highly aggregated and showed a preferential direction around adults. At the microhabitat scale, most seeds accumulated under adult maples. However, there were no more seeds under trees and shrubs other than maple than in open interspaces, implying that established vegetation does not disrupt patterns of seed deposition by physically trapping seeds. When compared with patterns of seedling establishment, limited dispersal ability and inter‐annual spatial concordance in seed rain patterns suggest that several potentially safe sites for recruitment have a very low probability of receiving seeds in most maple populations. These findings are especially relevant for rare species such as Acer opalus subsp. granatense, and illustrate how dispersal studies are not only crucial for our understanding of plant population dynamics but also to provide conservation directions.     

Populational dynamics of three annual species of alpine plants in the Rocky Mountains

Summary Survivorship, fecundity, and seed bank size were measured over two years in alpine populations of the annuals, Koenigia islandica, Polygonum confertiflorum, and P. douglasii. The major loss of individuals from all populations occurred between seed dispersal and germination. Survival of vegetative plants to maturity was high in all species, usually above 80%, and the average number of seeds produced per plant was less than 10 in all species in both years. Seed banks existed for all three species but were small since more than 80% of the viable seeds in the soil germinated in the spring. Both survival and fecundity were negatively correlated with density in P. confertiflorum. These patterns of population dynamics are similar to those found in many annuals of temperate environments.     

Geographic variation in seed traits within and among forty‐two species of Rhododendron (Ericaceae) on the Tibetan plateau: relationships with altitude,habitat, plant height,and phylogeny

Seed mass and morphology are plant life history traits that influence seed dispersal ability, seeding establishment success, and population distribution pattern. Southeastern Tibet is a diversity center for Rhododendron species, which are distributed from a few hundred meters to 5500 m above sea level. We examined intra‐ and interspecific variation in seed mass and morphology in relation to altitude, habitat, plant height, and phylogeny. Seed mass decreased significantly with the increasing altitude and increased significantly with increasing plant height among populations of the same species. Seed mass differed significantly among species and subsections, but not among sections and subgenera. Seed length, width, surface area, and wing length were significantly negative correlated with altitude and significantly positive correlated with plant height. Further, these traits differed significantly among habitats and varied among species and subsection, but not among sections and subgenera. Species at low elevation had larger seeds with larger wings, and seeds became smaller and the wings of seeds tended to be smaller with the increasing altitude. Morphology of the seed varied from flat round to long cylindrical with increasing altitude. We suggest that seed mass and morphology have evolved as a result of both long‐term adaptation and constraints of the taxonomic group over their long evolutionary history.     

The role of avian ‘seed predators’ as seed dispersers

Seed dispersal is a central process in plant ecology with consequences for species composition and habitat structure. Some bird species are known to disperse the seeds they ingest, whereas others, termed ‘seed predators’, digest them and apparently play no part in dispersal, but it is not clear if these are discrete strategies or simply the ends of a continuum. We assessed dispersal effectiveness by combining analysis of faecal samples and bird density. The droppings of seed dispersers contained more entire seeds than those of typical seed predators, but over a quarter of the droppings of seed predators contained whole seeds. This effect was further magnified when bird density was taken into account, and was driven largely by one frequent interaction: the Chaffinch Fringilla coelebs, a typical seed predator and the most abundant bird species in the area and dispersed seeds of Leycesteria formosa, a non‐native plant with berry‐like fruits. These results suggest the existence of a continuum between seed predators and seed dispersers.     

Positive effects of flower abundance and synchronous flowering on pollination success,and pollinia dispersal in rewardless Changnienia amoena (Orchidaceae)

Pollination success and pollen dispersal in natural populations depend on the spatial‐temporal variation of flower abundance. For plants that lack rewards for pollinators, pollination success is predicted to be negatively related to flower density and flowering synchrony. We investigated the relationships between pollination success and flower abundance and flowering synchrony, and estimated pollinia dispersal distance in a rewardless species, Changnienia amoena (Orchidaceae). The results obtained in the present study revealed that male pollination success was negatively influenced by population size but was positively affected by population density, whereas female pollination success was independent of both population size and density. Phenotypic analysis suggested that highly synchronous flowering was advantageous through total pollination success, which is in contrast to previous studies. These results indicate that pollination facilitation rather than competition for pollinator visits occurs in this rewardless plant. The median distance of pollinia dispersal was 11.5 m (mean distance = 17.5 m), which is comparable to that of other rewardless plants but longer than for rewarding plants. However, pollen transfer occured mainly within populations; pollen import was a rare event. Restricted gene flow by pollinia and seeds probably explains the previous population genetic reporting a high degree of genetic differentiation between populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 477–488.     

Role of Brycinus lateralis (Teleostei: Alestidae) in dispersal and germination of Nymphaea nouchali (Angiospermae: Nymphaeaceae) seeds on a seasonal floodplain of the Okavango Delta,Botswana

Seed passage through the gut of vertebrates can be important for seed dispersal, but might influence seed viability. The ability of seeds to germinate after ingestion by seed-eating fish is important for the population dynamics of some plant species, and significant in the evolution of plant–fish interactions. Certain fish in the Okavango Delta, Botswana, are fruit- and seed-eaters and could act as seed dispersers. We sampled 14 fish species in 2013, finding Nymphaea nouchali var. caerulea seeds in the digestive tracts of eight, most commonly in the striped robber Brycinus lateralis. Seeds extracted from the gut of this species had an overall mean germination success of 11.7%. This fish species might well be a legitimate seed disperser, having a positive effect on seed dispersal from parent plants in the Okavango Delta. The current study represents one of the first investigations of the likelihood of seed dispersal by fish on the African continent.     

Primate seed dispersal leaves spatial genetic imprint throughout subsequent life stages of the Neotropical tree Parkia panurensis

Key message

The Neotropical tree Parkia panurensis shows a spatial genetic structure from the seed to the adult stage that is most likely the outcome of the seed dispersal provided by primates.

Abstract

Seed dispersal and pollination determine the gene flow within plant populations. In addition, seed dispersal creates the template for subsequent stages of plant recruitment. Therefore, the question arises whether and how seed dispersal affects the spatial genetic structure (SGS) of plant populations. In this study, we used microsatellites to analyse the SGS of the Neotropical tree Parkia panurensis (Fabaceae). This plant species is a major food resource for primates and its seeds are mainly dispersed by primates. Seeds were collected during behavioural observations of a tamarin mixed-species troop in north-eastern Peru. Additionally, leaf samples of juveniles and of adults trees of this species were collected throughout the home range of the tamarin troop. A significant SGS for embryos (located within the dispersed seeds) and for non-reproductive plants are found up to a distance of 300 m. This matches the distance within which most seeds are dispersed. In the adult stage, the scale of a significant SGS is reduced to 100 m. While we cannot explain this scale reduction, our study provides the first evidence that primate seed dispersal does influence the SGS of a tropical tree species.     

GENETIC CONSEQUENCES OF SEED DISPERSAL IN THREE SYMPATRIC FOREST HERBS. I. HIERARCHICAL POPULATION-GENETIC STRUCTURE

To examine the effects of seed dispersal on spatial genetic structure, we compare three sympatric species of forest herbs in the family Apiaceae whose fruits differ widely in morphological adaptations for animal-attached dispersal. Cryptotaenia canadensis has smooth fruits that are gravity dispersed, whereas Osmorhiza claytonii and Sanicula odorata fruits have appendages that facilitate their attachment to animals. The relative seed-dispersal ability among species, measured as their ability to remain attached to mammal fur, is ranked Sanicula > Osmorhiza > Cryptotaenia. We use a nested hierarchical sampling design to analyze genetic structure at spatial scales ranging from a few meters to hundreds of kilometers. Genetic differentiation among population subdivisions, estimated by average genetic distance and hierarchical F-statistics, has an inverse relationship with dispersal ability such that Cryptotaenia > Osmorhiza > Sanicula. In each species, genetic differentiation increases with distance among population subdivisions. Stochastic variation in gene flow, arising from seed dispersal by attachment to animals, may partly explain the weak relationship between pairwise spatial and genetic distance among populations and heterogeneity in estimates of single locus F-statistics. A hierarchical island model of gene flow is invoked to describe the effects of seed dispersal on population genetic structure. Seed dispersal is the predominant factor affecting variation in gene flow among these ecologically similar, taxonomically related species.     

Effects of different abiotic and biotic factors on spatial primary seed dispersal in the semachorous species Scrophularia canina

Seed dispersal is one of the most important steps in the plant life cycle. However, there is, generally, a lack of fieldworks focused on wind dispersal and especially on semachorous dispersal (seeds spread when the fruits are shaken by wind and other vectors, such as animals), including boleochorous dispersal. Therefore, we aimed to determine how different types of wind and animals affected seed dispersal under natural conditions in the widespread species Scrophularia canina. We evaluated the effects of wind gusts (simulating them using a leaf blower) and wild animals (using differently sized dogs) on seed dispersal in a population located in south-western Europe. We found that S. canina is a semachorous species, and its spatial seed dispersal was affected by wind gust speed and direction, plant structure and vector type. The results also revealed the presence of xerochasy, individual anisotropy with strong winds, and primary short-distance dispersal associated with successional processes independent of the vector. Additionally, there was a masking effect of plant structure on the seed shadow outline. It is essential to conduct fieldworks to reveal what actually happens in nature, taking into account the characteristics determining seed dispersal. In addition, in these works it is important to find out what factors affect seed distributions of anemochorous and semachorous species.     

Effects of ant behaviour and presence of extrafloral nectaries on seed dispersal of the Neotropical myrmecochore Turnera ulmifolia L. (Turneraceae)

This paper describes the myrmecochory system of Turnera ulmifolia in a coastal sand dune matorral in Mexico. Turnera ulmifolia has elaiosome‐bearing seeds and extrafloral nectaries (EFNs). In ten quadrants (4 × 15 m) ant–seed interaction was monitored, and an interaction intensity index calculated and correlated with the number of seedlings. Seed removal rates by ants were surveyed every 2 h for 24 h, the ants being observed both on and beneath the plants. The role of the elaiosome in seed removal was evaluated by offering seeds with and without elaiosomes, and elaiosomes only. Finally, the effect of ant manipulation in seed germination was evaluated. There were 25 ant species associated with seeds and/or EFNs, the most frequently recorded being Monomorium cyaneum and Forelius analis. There was a positive correlation between the intensity index and seedling number per quadrant. There was significantly higher mean seed removal during the day than during the night (19.3% and 12.3%, respectively), and from beneath than on the plant (21.9% and 9.5%, respectively). The preference for elaiosomes only was also greater during the diurnal period, and when gathered on, rather than beneath, the plant. Seed manipulation by F. analis enhanced germination by T. ulmifolia. Seed removal, dispersal distances, seed predation and germination were largely determined by ant behaviour. The presence of EFNs may be influencing seed removal on the plant by attracting a specific assemblage of omnivorous ants. Among such assemblages associated with T. ulmifolia we encountered a variety of behaviours, with ant species either good at defending plants but bad at dispersing seeds, or vice versa. We discuss the way in which these two rewards, and the processes involved (defence and dispersion), could have interacted with each other and evolved. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86 , 67–77.     

Seed dispersal by lizards on a continental‐shelf island: predicting interspecific variation in seed rain based on plant distribution and lizard movement patterns

Aim We estimated the patterns of seed deposition provided by the eyed lizard, Timon lepidus, and evaluated whether these patterns can be generalized across plant species with different traits (fruit and seed size) and spatial distributions. Location Monteagudo Island, Atlantic Islands National Park (north‐western Spain). Methods We radio‐tracked seven lizards for 14 days and estimated their home ranges using fixed kernels. We also geo‐referenced all fruit‐bearing individuals of four plant species dispersed by eyed lizards in the study area (Corema album, Osyris alba, Rubus ulmifolius and Tamus communis), measured the passage time of their seeds through the lizard gut, and estimated seed predation in four habitats (bare sand, grassland, shrub and gorse). Seed dispersal kernels were estimated using a combination of these data and were combined with seed predation probability maps to incorporate post‐dispersal seed fate (‘seed survival kernels’). Results Median seed gut‐passage times were around 52–98 h, with maximum values up to 250 h. Lizards achieved maximum displacement in their home ranges within 24–48 h. Seed predation was high (80–100% of seeds in 2 months), particularly under Corema shrub and gorse. Seed dispersal kernels showed a common pattern, with two areas of preferential seed deposition, but the importance of these varied among plant species. Interspecific differences among dispersal kernels were strongly reduced by post‐dispersal seed predation; hence, seed survival kernels of the different plant species showed high auto‐ and pairwise‐correlations at small distances (< 50 m). As a result, survival to post‐dispersal seed predation increased with dispersal distance for O. alba and T. communis, but not for C. album. Main conclusions Seed dispersal by lizards was determined primarily by the interaction between the dispersers’ home ranges and the position of the fruit‐bearing plants. As a result, seed rain shared a common template, but showed considerable variation among species, determined by their specific spatial context. Seed predation increased the spatial coherence of the seed rain of the different species, but also resulted in contrasting relationships between seed survival and dispersal distance, which may be of importance for the demographic and evolutionary processes of the plants.     

What are the consequences of ant–seed interactions on the abundance of two dry-fruited shrubs in a Mediterranean scrub?

Strong interactions between dry-fruited shrubs and seed-harvesting ants are expected in early successional scrubs, where both groups have a major presence. We have analysed the implications of the seed characteristics of two dry-fruited shrub species (Coronilla minima and Dorycnium pentaphyllum) on seed predation and dispersal mediated by harvester ants and the consequences of these processes on spatio-temporal patterns of plant abundance in a heterogeneous environment. We found that large C. minima seeds were collected much more (39%) than small D. pentaphyllum seeds (2%). However, not all of the removed seeds of these plant species were consumed, and 12.8% of the seeds were lost along the trails, which increased dispersal distances compared with abiotic dispersal alone. Seed dropping occurred among all microhabitats of the two plant species, but especially in open microhabitats, which are the most suitable ones for plant establishment. The two plant species increased their presence in the study area during the study period: C. minima in open microhabitats and D. pentaphyllum in high vegetation. The large size of C. minima seeds probably limited the primary seed dispersal of this species, but may have allowed strong interaction with ants. Thus, seed dispersal by ants resulted in C. minima seeds reaching more suitable microhabitats by means of increasing dispersal distance and redistribution among microhabitats. In contrast, the smaller size of D. pentaphyllum seeds arguably allows abiotic seed dispersal over longer distances and colonization of all types of microhabitats, although it probably also limits their interaction with ants and, consequently, their redistribution in suitable microhabitats. We suggest that dyszoochory could contribute to the success of plant species with different seed characteristics in scrub habitats where seeds are abundantly collected by seed-harvesting ants.     

Plant recruitment bottlenecks in temperate forest fragments: seed limitation and insect herbivory

Numerous studies have documented declines in plant diversity in response to habitat loss in fragmented landscapes. However, determining the mechanisms that lead to species loss is challenging using solely a correlative approach. Here we link correlative assessments of plant community composition with seed additions for a focal species to test the hypothesis that distributions of forests plants within a fragmented landscape are limited by seed dispersal. Woody plant species richness of fragments declined as fragments (n=26) became more isolated by agricultural fields. We predicted that if these isolation effects were driven by poor dispersal rather than other effects associated with habitat loss, then plants should vary in their response to isolation in relation to their seed size (i.e., stronger effects for plants with larger seeds). As predicted under this dispersal limitation hypothesis, sensitivity of bird-dispersed shrubs to isolation was related to their seed mass, with species with heavy seeds (e.g., Lindera benzoin) exhibiting stronger declines in presence across isolation gradients than species with light seeds. Seed addition experiments were performed for Lindera benzoin in two high isolation forest fragments (nearest neighbor mean distance=803 m) where Lindera was naturally absent, and two low isolation fragments (nearest neighbor mean distance=218 m) with naturally occurring Lindera populations. Seed addition and control plots (n=50 1 m² plots per fragment) were monitored for 13 censuses over 3 years. Across all four fragments, seed additions resulted in significant increases in Lindera seedling recruitment with no differences in final seedling establishment among fragments. However, insect herbivory was higher on Lindera seedlings in high isolation compared to low isolation fragments and was negatively correlated with seedling survival over some years. Consistent with prior work, our results confirm that seed dispersal plays a significant role in affecting plant diversity in fragmented landscapes. However, results also suggest the need for a better understanding of how additional processes, such as herbivory, may be altered as habitat is lost and what effects such changes have for forest plants.     

Seed and microsite limitations of recruitment and the impacts of post-dispersal seed predation at the within population level

Seed predation may cause important seed losses in plant populations, but its impact on the dynamics of populations will depend on the degree of seed or microsite limitations for recruitment. Seed losses will only affect recruitment if it is seed limited. The nature of recruitment limitation (seeds or microsites) is usually ascribed to whole plant populations but it may vary within populations among microhabitats and habitats. Thus, the potential impact of seed predation will also vary within the population, being highest where recruitment is seed limited. The impact to the whole population will depend on the spatial concordance between the intensity of seed predation and that of seed limitation. Recruitment limitations (with seed addition experiments), seed predation (with seed removal experiments), and the dynamics of seed availability in the soil (with soil samples taken both after seed dispersal and before the following dispersal event) of the shrub Corema album (Empetraceae) were investigated in dunes in NW Spain, at microhabitats ‘open ground’, ‘underneath C. album ♀’, and ‘underneath C. album ♂’ at two habitats, sparse and dense scrub. The nature of recruitment limitation (seeds vs. microsites) varied within the population. It was seed limited in the microhabitat ‘open ground’ and microsite limited under shrub cover. The spatial patterns of seedling recruitment were unrelated to seed availability but strongly affected by germination requirements. The spatial discordance between seed availability and recruitment implies a crucial constraint for processes affecting seed availability (seed predation but also e.g., dispersal) to impact recruitment. They will not affect its spatial pattern but only its quantity as long as they act in those sites selected by seeds to germinate. Seed predation was highest underneath mother plants and lowest in open ground. Thus, its potential impact is low, as it is centred where recruitment is not seed limited. This study shows that the analysis of seed predation in relation to recruitment limitations at smaller spatial scales within the population provides more insight to understand its impact.     

Seed Dispersal by Avian Frugivores: Non‐random Heterogeneity at Fine Scales

Seed dispersal studies have primarily examined dispersal as a function of distance from the parent tree and/or heterogeneity in dispersal due to animal use of nesting, roosting and sleeping sites. However, non‐random heterogeneity in seed dispersal is also likely to result from the post‐foraging behavior and movement of frugivores which prefer certain trees. To characterize variation in seed rain at fine scales, we studied the dispersal curve of Prunus ceylanica, a primarily bird‐dispersed species. We compared seed rain at conspecifics, heterospecific fruiting trees with similar frugivore assemblages, emergent trees, and the landscape surrounding these trees. Seed rain of P. ceylanica was found to peak globally under the canopy of conspecifics but to peak locally under the canopy and immediate neighborhood of heterospecific fruiting trees. Our results demonstrate that seed rain is highly clumped even at fine spatial scales. A large proportion of seeds are dispersed in specific, localized regions. This variation can have important implications for plant population dynamics and might significantly alter the impact of post‐dispersal processes. Seed dispersal models may need to incorporate this heterogeneity to explain manifestations of spatially explicit dynamics like mixed species ‘orchards’.     

The influence of seed dispersal mechanisms on the genetic structure of tropical tree populations

Seed dispersal mechanisms should have a direct impact on the genetic structure of populations. Species whose seeds are dispersed near the maternal plant (e.g. gravity or wind dispersal) or species whose seeds are deposited in clumps or patches should have more fine-scale genetic structure than species whose seeds are dispersed singly by mobile animals. Furthermore, due to the overlap of seed shadows, species with high adult densities should have less genetic structure than species with lower densities. Allozyme analyses of three tropical tree species belonging to the moist tropical forest of Barro Colorado Island, Republic of Panama, were used to describe variation in the scale and intensity of genetic structure within their populations. The genetic structure of seedlings and immature trees in the low-density, wind-dispersed species (Platypodium elegans) was the coarsest and strongest whereas genetic structure in a population of Swartzia simplex var. ochnacea (high density, bird-dispersed) was both the finest and the weakest. The genetic structure of Alseis blackiana, a high-density, wind-dispersed species was intermediate in both degree and scale. In P. elegans and A. blackiana, which had J shaped size distributions, the significant genetic structure seen in the smaller and intermediate diameter classes disappeared in the largest diameter class. The loss of genetic structure was not observed in S. simplex, a species with a more even size distribution.