Seed dispersal spectra: a comparison of temperate plant communities

Abstract. We compare the dispersal spectra of diaspores from varied plant communities in Australia, New Zealand, and North America, assigning dispersal mode to each diaspore type on the basis of apparent morphological adaptations. Species with ballistic and external dispersal modes were uncommon in most communities we surveyed. Ant dispersal was also rather uncommon, except in some Australian sclerophyll vegetation types. The frequency of vertebrate dispersal ranged up to 60% of the flora, the highest frequencies occurring in New Zealand forests. Wind dispersal ranged as high as 70% of the flora, with the highest values in Alaska, but usually comprised 10–30% of the flora. Many species in most communities had diaspores with no special morphological device for dispersal. Physiognomically similar vegetation types indifferentbiogeographic regions usually had somewhat dissimilar dispersal spectra. The frequency of dispersal by vertebrates often increased and the frequency of species with no special dispersal device decreased along gradients of increasing vertical diversity of vegetation structure. Elevation and moisture gradients also exhibited shifts in dispersal spectra. Within Australia, vertebrate- and wind-dispersal increased in frequency along a soil-fertility gradient, and dispersal by ants and by no special device decreased. Habitat breadths (across plant communities) and microhabitat breadths (within communities) for species of each major dispersal type did not show consistent differences, in general. Ant-dispersed species often had lower cover-values than other species in several Australian vegetation types. We discuss the ecological bases of these differences in dispersal spectra in terms of the availability of dispersal agents, seed size, and other ecological constraints. Seed size is suggested to be one ecological factor that is probably of general relevance to the evolution of dispersal syndromes.     

Contemporary pollen and seed dispersal in a Prunus mahaleb population: patterns in distance and direction

Pollination and seed dispersal determine the spatial pattern of gene flow in plant populations and, for those species relying on pollinators and frugivores as dispersal vectors, animal activity plays a key role in determining this spatial pattern. For these plant species, reported dispersal patterns are dominated by short-distance movements with a significant amount of immigration. However, the contribution of seed and pollen to the overall contemporary gene immigration is still poorly documented for most plant populations. In this study we investigated pollination and seed dispersal at two spatial scales in a local population of Prunus mahaleb (L.), a species pollinated by insects and dispersed by frugivorous vertebrates. First, we dissected the relative contribution of pollen and seed dispersal to gene immigration from other parts of the metapopulation. We found high levels of gene immigration (18.50%), due to frequent long distance seed dispersal events. Second, we assessed the distance and directionality for pollen and seed dispersal events within the local population. Pollen and seed movement patterns were non-random, with skewed distance distributions: pollen tended moved up to 548 m along an axis approaching the N-S direction, and seeds were dispersed up to 990 m, frequently along the SW and SE axes. Animal-mediated dispersal contributed significantly towards gene immigration into the local population and had a markedly nonrandom pattern within the local population. Our data suggest that animals can impose distinct spatial signatures in contemporary gene flow, with the potential to induce significant genetic structure at a local level.     

Dispersal of aquatic organisms by waterbirds: a review of past research and priorities for future studies

1. Inland wetlands constitute ecological islands of aquatic habitat often isolated by huge areas of non-suitable terrestrial habitats. Although most aquatic organisms lack the capacity to disperse by themselves to neighbouring catchments, many species present widespread distributions consistent with frequent dispersal by migratory waterbirds.
2. A literature review indicates that bird-mediated passive transport of propagules of aquatic invertebrates and plants is a frequent process in the field, at least at a local scale. Both endozoochory (internal transport) and ectozoochory (external transport) are important processes.
3. The characteristics of the dispersed and the disperser species that facilitate such transport remain largely uninvestigated, but a small propagule size tends to favour dispersal by both internal and external transport.
4. We review the information currently available on the processes of waterbird-mediated dispersal, establishing the limits of current knowledge and highlighting problems with research methods used in previous studies. We also identify studies required in the future to further our understanding of the role of such dispersal in aquatic ecology.     

The link between behavioural type and natal dispersal propensity reveals a dispersal syndrome in a large herbivore

When individuals disperse, they modify the physical and social composition of their reproductive environment, potentially impacting their fitness. The choice an individual makes between dispersal and philopatry is thus critical, hence a better understanding of the mechanisms involved in the decision to leave the natal area is crucial. We explored how combinations of behavioural (exploration, mobility, activity and stress response) and morphological (body mass) traits measured prior to dispersal were linked to the subsequent dispersal decision in 77 roe deer Capreolus capreolus fawns. Using an unusually detailed multi-trait approach, we identified two independent behavioural continuums related to dispersal. First, a continuum of energetic expenditure contrasted individuals of low mobility, low variability in head activity and low body temperature with those that displayed opposite traits. Second, a continuum of neophobia contrasted individuals that explored more prior to dispersal and were more tolerant of capture with those that displayed opposite traits. While accounting for possible confounding effects of condition-dependence (body mass), we showed that future dispersers were less neophobic and had higher energetic budgets than future philopatric individuals, providing strong support for a dispersal syndrome in this species.     

Measuring dispersal and detecting departures from a random walk model in a grasshopper hybrid zone

Abstract. 1. The grasshopper species Chorthippus brunneus and C. jacobsi form a complex mosaic hybrid zone in northern Spain. Two mark–release–recapture studies were carried out near the centre of the zone in order to make direct estimates of lifetime dispersal.
2. A model framework based on a simple random walk in homogeneous habitat was extended to include the estimation of philopatry and flying propensity. Each model was compared with the real data, correcting for spatial and temporal biases in the data sets.
3. All four data sets (males and females at each site) deviated significantly from a random walk. Three of the data sets showed strong philopatry and three had a long dispersal tail, indicating a low propensity to move further than predicted by the random walk model.
4. Neighbourhood size estimates were 76 and 227 for the two sites. These estimates may underestimate effective population size, which could be increased by the long tail to the dispersal function. The random walk model overestimates lifetime dispersal and hence the minimum spatial scale of adaptation.
5. Best estimates of lifetime dispersal distance of 7–33 m per generation were considerably lower than a previous indirect estimate of 1344 m per generation. This discrepancy could be influenced by prezygotic isolation, an inherent by-product of mosaic hybrid zone structure.     

Bryophyte dispersal by flying foxes: a novel discovery

This research provides the first evidence of dispersal of bryophytes and associated microorganisms through ingestion by a highly mobile vertebrate vector, the spectacled flying fox (Pteropus conspicillatus). Bryophyte fragments were found in faeces collected at four P. conspicillatus’ camps in the Wet Tropics bioregion, northeastern Australia. These fragments were viable when grown in culture; live invertebrates and other organisms were also present. Our study has significantly increased understanding of the role of flying foxes as dispersal vectors in tropical forests.     

Social constraints limit dispersal and settlement decisions in a group-living bird species

Dispersal is a fundamental process affecting the genetic structureof populations, speciation, and extinction. Nevertheless, ourunderstanding of the evolution of dispersal is limited by ourpaucity of knowledge on dispersal decisions at the individuallevel. We investigated the effect of interactions between residentsand juvenile dispersers on individual dispersal and settlementdecisions in Siberian jays (Perisoreus infaustus). In this group-livingbird species, some offspring remain on the parental territoryfor up to 3 years (retained juveniles) whereas other offspringdisperse within 2 months of fledging (dispersers). We foundthat retained juveniles constrained settlement decisions ofdispersers by aggressively chasing dispersers off their territory,resulting in dispersers continuing to disperse and settlingin groups without retained juveniles. Experimental removal ofmale breeders during the dispersal period also demonstratedthat dispersers were unable to settle in high-quality breedingopenings, which were instead filled by older nonbreeding residents.Rather, dispersers immigrated into groups without retained offspringwhere they became subordinate group members, queuing for a breedingopening. Also, they preferably settled in groups with shortqueues where no same-sex juveniles were present. Dispersal didnot inflict a cost to dispersers through increased mortality.However, the presence of immigrants was costly for breedersbecause it increased the rate of conflicts during the breedingseason which negatively affected nestling condition. These resultsdemonstrate that resident individuals constrain both dispersaland settlement decisions of dispersers. Social interactionsbetween residents and dispersers can thus be a key factor tounderstand the evolution of dispersal.     

Dispersal, interpatch movements, and survival in a shrubland breeding bird community

ABSTRACT Dispersal events can affect the distribution, abundance, population structure, and gene flow of animal populations, but little is known about long‐distance movements due to the difficulty of tracking individuals across space. We documented the natal and breeding dispersal of shrubland birds among 13 study sites in a 1000 km² area in southeastern Ohio. In addition, we radio‐marked and tracked 37 adult males of one shrubland specialist, the Yellow‐breasted Chat (Icteria virens). We banded 1925 juveniles and 2112 adults of nine shrubland species from 2002 to 2005. Of these, 33 (1.7%) juveniles were encountered in subsequent years (2003–2006) as adults (natal dispersal) and 442 (20.9%) birds initially banded as breeding adults were re‐encountered in subsequent years (breeding dispersal). Apparent survival of juvenile shrubland birds on their natal patches was 0.024 (95% CI 0.016–0.036). After accounting for the probability of detection, we found that 21% of birds banded as juveniles and recaptured as adults returned to their natal patches, whereas 78% of adult birds showed fidelity to the patch where they were originally captured. Moreover, natal dispersers tended to move farther than breeding dispersers (corrected natal median = 1.7 km ± 0.37; corrected breeding median = 0.23 km ± 0.10). We used our estimates of natal dispersal and annual apparent survival to estimate true survival at 0.11 (95% CI 0.07–0.18) for juveniles in their first year. However, this estimate was only applicable for birds dispersing within 7 km of their natal patches. Interpatch movements of radio‐marked Yellow‐breasted Chats were not uncommon, with 13 of 37 males located in more than one habitat patch. Overall, we observed low natal philopatry, but high adult site fidelity for shrubland birds in our study area. Considering the frequency of short‐distance movements observed (median = 531 m, range = 88–1045 m), clustering of patches within 1 km might facilitate use of shrubland habitat.     

Going against the flow: a case for upstream dispersal and detection of uncommon dispersal events

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Directed vertebrate-mediated seed dispersal of a fleshy-fruited amaryllid in a heterogenous habitat

Most plants with fleshy fruits have seeds that are ingested by animals, but a less well-understood mode of seed dispersal involves fleshy fruits containing seeds that are discarded by frugivorous animals because they are too large or toxic to be ingested. We studied the seed dispersal biology of Haemanthus deformis, an amaryllid lily species found in a mosaic of bush clumps in a grassland matrix in South Africa. We asked whether seed dispersal is directed in and among bush clumps and whether germination and survival are greater for seeds dispersed to bush clumps than for those dispersed into grassland. Using camera trapping, we found that fruits are consumed mainly by birds and rodents. The pulp was removed from the seeds which were then discarded without ingestion. While many seeds were dispersed close to the parent plant, most (c. 78.5%) were dispersed further than 1 m away from the parent plant. Longer distance dispersal resulted mainly from birds flying off with fruits in their bill or from rodents engaging in scatter-hoarding behavior. Seedling survival was most successful within bush clumps as compared to grasslands and shade was identified as a primary requirement for seedling survival. Seeds from which the fruit pulp had been removed germinated faster than those in intact fruits. Haemanthus deformis deploys a system of directed seed dispersal, whereby both birds and rodents contribute to the dispersal of seeds within patchy bush clumps that are favorable for seedling survival.     

Spatial and environmental effects on a rock‐pool metacommunity depend on landscape setting and dispersal mode

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Spatial distribution and temporal dynamic of the seed pool in a Danish lowland stream

In this paper we quantify the seed pools in four locations down through a small stream in Denmark. The objectives of the study were (1) to test if the number of plant taxa in the seed pool increases downstream in the stream system; (2) to test if the seed density in the pool is highest in autumn; and (3) to analyze to what extent the seed pool contributes to re-establishment of vegetation in spring. Counting method showed no increase in species taxa in the seed pool down through the stream system. Neither did we find clear seasonal patterns in seed abundance in the seed pool. By concurrent field and laboratory observations in spring we found that seed pools can function as a source for re-establishing the vegetation in spring. However, the actual germination rate in the field was <100 and <10 times the germination rate in the laboratory for aquatic and terrestrial plants, respectively. The results suggest that small lowland streams contain large seed pools in deposition areas throughout the year, and that seed pools are much more important for the re-establishment of riparian stream vegetation in spring than submerged vegetation.     

Myrmecochorous dispersal distances: a world survey

Abstract. Myrmecochorous dispersal distances are reviewed; the seed dispersal curve generated by ants shows a characteristic peak at short distances and a long tail, a shape suited to small densities of safe sites. Mean global distance is of 0.96 m (n= 2524) with a range of 0.01–77 m. Data have been broken down by geography (Northern hemisphere v. Southern hemisphere), taxonomy (ant subfamilies) and ecology (vegetation: sclerophyllous v. mesophyllous). Although a statistical difference exists between dispersal curves from the Northern hemisphere and the Southern hemisphere, this may be an artefact of lack of data from mesophyllous myrmecochores from this hemisphere. The four ant subfamilies do show also numerical differences but could not be subjected to statistical analysis. A difference between the shape of dispersal curve for sclerophyllous myrmecochores and mesophyllous myrmecochores has also been detected. We hypothesize that this difference is related to the myrmecological communities from both types of vegetation: dispersing ants from sclerophyllous vegetation would have smaller nest densities and/or bigger foraging areas than dispersing ants from mesic environments.     

Soil seed bank and driftline composition along a successional gradient on a temperate salt marsh

Abstract. This study focuses on the relationship between vegetation succession and soil seed bank composition on the Schiermonnikoog (The Netherlands) salt marsh over 100 yr. The importance of driftline material in seed dispersal and the relationship with succession is also investigated. The results indicate that the majority of species have a transient or short‐term seed persistent bank. Seeds of most species are able to float over the salt marsh and become concentrated in the driftline higher up the marsh. After plants have established a seed bank forms, which disappears when vegetation is replaced by later‐successional species. Exceptions are Spergularia mar‐itima, which is still present in the seed bank of late successional stages, and Juncus gerardi and Glaux maritima, which appear in the seed bank of early successional stages, but are absent in the vegetation. Based on the results of this study constraints and possibilities for salt‐marsh restoration by de‐embankment are discussed.     

Dispersal influences genetic and acoustic spatial structure for both males and females in a tropical songbird

Animals exhibit diverse dispersal strategies, including sex‐biased dispersal, a phenomenon common in vertebrates. Dispersal influences the genetic structure of populations as well as geographic variation in phenotypic traits. Patterns of spatial genetic structure and geographic variation may vary between the sexes whenever males and females exhibit different dispersal behaviors. Here, we examine dispersal, spatial genetic structure, and spatial acoustic structure in Rufous‐and‐white Wrens, a year‐round resident tropical bird. Both sexes sing in this species, allowing us to compare acoustic variation between males and females and examine the relationship between dispersal and song sharing for both sexes. Using a long‐term dataset collected over an 11‐year period, we used banding data and molecular genetic analyses to quantify natal and breeding dispersal distance in Rufous‐and‐white Wrens. We quantified song sharing and examined whether sharing varied with dispersal distance, for both males and females. Observational data and molecular genetic analyses indicate that dispersal is female‐biased. Females dispersed farther from natal territories than males, and more often between breeding territories than males. Furthermore, females showed no significant spatial genetic structure, consistent with expectations, whereas males showed significant spatial genetic structure. Overall, natal dispersal appears to have more influence than breeding dispersal on spatial genetic structure and spatial acoustic structure, given that the majority of breeding dispersal events resulted in individuals moving only short distances. Song sharing between pairs of same‐sex animals decreases with the distance between their territories for both males and females, although males exhibited significantly greater song sharing than females. Lastly, we measured the relationship between natal dispersal distance and song sharing. We found that sons shared fewer songs with their fathers the farther they dispersed from their natal territories, but that song sharing between daughters and mothers was not significantly correlated with natal dispersal distance. Our results reveal cultural differences between the sexes, suggesting a relationship between culture and sex‐biased dispersal.     

Extensive pollen dispersal in a bird-pollinated shrub, Calothamnus quadrifidus, in a fragmented landscape

Pollen dispersal was investigated in six populations of Calothamnus quadrifidus, a bird-pollinated shrub in the fragmented agricultural region of southern Western Australia. Paternity analysis using six microsatellite loci identified a pollen source within populations for 67% of seedlings, and the remainder were assumed to have arisen from pollen sources outside the populations. Outcrossing was variable, ranging from 5% to 82%, and long-distance pollen dispersal was observed in all populations with up to 43% of pollen sourced from outside the populations over distances of up to 5 km. This extensive pollen immigration was positively associated with population size but not isolation. Comparison of two populations of similar size but different density showed greater internal pollination and less selfing in the denser population, suggesting an influence of density on pollinator behaviour. The study revealed extensive long-distance pollen dispersal for C. quadrifidus within this fragmented agricultural landscape and highlighted the interaction between reserve populations and isolated road verge remnants in maintaining genetic connectivity at the landscape scale.     

Community seed rain patterns and a comparison to adult community structure in a West African tropical forest

We examined the seed rain throughout a twelve month period in a lowlandtropical forest in Cameroon, West Africa, 1996–97. Traps (0.5m², n = 216) were erected throughout a 25km² area in 24 randomly placed clusters of nine trapseach. Fruits and seeds that landed in traps were collected every 7–10daysand classified by species and dispersal type. More than 32,000 seeds fromapproximately 200 species fell into the traps, an average of 297 seedsm^–2 yr^–1 Thirty species represent 82%of the total seed rain while an additional 175 species comprise the remaining18%. When we compared the adult community to the seed rain community within thesame plots, we found no apparent correlation between seed rain patterns andadult community structure for this year of study. Furthermore, only 49% of theadult tree community produced and dispersed seed into traps in this year. Morethan 100 species (52%) found in the seed rain represented long-distanceimmigrant seed rain. Seed rain was highly variable at several scales, bothspatially and seasonally, although seeds arrived in traps during eachcollectionperiod. Cluster analyses showed that traps within plots were seldom moresimilarto one another than traps between plots. While 82% of the tree species in thecommunity are thought to be animal dispersed, only 28% of all seeds that fellinto traps had been obviously handled (bitten, chewed, or passed)by animals. Tests for fruit and seed removal by predators or dispersers found5%or less removal rate from traps.     

Spatial genetic structure within a metallicolous population of Arabidopsis halleri, a clonal, self-incompatible and heavy-metal-tolerant species

Arabidopsis halleri, a close wild relative of A. thaliana, is a clonal, insect-pollinated herb tolerant to heavy metals (Zn, Pd, Cd) and a hyperaccumulator of Zn and Cd. It is of particular interest in the study of evolutionary processes and phytoremediation. However, little is known about its population gene flow patterns and the structure of its genetic diversity. We used five microsatellite loci to investigate the genetic structure at a fine spatial scale (10 cm to 500 m) in a metallicolous population of A. halleri. We also studied the contributions made by clonal propagation and sexual reproduction (seed and pollen dispersal) to the genetic patterns. Clonal diversity was high (D(G) > 0.9). Clonal spread occurs only at short distances (< 1 m). Both clonal spread and limited dispersal, associated with sexual reproduction, contribute to the significant spatial genetic structure revealed by spatial autocorrelation analysis. The shape of the autocorrelogram suggests that seed dispersal is restricted and pollen flow extensive, which may be related to intense activity by insect pollinators. Clonal spread was more extensive in the lowly polluted zone than in the highly polluted zone. This cannot be interpreted as a strategy for promoting the propagation of adapted genotypes under the harshest ecological constraints (highest heavy metal concentrations). The higher fine-scale spatial genetic structure found in the lowly polluted zone can be ascribed to plant densities that were lower than in the highly polluted zone. No evidence of genetic divergence due to spatial heavy metal heterogeneity was found between lowly and highly polluted zones.