Arrival Of Seeds Research Articles

Spillover of avian seed dispersers between secondary forests and degraded areas in a tropical island

Seed arrival to degraded areas often represents a limiting factor to forest regeneration, but seed-dispersing birds able to move into such areas may help to overcome such constraint. As the number of degraded areas prone to regeneration is increasing in the tropics, it is important to know which bird species and associated traits make such spillover movements. We studied the interactions between frugivorous birds and eight plant species in contiguous degraded and secondary forest areas in a Brazilian land-bridge island, evaluating the potential of avian cross-habitat spillover to disperse seeds to degraded areas and thus contribute to their vegetation recovery. We recorded 21 and 17 bird species removing fruits in degraded areas and secondary forests, respectively. Avian communities dispersing seeds in both habitats are dominated by habitat generalist species. Visitation and fruit removal rates did not differ between the two habitats. Avian cross-habitat spillovers were more frequent from secondary forests to degraded areas than the reverse. A few bird species (Dacnis cayana, Elaenia flavogaster, and Turdus spp.) stood out in the spillover between habitats due to their generalist habitat occupancy and capacity for foraging in disturbed areas. We likewise identified the plants that most attracted birds (including an exotic palm, Livistona chinensis) so as to pinpoint the plant species that may occasionally be the focus of management actions to enhance the arrival of seeds to degraded areas. We conclude that the spillover dynamics reported herein shed some light on how tropical forest regeneration could benefit from native species’ spatial behaviour, offering a biological alternative to overcome practical issues, such as seed limitation in degraded environments.

Spatial and temporal drivers of post-fire tree establishment and height growth in a managed forest landscape

BackgroundIn seed-obligate conifer forests of the western US, land managers need a better understanding of the spatiotemporal variability in post-fire recovery to develop adaptation strategies. Successful establishment of post-fire seedlings requires the arrival of seeds and favorable environmental conditions for germination, survival, and growth. We investigated the spatiotemporal limitations to post-fire seedling establishment and height growth in dry to moist mixed conifer forests with and without post-fire forest management treatments (salvage logging, grass seeding) in areas burned from low to high severity. In 2011, we measured post-fire seedling establishment year, juvenile density (seedlings and saplings), and height growth (annual and total) in 50 plots with six conifer species in the School Fire (2005), Blue Mountains, WA, USA. In 2021, we remeasured the plots for post-fire juvenile density and height growth.ResultsPost-fire juvenile tree densities appeared sufficient for self-replacement of forest (> 60 stems ha−1) in 96% of plots in 2021 (median 3130 stems ha−1), but densities were highly variable (range 33–100,501 stems ha−1). Annual seedling establishment was positively correlated with cooler, wetter climate conditions during the summer of germination (July–September) and the growing season of the subsequent year (April–September) for multiple tree species. We found lower juvenile densities at greater distances to seed sources and with higher grass cover, while salvage logging had no effect. Annual height growth was shorter on warmer, drier topographic positions for three species, whereas annual height growth was associated with climate variability for one species. Shifts in height class structure from 2011 to 2021 were, in part, explained by differences among species in annual height growth.ConclusionsAbundant and widespread tree seedling establishment for multiple conifer species after fire was strong evidence that most burned sites in the present study are currently on a trajectory to return to forest. However, post-fire establishment may be constrained to brief periods of cooler, wetter climate conditions following future fires. Long-term monitoring of post-fire recovery dynamics is needed to inform management activities designed to adapt forests to climate change and future disturbances, which will collectively shape future forest structure and composition.

Seed Rain in a Degraded Mining Area: The Role of Bird Perches and Pioneer Trees

Mining causes changes in natural areas, from the elimination of vegetation and the seed bank to physical changes in the soil and water dynamics. The recovery of such impacted areas depends on both the natural process of secondary succession and the use of techniques that assist the return of the flora. Aiming to evaluate the seed rain in an area of basaltic rock mining, 18 seed collectors of 0.5 m2 were installed placed under artificial perches, pioneer trees (natural perches) and control collectors. After 12 months, 8976 seeds of 33 plant species were quantified with an average density of 997 seeds m2 year-1 that differed statistically between the different perches. Artificial perches were more efficient in facilitating the seed rain, the most abundant of which were Cecropia pachystachya Trécul (n=3218), Andropogon bicornis L. (n=1417), Urochloa plantaginea (Link) R.D.Webster (n=1179), and Leucaena leucocephala (Lam.) de Wit (n=1138) were more abundant and represented 77.4% of total collected seeds and 12.1% of species richness. The installation of artificial perches should be encouraged in degraded areas, because they facilitate the visitation of birds and seed dispersal, contributing both to the arrival of seeds and to an increase in seed richness in the area.

Seed rain in a restoration site and in an adjacent remnant of Seasonal Atlantic Forest

A sucessão ecológica depende da chuva de sementes. Embora a simples chegada de sementes não garanta o estabelecimento, conhecer a chuva de sementes é o primeiro passo para avaliar o processo sucessional em sítios sob restauração florestal. Por essa razão, foi amostrada a chuva de sementes em reflorestamento e em remanescente de floresta estacional semidecidual no sul do Brasil, a fim de descrever o processo. Em cada área, 18 armadilhas de sementes (1 m2) foram alocadas e visitadas, mensalmente, por dois anos. Todas as sementes amostradas foram identificadas, contadas e classificadas quanto à forma de vida, grupo ecológico e síndrome de dispersão. O remanescente florestal apresentou maior riqueza de espécies e abundância de propágulos que o reflorestamento. A composição de espécies também diferiu entre as áreas. A chuva de sementes, de ambas as áreas, apresentou maior proporção de árvores e de espécies intolerantes à sombra. Várias espécies não plantadas no reflorestamento, possivelmente dispersadas a partir do remanescente florestal, foram registradas no sítio em restauração. A análise dos resultados sugere que a chuva de semente não é um impedimento ao processo sucessional no reflorestamento. Entretanto, o sítio deve ser monitorado por mais tempo a fim de avaliar o estabelecimento, e não somente a chegada de sementes de espécies sucessionais tardias.

Effects of habitat fragmentation on frugivorous birds and on seed removal from Pistacia lentiscus in two contrasting fruiting seasons

Successful animal seed dispersal is the result of the interaction between frugivore behavior and the distribution of food resources, which can vary over space and time. We evaluated the interaction between avian frugivores and the masting shrub Pistacia lentiscus in both connected and isolated forest fragments along the Guadalquivir valley (south Iberian Peninsula) in two contrasting fruiting seasons (mast and non-mast). We compared the abundance and composition of avian dispersers and seed predators, recorded fruit removal rates by dispersers and predators, and investigated potential changes in the arrival of seeds to forest fragments. Our results showed that there was a greater abundance of dispersers in connected than in isolated fragments in the mast season (i.e. high fruit availability) but the opposite pattern in the non-mast season (i.e. low fruit availability). The effects of habitat fragmentation were more noticeable on medium-sized than small bird dispersers. Medium-sized birds were more abundant in isolated forest fragments but their abundance was highly dependent on the season, while small bird dispersers were apparently unaffected by fragmentation. Seed removal rates were higher in isolated than in connected fragments in the mast season but not in the non-mast season; predation rates were negligible in both seasons. Seed arrival was consistent between seasons and microhabitat where the dispersed seeds were deposited, most seeds being dispersed under the canopy of female Pistacia plants. We highlight the influence of the interplay between seasonality and landscape configuration on patterns of frugivore-mediated seed dispersal. Thus, despite its direct link to plant–frugivore interaction, the effects of habitat fragmentation on frugivores and plants were decoupled in our study system. These decoupled responses seemed to be chiefly due to the high feeding dependence of dispersers on P. lentiscus fruits.

Human‐mediated dispersal as a driver of vegetation dynamics: A conceptual synthesis

AbstractQuestionHuman‐mediated dispersal (HMD) comprises human‐vectored dispersal (HVD; direct movement of organisms by people) and human‐altered dispersal (HAD; indirect change in dispersal patterns through human alteration of ecosystems). In the vegetation dynamics literature, human influence has primarily been studied in terms of perturbations to natural communities. Except for non‐native invasions, the role of HMD in vegetation dynamics has rarely been considered. Given the increasing human population and its pervasive impacts across the world, it is necessary to understand the different ways in which HMD drives changes in vegetation dynamics. Importantly how large are these influences and how do they disrupt natural processes?MethodWe reviewed studies examining aspects of HMD in relation to vegetation dynamics and used the broader literature to inform a conceptual synthesis of the impacts of HMD on vegetation dynamics.Results & ConclusionsThe propensity to be affected by HMD varies among species, and this is related to seed and plant traits. Together, these effects combine to determine whether HMD disrupts or enhances seed dispersal into a community. The ultimate consequences of changed arrival of seeds into a community are determined by the strength of the environmental and biotic filters, which govern the establishment and persistence of species. The effect of accidental HVD depends whether it follows the same rules as for natural dispersal; indeed humans might replace lost natural dispersers and thus enhance community resilience. Intentional HVD through sowing or planting will generally be highly disruptive especially as it often involves associated management. Traditionally, HAD has been considered to disrupt vegetation dynamics through, e.g., fragmentation or loss of natural dispersers. However, an HMD perspective can inform actions related to HAD that increase resilience, e.g., green infrastructure or vegetation management. Our framework encourages researchers to consider HMD holistically, to understand how the increasing human footprint might affect vegetation dynamics and resilience under future change.

Специфіка анемохорії багаторічних видів бур’янів на орних землях

One of the most perfect ways to spread the seeds of weeds is the transfer of fruits and seeds by wind (anemohoria). Such powerful perennial types of weeds as the Syrian woolblast - Asclepius siriaca L. and the currant pink - Cirsium arvense L. also use anemochoria to spread its offspring to new territories. The aim of the research was : To carry out in 2015-2018 the estimation of the intensity of the processes of seed and fruit dispersal with anemon-species species of perennial weeds on the arable land of the Kyiv region. Methods and conditions for conducting research . The studies were field-based. They were performed in the laboratory of herbology at the Institute of Bioenergetic Cultures and Sugar Beet NAN. For researches among a large number of anemohorous weed species, the Syriac wafer and thistle rose were chosen. The calculations of the intensity of the processes of scattering and translocation of seeds of anemofor species of weeds were carried out within 20 days of the time they were reached and started to be separated by airflows (leaflet opening or opening of baskets with seeds on weed plants). On the determined scheme of experiments distances from model plants certain types of weeds: 16; 32; 64; 125; 250; 500; 1000m set up catching tables (an area of 0.25m2 each). At each distance, 4 hatching tables were displayed. Their total surface area is 1m2. Experiments were carried out in 4 repetitions. The obtained results were analyzed over the years and the general laws were determined and statistically processed and generalized. Results and discussion As a result of the recorded records, the distribution of seeds of Syriac waders found that at a distance of up to 16 m from maternal plants an average of 16.31 units was recorded for 20 days. seed / m 2 . At a distance of 1000 meters from the place of formation of seeds on maternal plants, the number of weed seeds per m 2 , which occupied the surface of the field decreased to 0.004 st/m 2 . Compared with the indicators at the first distance (16m), the intensity of the arrival of seeds of the Syrian driller per unit area of the field decreased 4077.5 times. Investigation of the peculiarities of the distribution of pinkish-thistle seeds was found on the tables, which were located at a distance of 16 m from maternal weed plants, an average of 31.54 st/m 2 of weed seeds was recorded. At a distance of 500 meters from the mother plants, the level of settling of arable land with thistle seeds rose to 0,028 st/m 2 . At a distance of 1000 meters from the mother plants, the area occupied by the seeds of thistle rose was 0.006 seeds/m 2 or 5256.7 times less than the presence of seeds at a distance of 16 meters. Conclusions . The presence of specific adaptations in the fruits and seeds that provides them with high sailing, allows the flow of air to successfully translate and populate such perennial species of weeds large areas. Even at a distance of 1000 m from the mother plants for each m 2 of the surface of the field comes from 0.004 to 0.006 seeds of such dangerous and highly competitive types of weeds.

Size-Dependent Patterns of Seed Rain in Gaps in Temperate Secondary Forests, Northeast China

Secondary forests have become the major forest type worldwide, and are experiencing various disturbances and exhibiting obvious vegetation degradation (e.g., reduced biodiversity and decreased productivity) compared with primary forests. Forest gap is a common small-scale disturbance in secondary forests. Promoting natural regeneration under gap disturbance is an important approach to recover biodiversity and ecosystem services for temperate secondary forests. The gap size is the crucial characteristic controlling natural regeneration of many tree species. However, little is known about the spatiotemporal pattern of seed rain for gravity-dispersed and wind-dispersed tree species in gaps of varying sizes. The objectives of this study were to determine how seed rain of dominant tree species depend on gap size, and consequently, to explore some gap-based silviculture solutions for restoring secondary forests from the view of seed dispersal. The spatial distribution of seed rain in gaps with three sizes (large gaps of 250–350 m2, medium gaps of 150–250 m2, and small gaps of <150 m2), the temporal dynamics of seed rain over three years, and the relationship between seed rain and soil seed banks were explored in temperate secondary forests. The results showed that more than 90% of the seeds in seed rain were wind-dispersed, and their seed rain density and the contribution of seed rain to soil seed bank in medium gaps reached the highest (p = 0.03). The results suggest that establishing medium-sized gaps (i.e., gap size with 150–250 m2) in the secondary forests is more favorable for improving the natural regeneration potential (arrival of seeds and forming soil seed bank) of gap-dependent and wind-dispersed species (e.g., Acer mono) in gaps.

Seed movement between the native forest and monoculture tree plantations in the southern Atlantic forest: A functional approach

The native vegetation within tree plantations increases the suitability for native biodiversity; however, the regeneration of this vegetation depends on the movement of seeds from the native forest by vertebrates and wind. In the present study, we examined functional patterns of seed dispersal between the native forest and tree plantations with different degrees of contrast. We expected that the movement of seeds between the native forest and plantations would depend on the degree of edge contrast, the dispersal syndrome, and the size/weight of seeds. We sampled the seed rain by using seed traps, and measured vegetation structure in the ecotone between four different plantations and the native forest (300 m inside both the native forest and the plantations) in the Atlantic forest of Argentina during a 12-month period. We weighed wind-dispersed seeds and measured vertebrate-dispersed seeds. Edge effects acted as a filter for seed size/weight of both vertebrate- and wind-dispersed seeds. The abundance and functional diversity of seeds arriving at tree plantations increased with plantation age; large seeds were more sensitive to habitat disturbance than small seeds, independently of the dispersal syndrome. Our results highlight that seed movements between the native forest and human-created habitats largely depend on the interaction between dispersal syndrome, seed size, distance to the edge and habitat contrast. Our results also showed that long-term plantation cycles will increase the functional diversity of seeds in the seed bank and facilitate the regeneration of the native vegetation, and that small mature stands close to the native forest will largely facilitate the arrival of seeds and increase the suitability for native fauna.

Rainforest seed rain into abandoned tropical Australian pasture is dependent on adjacent rainforest structure and extent

AbstractIt is well known that the recovery of abandoned tropical pastures to secondary rainforest benefits from the arrival of seeds from adjacent rainforest patches. Less is known, however, about how the structural attributes of adjacent rainforest (e.g. tree density, canopy cover and tree height) impact seed rain patterns into abandoned pastures. Between 2011 and 2013, we used seed traps and ground seed surveys to track the richness and abundance of rainforest seeds entering abandoned pastures in Australia's wet tropics. We also tested how seed rain diversity is related to the distance from forest, the proportion of forest cover in the landscape and several structural attributes of adjacent forest patches, specifically average tree height, canopy cover, tree species richness and density. Almost no seeds were captured in elevated pasture seed traps, even near forest remnants. Abundant forest seeds were found in ground surveys but only within 10 m of forest edges. In ground surveys, seeds from wind‐dispersed species were more abundant, but less species rich, than animal‐dispersed species. A survey of pasture seedling recruits suggested that some forest seeds must be dispersing more than 10 m into pasture at very low frequencies, but only a few species are establishing there. Recruits were predominantly animal‐dispersed not wind‐dispersed species. In addition to distance from forest and the proportion of forest within a 100‐ to 200‐m radius of sampling sites, the richness and density of adjacent forest trees were the most important factors for explaining the probability of seed occurrence in abandoned pastures. Results suggest that without some restoration assistance, the recovery of abandoned pastures into secondary rainforest in Australia's tropical rainforests will likely be limited, at least in part, by a very low rate of seed dispersal away from forest edges and by the diversity and density of trees in adjacent remnant forests.

Limited Seed Dispersal May Explain Differences in Forest Colonization by the Japanese Raisin Tree (Hovenia Dulcis Thunb.), an Invasive Alien Tree in Southern Brazil

In South American forests, the Japanese raisin tree ( Hovenia dulcis Thunb.) is an invasive tree that produces capsuled fruits attached to peduncles that grow thick and fleshy when ripe. Several frugivorous birds are reported to be dispersal agents of H. dulcis. The spread of H. dulcis propagules to non-invaded sites was evaluated at a local spatial scale in deciduous forests in Southern Brazil, in order to propose management strategies to prevent the arrival of propagules of such species in non-invaded sites. Seed arrival via birds was assessed by five 0.6 × 0.6 m seed traps in each of the 10 × 10 m plots in non-invaded sites at different distances from invaded sites. Seed arrival was also compared between control plots and plots where all H. dulcis trees were felled within a 30m radius from the center of plots. A limitation in short distance seed dispersal by birds was observed in the study area. There was a negative exponential relationship between number of seeds and distance from H. dulcis trees to non-invaded sites, and a positive exponential dependence of the number of seeds found in seed traps on the basal area of H. dulcis. Felling H. dulcis trees was effective in reducing the arrival of seeds in non-invaded sites, resulting in significantly fewer seeds in managed plots than in control plots.

Community-wide spatial and temporal discordances of seed-seedling shadows in a tropical rainforest.

Several factors decrease plant survival throughout their lifecycles. Among them, seed dispersal limitation may play a major role by resulting in highly aggregated (contagious) seed and seedling distributions entailing increased mortality. The arrival of seeds, furthermore, may not match suitable environments for seed survival and, consequently, for seedling establishment. In this study, we investigated spatio-temporal patterns of seed and seedling distribution in contrasting microhabitats (bamboo and non-bamboo stands) from the Brazilian Atlantic Forest. Spatial distribution patterns, spatial concordance between seed rain and seedling recruitment between subsequent years in two fruiting seasons (2004–2005 and 2007–2009), and the relation between seeds and seedlings with environmental factors were examined within a spatially-explicit framework. Density and species richness of both seeds and seedlings were randomly distributed in non-bamboo stands, but showed significant clustering in bamboo stands. Seed and seedling distributions showed across-year inconsistency, suggesting a marked spatial decoupling of the seed and seedling stages. Generalized linear mixed effects models indicated that only seed density and seed species richness differed between stand types while accounting for variation in soil characteristics. Our analyses provide evidence of marked recruitment limitation as a result of the interplay between biotic and abiotic factors. Because bamboo stands promote heterogeneity in the forest, they are important components of the landscape. However, at high densities, bamboos may limit recruitment for the plant community by imposing marked discordances of seed arrival and early seedling recruitment.

The importance of long-distance seed dispersal for the demography and distribution of a canopy tree species.

Long-distance seed dispersal (LDD) is considered a crucial determinant of tree distributions, but its effects depend on demographic processes that enable seeds to establish into adults and that remain poorly understood at large spatial scales. We estimated rates of seed arrival, germination, and survival and growth for a canopy tree species (Miliusa horsfieldii), in a landscape ranging from evergreen forest, where the species' abundance is high, to deciduous forest, where it is extremely low. We then used an individual-based model (IBM) to predict sapling establishment and to compare the relative importance of seed arrival and establishment in explaining the observed distribution of seedlings. Individuals in deciduous forest, far from the source population, experienced multiple benefits (e.g., increased germination rate and seedling survival and growth) from being in a habitat where conspecifics were almost absent. The net effect of these spatial differences in demographic processes was significantly higher estimated sapling establishment probabilities for seeds dispersed long distances into deciduous forest. Despite the high rate of establishment in this habitat, Miliusa is rare in the deciduous forest because the arrival of seeds at long distances from the source population is extremely low. Across the entire landscape, the spatial pattern of seed arrival is much more important than the spatial pattern of establishment for explaining observed seedling distributions. By using dynamic models to link demographic data to spatial patterns, we show that LDD plays a pivotal role in the distribution of this tree in its native habitat.

Roles of Seed and Establishment Limitation in Determining Patterns of Afrotropical Tree Recruitment

Quantifying the relative importance of the multiple processes that limit recruitment may hold the key to understanding tropical tree diversity. Here we couple theoretical models with a large-scale, multi-species seed-sowing experiment to assess the degree to which seed and establishment limitation shape patterns of tropical tree seedling recruitment in a central African forest. Of five randomly selected species (Pancovia laurentii, Staudtia kamerunensis, Manilkara mabokeensis, Myrianthus arboreas, and Entandophragma utile), seedling establishment and survival were low (means of 16% and 6% at 3 and 24 months, respectively), and seedling density increased with seed augmentation. Seedling recruitment was best explained by species identity and the interaction of site-by-species, suggesting recruitment probabilities vary among species and sites, and supporting the role of niche-based mechanisms. Although seed augmentation enhanced initial seedling density, environmental filtering and post-establishment mortality strongly limited seedling recruitment. The relative importance of seed and establishment limitation changed with seed and seedling density and through time. The arrival of seeds most strongly affected local recruitment when seeds were nearly absent from a site (∼ 1 seed m2), but was also important when seeds arrived in extremely high densities, overwhelming niche-based mortality factors. The strength of seed limitation and density-independent mortality decreased significantly over time, while density-dependent mortality showed the opposite trend. The varying strengths of seed and establishment limitation as a function of juvenile density and time emphasize the need to evaluate their roles through later stages of a tree’s life cycle.

Where do seeds go when they go far? Distance and directionality of avian seed dispersal in heterogeneous landscapes

Seed dispersal at large scales strongly influences plant population dynamics. Still, ecologists have rarely measured seed dispersal at relevant scales, and the role of habitat types in affecting seed dispersal at long distances remains unexplored. We studied seed dispersal of Ilex aquifolium and Crataegus monogyna in northern Spain, hypothesizing that seeds would be recovered at higher rates and at longer distances (LDD) at habitats with fleshy-fruited trees, compared to habitats with other tree types or at open habitats. We tracked seeds in eight landscapes by enriching trees with 15N isotopes at the center of landscapes, and then detected 15N-marked seeds by sampling at distances of up to 700 m. We found that seeds arrive in greater densities and at longer distances in habitats with trees, particularly fleshy-fruited types, producing different LDD probabilities for each habitat. Results also show a disproportional arrival of seeds in habitats similar to those of mother plants, which should affect seed establishment and the genetic diversity of plant neighborhoods. Findings reveal the strong dependence of seed dispersal on the existing templates that guide the movements of avian dispersers in heterogeneous landscapes and also suggest that LDD above tree lines and beyond hard habitat edges can be difficult.

Composition and clumping of seeds deposited by frugivorous birds varies between forest microsites

More frequent deposition of seeds by frugivores beneath plants in fruit could impose spatial limits to the distribution of plants dispersed by animals and contribute to species coexistence. Also, differences in diet and use of microhabitats by seed dispersers could promote spatial variation in the combination of seed species deposited. We investigated patterns of seed deposition ofMiconia fosteriandMiconia serrulata(Melastomataceae) by birds in the Amazon. The goal was to determine how distribution and abundance of fruiting plants, both con‐ and hetero‐specifics, affect the spatial variability in clumping and composition of multi‐specific seed deposition. We established two 9‐ha plots in undisturbed terra‐firme understory in the Ecuadorian Amazon. Seed rain was sampled with seed traps located in four microsites: below plants of the focal species, belowAnthurium eminens(Araceae), and in randomly selected microsites. We examined seed deposition in these microsites in relation to habitat, fruiting neighborhood (fruit abundance, and distance to and density of plants of the target species), and crop size ofM. fosteriorM. serrulatato determine if microsites differed in abundance and species composition of seeds. Seed traps below plants in fruit received more seeds than did randomly located traps. Seeds of the target species were, moreover, more commonly deposited below con‐ rather than hetero‐specific plants. Seed aggregation below fruiting plants increased in forest neighborhoods where the abundance of fruits and the combination of fruiting plant species contributed to the arrival of seeds. Microsites differed notably in the combination of seeds deposited by frugivores, and differences were less pronounced among microsites that received seeds ofM. fosteriandM. serrulatathan among all microsites where at least some seed species were deposited by birds. We demonstrate that two closely related, ecologically similar species possess many similarities in their patterns of seed deposition and in the factors that affect those patterns. The combination of seed species deposited below foci of dispersal depended on the fruiting plant species, and the spatial patterns of seed deposition varied with the location of the microsite and the combination of co‐dispersed species in the neighborhood. Similar species that share the same dispersers were confronted with different combinations of seeds depending on the microsite where they arrived, which could promote forest heterogeneity in the combination of plant species.

Incorporating density dependence into the directed‐dispersal hypothesis

The directed-dispersal (DrD) hypothesis, one of the main explanations for the adaptive value of seed dispersal, asserts that enhanced (nonrandom) arrival to favorable establishment sites is advantageous for plant fitness. However, as anticipated by the ideal free distribution theory, enhanced seed deposition may impair site suitability by increasing density-dependent mortality, thus negating the advantage postulated by the DrD hypothesis. Although the role of density effects is thoroughly discussed in the seed-dispersal literature, this DrD paradox remains largely overlooked. The paradox, however, may be particularly pronounced in animal-mediated dispersal systems, in which DrD is relatively common, because animals tend to generate local seed aggregations due to their nonrandom movements. To investigate possible solutions to the DrD paradox, we first introduce a simple analytical model that calculates the optimal DrD level at which seed arrival to favorable establishment sites yields maximal fitness gain in comparison to a null model of random arrival. This model predicts intermediate optimal DrD levels that correspond to various attributes of the plants, the dispersers, and the habitat. We then use a simulation model to explore the temporal dynamics of the invasion process of the DrD strategy in a randomly dispersed population, and the resistance of a DrD population against invasion of other dispersal strategies. This model demonstrates that some properties of the invasion process (e.g., mutant persistence ratio in the population and generations until initial establishment) are facilitated by high DrD levels, and not by intermediate levels as expected from the analytical model. These results highlight the need to revise the DrD hypothesis to include the countering effects of density-dependent mortality inherently imposed by enhanced arrival of seeds to specific sites. We illustrate how the revised hypothesis can elucidate previous results from empirical studies reporting little or no support for the DrD hypothesis, and we suggest its incorporation in designing empirical studies of plant recruitment and in management practices.

Do plant-community-based grazing regimes lead to epizoochorous dispersal of high proportions of target species?

The contribution of large-herbivore epizoochory to the transfer of seeds within and between areas is thought to be significant. But often seeds of ubiquitous species are dispersed, which may enhance ruderalization processes. In order to study the dispersal of target species by sheep, we employed a community-based grazing approach followed by intra- and inter-area sheep transfers (maximum transfer distance 3 km). In case of inter-area transfers, well-developed target communities of an open inland sand ecosystem are used as “source”, linked to less-developed sand habitats (“sink”) via sheep. Also other factors determining which species become dispersed under field conditions were tested: seed surface structure, seed mass, plant seed-releasing height and animal behaviour. Finally the influence of animal movement on seed detachment and the actual arrival of seeds within a “sink” were studied. Sheep transfers resulted in the dispersal of 56 seed species, dominated by Red List (seven species) and other target species. Quantitatively, most transported seeds belonged to target species, whereas graminoid competitors were highly under-represented. Morphological traits enhance the attachment probability regardless of seed mass. But for seeds without these epizoochory-facilitating traits, mass seems to affect attachment negatively. Plant height affected the number of species present in sheep coats but not the seed quantities. Probably certain species in the vegetation produced large numbers of seeds, e.g. low-growing Medicago minima with seed surface structures and high-growing species Verbascum phlomoides without seed surface structures. Also, although transfer half-times were three times lower than grazing half-times, naturally attached Stipa capillata and Agrimonia procera seeds showed no significant detachment rates during transfer, whereas considerable losses were found during grazing. Other 3-km sheep transfers did not result in significant losses of either epizoochorously transported seed or species quantities. Our study shows that community-based grazing can lead to the dispersal of especially target species. Besides that, different habitat fragments can be connected to each other via sheep without significant seed losses along the way.

The Contribution of Endozoochory to the Colonization and Vegetation Composition of Recently Formed Sand Coastal Dunes

The objective of this study was to determine whether endozoochory contributes to the dispersal and colonization of plant species in recently formed coastal dunes. At least 5.7% of species present in the study area are being dispersed by wild rabbits (Oryctolagus cuniculusL.). Most dispersed species are perennial herbs with small seeds size. The continuous input of seeds through rabbit feces into newly created areas would ensure the constant arrival of seeds and would facilitate colonization. Therefore, endozoochorous dispersal may play a relevant role for the structure and composition of dune plant communities.

Is seed availability enough to ensure colonization success?: An experimental study in road embankments

We tested the hypothesis that seed availability is a limiting factor for plant colonization of road embankments under Mediterranean climate conditions. Experimental sowing on 10 road embankments was carried out to compare the colonization success of plants that successfully colonize the road embankment and species that appear only occasionally in the road embankments. After sowing, we measured plant establishment, biomass production, and reproductive capacity of the species. The species that appear only occasionally in the road embankments had lower emergence rates (l.l ± 0.3%) than species that were successful colonizers (18.8 ± 2.9%). None of the species of the former group survived or reproduced. The results did not support the hypothesis that seed availability was the main factor limiting plant colonization in the road embankments. We concluded that the arrival of seeds to road embankments under Mediterranean climate conditions was not enough to ensure colonization success of plants. Other factors, like hydric stress, appeared to affect seedling establishment and plant growth. Reclamation measures such as species selection should be taken in account to ensure revegetation success of road embankments.