Relationship between Position on the Parent Plant and Dormancy Characteristics of Seeds of Three Cultivars of Celery (Apium graveolens)

Germination and seedling emergence studies were made on seeds harvested from four different umbel positions of three cultivars of celery (Apium graveolens L.). Although heavier seeds were produced from primary umbels than from other umbels, these were less viable as measured by the germination percentage at I8°C in the light. However, germination of viable seeds from quaternary umbels was lower than that of seeds from primary umbels at 18°C in the dark when incubated with GA₄, (2 × 10 ^?4M) and seed from secondary and tertiary umbels tended to be intermediate in response. All viable seeds germinated when N⁶-benzyladenine (10^?2M) was used in combination with GA₄. Seeds from quaternary umbels of two of the cultivars had a lower high-temperature limit for germination in the dark than did seeds from other umbels. In glasshouse experiments the emergence of viable ‘quaternary’ seeds of these cultivars was higher than that of ‘primary’ seeds. Under these conditions the time to 50% of the final emergence as determined after 42 days was similar for seeds from all umbel positions within each cultivar. In two varieties seedling weights were greater from seeds of primary as compared to quaternary umbels, and in general, the largest seedlings arose from the heaviest seeds and the smallest from the lightest seeds in all three cultivars.     

Leaving home ain't easy: non-local seed dispersal is only evolutionarily stable in highly unpredictable environments

It is widely understood that in the presence of asynchronous environmental variation, seeds disperse to escape disturbances, avoid crowding or colonize newly favourable habitat before a superior competitor can arrive. If seeds are dispersing for any of these reasons, it seems intuitive that they should travel far enough to reach conditions uncorrelated with their natal environment: why ‘escape in space’ only to land somewhere more or less like where they started? However, in this paper, I present a series of mathematical experiments which show that the evolutionarily stable mean dispersal distance remains well short of the spatial correlation length of the environmental variation, regardless of disturbance severity, coevolution with a superior competitor or the presence of a small fraction of seeds which travel well beyond the mean distance. Non-local dispersal arises only as part of a polymorphism that evolves when favourable conditions are fleeting. To the degree that non-local dispersal is a response to environmental variation, it appears to be a response to environmental unpredictability.     

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

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

Body size determines rates of seed dispersal by giant king crickets

New Zealand is home to giant king crickets called weta, which are the only insects known to consume fleshy-fruits and disperse seeds after gut passage. Although they disperse seeds in viable condition after consumption, their importance as seed dispersers is unknown. We conducted a series of field observations and laboratory experiments to investigate intraspecific variation in the capacity of Wellington tree weta (Hemidenina crassidens) to disperse seeds of tree fuchsia (Fuchsia excorticata). We asked three questions. How frequently do weta disperse fuchsia seeds? Do seed passage rates differ between sexes and different-sized weta? Might weta select for particular seed sizes via differential seed mortality after ingestion? A total of 2,272 F. excorticata seedlings germinated from 241 scats (i.e., faecal pellets) that were collected from the field. Experimental results showed that, on average, 15% of seeds ingested by weta successfully germinated, whereas 75% germinated in control trials. Larger weta dispersed greater numbers of seeds in experimental trials, while no differences in dispersal rates were observed between sexes. Regardless of sex and size, weta preferentially dispersed larger seeds. When interpreted collectively, results indicate that (1) weta are frequent seed dispersers of F. excorticata, although many seeds are destroyed during ingestion, (2) larger-bodied weta consistently disperse greater quantities of seeds, which is unusual in seed dispersal mutualisms, and (3) weta preferentially disperse larger seeds, suggesting that they might interact evolutionarily with New Zealand plants.     

Evolution of condition-dependent dispersal under kin competition

Dispersers often differ in body condition from non-dispersers. The social dominance hypothesis explains dispersal of weak individuals, but it is not yet well understood why strong individuals, which could easily retain their natal site, are sometimes exposed to risky dispersal. Based on the model for dispersal under kin competition by Hamilton and May, we construct a model where dispersal propensity depends on body condition. We consider an annual species that inhabits a patchy environment with varying patch qualities. Offspring body condition corresponds to the quality of the natal patch and competitive ability increases with body condition. Our main general result balances the fitness benefit from not dispersing and retaining the natal patch and the benefit from dispersing and establishing somewhere else. We present four different examples for competition, which all hint that dispersal of strong individuals may be a common outcome under the assumptions of the present model. In three of the examples, the evolutionarily stable dispersal probability is an increasing function of body condition. However, we found an example where, counterintuitively, the evolutionarily stable dispersal probability is a non-monotone function of body condition such that both very weak and very strong individuals disperse with high probability but individuals of intermediate body condition do not disperse at all.     

Compensatory reproduction in a biennial herb following insect defloration

Summary The ability of the biennial herb, Pastinaca sativa L. (wild parsnip), to respond to and compensate for destruction of primary umbel seeds by the larvae of Depressaria pastinacella (Lepidoptera: Oecophoridae) was analyzed by comparing umbel and seed production of damaged and undamaged plants collected from five populations. Plants with a basal stem diameter smaller than 8 mm suffer a reduction in seed set of about 50% when the primary umbel is destroyed but larger plants are able to compensate for loss of primary umbel seeds by increased seed set of tertiary umbels. Depending on plant size, this is due to either an increase in the number of tertiary umbels that reach maturity or an increase in the number of seeds per tertiary umbel. Although seeds of tertiary umbels are significantly smaller than those of primary or secondary umbels, their viability is equivalent to that of secondary seeds and may be greater than that of primary seeds. Characteristics of P. sativa's reproduction, such as the long flowering period and the initiation of more umbels than the plant is normally able to bring to maturity, are important to P. sativa's ability to compensate for the effects of herbivore damage.     

Does size matter for dispersal distance?

Aim The aim of this study is to answer the questions: (1) do small organisms disperse farther than large, or vice versa; and (2) does the observed pattern differ for passive and active dispersers? These questions are central to several themes in biogeography (including microbial biogeography), macroecology, metacommunity ecology and conservation biology. Location The meta‐analysis was conducted using published data collected worldwide. Methods We collected and analysed 795 data values in the peer‐reviewed literature for direct observations of both maximal dispersal distance and mass of the dispersing organisms (e.g. seeds, not trees). Analysed taxa ranged in size from bacteria to whales. We applied macroecology analyses based on null models (using Monte Carlo randomizations) to test patterns relative to specific hypotheses. Results Collected dispersal distance and mass data spanned 9 and 21 orders of magnitude, respectively. Active dispersers dispersed significantly farther (P < 0.001) and were significantly greater in mass (P < 0.001) than passive dispersers. Overall, size matters: larger active dispersers attained greater maximum observed dispersal distances than smaller active dispersers. In contrast, passive‐disperser distances were random with respect to propagule mass, but not uniformly random, in part due to sparse data available for tiny propagules. Conclusions Size is important to maximal dispersal distance for active dispersers, but not for passive dispersers. Claims that microbes disperse widely cannot be tested by current data based on direct observations of dispersal: indirect approaches will need to be applied. Distance–mass relationships should contribute to a resolution of neutral and niche‐based metacommunity theories by helping scale expectations for dispersal limitation. Also, distance–mass relationships should inform analyses of latitudinal species richness and conservation biology topics such as fragmentation, umbrella species and taxonomic homogenization.     

Can observation of climatic variables be used to predict the flight dispersal rates of Prostephanus truncatus?

Abstract 1 Attack by Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) on the maize and cassava stored by small‐holder farmers in Africa is sporadic, varying considerably within and between years. The risk that food in store will become infested is related to the number of beetles dispersing by flight. A means of predicting years with high dispersal rates is needed to warn farmers when to be vigilant. 2 The relationship between climatic variables and pheromone trap catches was observed in a forest‐savannah transition zone in Ghana. These observations were used to devise a model using a mix of biological and empirical rules that operate on temperature and humidity data. The predicted and actual trap catch deviated by only +5% to ?1% in years when there were high dispersal rates. 3 The first part of the model estimates the numbers of beetles with potential for dispersal. The second part predicts the proportion likely to disperse. This is based on the apparent effect that those P. truncatus developing under low temperature conditions (about 24 °C) have a lowered propensity for flight, a response previously observed in a related species. 4 The model was validated using climate data and trap catches from a woodland–savannah zone and a short grass steppe zone. With minor adjustment, the model worked well for these two habitats.     

Density-dependent dispersal in host-parasitoid assemblages

Most spatial population models assume constant rates of dispersal. However, in a given community, dispersal may not only depend on the density of conspecifics, i.e. density‐dependent dispersal, but also on the density of other species, a phenomenon we term ‘community‐dependent dispersal’. We co‐vary the densities of both the beetle host Callosobruchus chinensis and its parasitoid wasp, Anisopteromalus calandrae, in a laboratory study and record the proportions of each species that disperse within a two‐hour period. The parasitoid in these systems exhibits community‐dependent dispersal – dispersing more frequently when parasitoid density is high and larval host density is low. This supported our prediction that individuals should disperse according to competition for available resources. However, in this study the host's dispersal was independent of density. We suggest that this may be due to less intense selection acting on host dispersal strategies than on the parasitoid. We consider some possible consequences of community‐dependent dispersal for a number of spatial population processes. A well‐known host‐parasitoid metapopulation model is expanded so that it includes a greater range of dispersal functions. When the model is parameterised with the parasitoid community‐dependent dispersal function observed in the empirical study, similar population dynamics are obtained as when fixed‐rate dispersal functions are applied. The importance of dispersal functions for invasions of both competitive and host‐parasitoid systems is also considered. The model results demonstrate that understanding how individuals disperse in response to different species’ population densities is important in determining the rate of spread of an invasion. We suggest that more empirical studies are needed to establish what determines dispersal rate and distance in a range of species, combined with theoretical studies investigating the role of the dispersal function in determining spatial population processes.     

Breeding dispersal in an isolated population of Spotted Owls Strix occidentalis: evidence for improved reproductive output

Breeding dispersal among territorial species is of interest to population biologists because leaving a territory carries fundamental risks to the dispersing individuals, and this may not outweigh the costs of maintaining the territory. Most studies of breeding dispersal have focused on species inhabiting spatially open populations, in which undetected emigration could impart a negative bias to estimates of dispersal. We studied breeding dispersal in an isolated (spatially closed) population of California Spotted Owl Strix occidentalis occidentalis in southern California for 12 years to assess factors that might correlate with breeding dispersal. Twenty‐nine per cent (n = 47) of territorial females and 19% (n = 35) of territorial males dispersed at least once during the study. Annually, 0–13% of the territorial females and 0–12% of the territorial males dispersed. Among a set of a priori and post hoc models related to breeding dispersal, the top a priori model indicated that birds having higher reproductive output relative to the population average were less likely to disperse. A post hoc model based on an index of territory quality was ranked higher than the top a priori model and indicated that birds occupying higher quality territories were less likely to disperse. These two models were correlated and represented short‐ and long‐term reproductive performance, respectively. Birds that dispersed also failed to fledge young in the year prior to dispersal, but the failure to fledge young did not, by itself, explain dispersal. Because Spotted Owls are long‐lived, they may ultimately improve their reproduction by dispersing given that they would have future opportunities to breed over the long term. Birds whose mates are likely to have died tended to improve their reproductive success, whereas the relationship between reproductive success of birds that divorced was less clear. Substantial variation in breeding dispersal was unexplained by our analysis so it is likely that the motivation to disperse was a complex process in this population.     

Dispersal patterns in black howler monkeys (Alouatta pigra): Integrating multiyear demographic and molecular data

Dispersal is a fundamental process in the functioning of animal societies as it regulates the degree to which closely related individuals are spatially concentrated. A species’ dispersal pattern can be complex as it emerges from individuals’ decisions shaped by the cost–benefit tradeoffs associated with either remaining in the natal group or dispersing. Given the potential complexity, combining long-term demographic information with molecular data can provide important insights into dispersal patterns of a species. Based on a 15-year study that integrates multiyear demographic data on six groups with longitudinal and cross-sectional genetic sampling of 20 groups (N = 169 individuals, N = 21 polymorphic microsatellite loci), we describe the various dispersal strategies of male and female black howler monkeys (Alouatta pigra) inhabiting Palenque National Park, Mexico. Genetically confirmed dispersal events (N = 21 of 59 males; N = 6 of 65 females) together with spatial autocorrelation analyses revealed that the dispersal pattern of black howlers is bisexual with strong sex-biases in both dispersal rate (males disperse more often than females) and dispersal distance (females disperse farther than males). Observational and genetic data confirm that both males and females can successfully immigrate into established groups, as well as form new groups with other dispersing individuals. Additionally, both males and females may disperse singly, as well as in pairs, and both may also disperse secondarily. Overall, our findings suggest multiple dispersal trajectories for black howler males and females, and longer multiyear studies are needed to unravel which demographic, ecological and social factors underlie individuals’ decisions about whether to disperse and which dispersal options to take.     

Human-mediated dispersal of seeds over long distances

Human activities have fundamental impacts on the distribution of species through altered land use, but also directly by dispersal of propagules. Rare long-distance dispersal events have a disproportionate importance for the spread of species including invasions. While it is widely accepted that humans may act as vectors of long-distance dispersal, there are few studies that quantify this process. We studied in detail a mechanism of human-mediated dispersal (HMD). For two plant species we measured, over a wide range of distances, how many seeds are carried by humans on shoes. While over half of the seeds fell off within 5m, seeds were regularly still attached to shoes after 5 km. Semi-mechanistic models were fitted, and these suggested that long-distance dispersal on shoes is facilitated by decreasing seed detachment probability with distance. Mechanistic modelling showed that the primary vector, wind, was less important as an agent of long-distance dispersal, dispersing seeds less than 250 m. Full dispersal kernels were derived by combining the models for primary dispersal by wind and secondary dispersal by humans. These suggest that walking humans can disperse seeds to very long distances, up to at least 10 km, and provide some of the first quantified dispersal kernels for HMD.     

Does an ant-dispersed plant,Viola reichenbachiana,suffer from reduced seed dispersal under inundation disturbances?

Many plant species use ants as seed dispersers. This dispersal mode is considered to be susceptible to disturbances, but the effect of natural, small-scale disturbances is still unknown. We investigated how small-scale disturbances due to inundation affect seed dispersal in Viola reichenbachiana, a dominant myrmecochorous herb in riparian forests. Inundation disturbances were high in depressions and low on hillocks of the forest floor. We found that V. reichenbachiana was similarly abundant at highly and less disturbed sites, contrary to other, non ant-dispersed species. We also found that the motivation of ants to disperse seeds was higher at highly disturbed sites. Nevertheless, the number of seeds dispersed was similar at highly disturbed and weakly disturbed sites and seedlings of V. reichenbachiana were equally abundant. We conclude that inundation disturbances do not interfere with mutualistic seed dispersal by ants in V. reichenbachiana, and suggest that this may possibly contribute to the persistence of V. reichenbachiana under inundation.     

Unexpected relationships and valuable mistakes: non‐myrmecochorous Prosopis dispersed by messy leafcutting ants in harvesting their seeds

Abstract Ants generally disperse seeds while feeding on fruits or structures attached to the seed. Seed dispersal as a by‐product of seed predation (dyszoochory) was recognized in specialized harvester ants, but not in ants predating seeds opportunistically. Leafcutting ants are the main herbivores in much of the Neotropics, and they have been reported to remove fruits and seeds, but their role as seed predators and dispersers has not been acknowledged. Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) is the most abundant tree species in the central Monte Desert, Argentina, and it is likely to depend on secondary animal dispersal. Mammalian frugivores are usually considered its main dispersers, but the opportunity for dispersal may be small since the removal of fruits and seeds by seed predators is very intense. The objective of this study was to identify which ant species interact with P. flexuosa fruits and to evaluate their relative importance as seed predators and dispersers. In a field experiment, whole and segmented pods were offered and several ant species exploiting the fruits were identified. Additionally, all pod segments remaining around nests of the three ant species able to remove them (the leafcutters Acromyrmex lobicornis Emery and Acromyrmex striatus Roger, and Pheidole bergi Mayr) were examined during and after the P. flexuosa primary dispersal season. Up to 753 pod segments and 90 sound seeds were found accumulated in a circle of 1 m radius over nests of A. lobicornis, and even more in an examined trail. Acromyrmex striatus left a smaller proportion of sound seeds and P. bergi left a smaller number of pod segments. All tendencies were similar during shorter known periods of accumulation. Leafcutting ants are acting as important seed predators, and ‘by mistake’ may be dispersing a key non‐myrmecochorous tree. This is an unexplored path in the seed dispersal cycle of P. flexuosa that challenges the tendency to predict interactions based on classifications made with other goals.     

Ontogenetic Behavior and Dispersal of Sacramento River White Sturgeon, <Emphasis Type="BoldItalic">Acipenser Transmontanus</Emphasis>, with a Note on Body Color

Synopsis We studied Sacramento River white sturgeon, Acipenser transmontanus, in the laboratory to develop a conceptual model of ontogenetic behavior and provide insight into probable behavior of wild sturgeon. After hatching, free embryos initiated a low intensity, brief downstream dispersal during which fish swam near the bottom and were photonegative. The weak, short dispersal style and behavior of white sturgeon free embryos contrasts greatly with the intense, long dispersal style and behavior (photopositive and swimming far above the bottom) of dispersing free embryos of other sturgeon species. If spawned eggs are concentrated within a few kilometers downstream of a spawning site, the adaptive significance of the free embryo dispersal is likely to move fish away from the egg deposition site to avoid predation and reduce fish density prior to feeding. Larvae foraged on the open bottom, swam <1 m above the bottom, aggregated, but did not disperse. Early juveniles initiated a strong dispersal with fish strongly vigorously swimming downstream. Duration of the juvenile dispersal is unknown, but the strong swimming likely disperses fish many kilometers. Recruitment failure in white sturgeon populations may be a mis-match between the innate fish dispersal and post-dispersal rearing habitat, which is now highly altered by damming and reservoirs. Sacramento River white sturgeon has a two-step downstream dispersal by the free embryo and juvenile life intervals. Diel activity of all life intervals peaked at night, whether fish were dispersing or foraging. Nocturnal behavior is likely a response to predation, which occurs during both activities. An intense black-tail body color was present on foraging larvae, but was weak or absent on the two life intervals that disperse. Black-tail color may be an adaptation for avoiding predation, signaling among aggregated larvae, or both, but not for dispersal.     

Informed dispersal in plants: Heterosperma pinnatum (Asteraceae) adjusts its dispersal mode to escape from competition and water stress

Informed‐dispersal theory (IDT) states that organisms may use information on the quality of the local environment when deciding whether to disperse or not. Dispersal is expected to occur from adverse patches where the costs of philopatry (lack of dispersal) are greater than those of dispersal. Evidence of informed dispersal in plants is scarce, and experiments under natural conditions are lacking. We tested IDT in a semiarid grassland using the annual forb Heterosperma pinnatum, which produces awned, zoochorous achenes and unawned, philopatric ones. We expected the proportion of awned seeds to increase under adverse conditions that reduce fitness, (i.e. under high competition and water stress). Heterosperma pinnatum was sown along natural moisture gradients with and without a shade that increased water availability. We found that competition (number of conspecific neighbors) and water stress reduced fitness. As expected, the proportion of highly dispersible seeds increased under such conditions, probably as a strategy to escape from unsuitable patches. Many plants species do not behave as expected by IDT. Our results suggest that experiments under natural conditions in systems where the assumptions of IDT are met may prove to be a wealthy source of evidence for informed dispersal in plants.     

The disperser dilemma in cooperatively breeding birds

In most cooperatively breeding birds, individuals do not breed with their natal group members. In order to breed, they have either to disperse into another group or wait for an opposite-sex individual to join their group. In most of these species, females disperse more than males. We develop a dynamic game-theoretic model to account for this asymmetry. When males are physically larger/heavier than females, this allows them to effectively welcome female immigrants into their natal group and overcome the local females' opposition more than vice versa. The model further assumes that the dispersal decision is not confined to a restricted time window, but is rather based on acquired information and responsive to opportunities. The model predicts that (i) females disperse more than males, and (ii) females are willing to tolerate more risks in dispersal than do males. The latter prediction is supported inter alia by the fact that in many cooperatively breeding birds, females disperse at a younger age, and further away from their natal group as compared to dispersing males.     

Dispersal Behavior and the Connectivity Between Wolf Populations in Northern Europe

ABSTRACT The isolated gray wolf (Canis lupus) population of the Scandinavian Peninsular is suffering from inbreeding depression. We studied dispersal of 35 wolves fitted with very high frequency (20) or Global Positioning System—global system for mobile (15) radiocollars in the neighboring Finnish wolf population. The growing wolf population in Finland has high numbers of dispersing individuals that could potentially disperse into the Scandinavian population. About half (53%) of the dispersing wolves moved total distances that could have reached the Scandinavian population if they had been straight-line moves, but because of the irregular pattern of movements, we detected no wolves successfully dispersing to the Scandinavian population. Dispersal to the Scandinavian population was also limited by high mortality of wolves in reindeer (Rangifer tarandus) management areas and by dispersal to Bothnian Bay at times of the year when ice was not present. We suggest that when a small wolf population is separated from source populations by distance, barriers, and human exploitation, wildlife managers could promote the population's viability by limiting harvest in the peripheral areas or by introducing wolves from the source population.     

How intraspecific variation in seed‐dispersing animals matters for plants

Seed dispersal by animals is a complex phenomenon, characterized by multiple mechanisms and variable outcomes. Most researchers approach this complexity by analysing context‐dependency in seed dispersal and investigating extrinsic factors that might influence interactions between plants and seed dispersers. Intrinsic traits of seed dispersers provide an alternative way of making sense of the enormous variation in seed fates. I review causes of intraspecific variability in frugivorous and granivorous animals, discuss their effects on seed dispersal, and outline likely consequences for plant populations and communities. Sources of individual variation in seed‐dispersing animals include sexual dimorphism, changes associated with growth and ageing, individual specialization, and animal personalities. Sexual dimorphism of seed‐dispersing animals influences seed fate through diverse mechanisms that range from effects caused by sex‐specific differences in body size, to influences of male versus female cognitive functions. These differences affect the type of seed treatment (e.g. dispersal versus predation), the number of dispersed seeds, distance of seed dispersal, and likelihood that seeds are left in favourable sites for seeds or seedlings. The best‐documented consequences of individual differences associated with growth and ageing involve quantity of dispersed seeds and the quality of seed treatment in the mouth and gut. Individual specialization on different resources affects the number of dispersed plant species, and therefore the connectivity and architecture of seed‐dispersal networks. Animal personalities might play an important role in shaping interactions between plants and dispersers of their seeds, yet their potential in this regard remains overlooked. In general, intraspecific variation in seed‐dispersing animals often influences plants through effects of these individual differences on the movement ecology of the dispersers. Two conditions are necessary for individual variation to exert a strong influence on seed dispersal. First, the individual differences in traits should translate into differences in crucial characteristics of seed dispersal. Second, individual variation is more likely to be important when the proportions of particular types of individuals fluctuate strongly in a population or vary across space; when proportions are static, it is less likely that intraspecific differences will be responsible for changes in the dynamics and outcomes of plant–animal interactions. In conclusion, focusing on variation among foraging animals rather than on species averages might bring new, mechanistic insights to the phenomenon of seed dispersal. While this shift in perspective is unlikely to replace the traditional approach (based on the assumption that all important variation occurs among species), it provides a complementary alternative to decipher the enormous variation observed in animal‐mediated seed dispersal.     

Dispersal-mediated selection on plant height in an autochorously dispersed herb

Dispersal ability is an important fitness component in most plant species. Therefore, some phenotypic traits can be selected due to their effect on dispersal. In this study I determine the potential for dispersal-mediated selection on plant height in an autochorous plant, Erysimum mediohispanicum (Brassicaceae). Selection was quantified by selection gradients, structural equation modeling and generalized additive models. I detected significant dispersal-mediated linear selection gradient on plant height, taller plants dispersing seeds farther. Nevertheless, the generalized additive models suggest that the selection on stalk height was non linear. Indeed, it detected a threshold in the effect of stalk height on dispersal ability; plants shorter than that threshold had an extremely short dispersal, whereas plants taller than that threshold dispersed the seeds very far. Furthermore, the structural equation modeling showed that stalk height indirectly affected dispersal distance through its significant effect on one reproduction-related fitness component, taller plants having greater fecundity. Selection on E. mediohispanicum stalk height occurs through two simultaneous paths, one via producing many seeds and the other through increasing probability of dispersing them far away.