Median Dispersal Distance Research Articles

Seed dispersal and seedling establishment of Sarracenia purpurea (Sarraceniaceae)

Plant ecologists continue to grapple with Reid's paradox, the observation that dispersal distances of most herbs and trees are too limited to account for their recolonization of northern latitudes following glacial recession. As global climate changes and natural habitats become increasingly fragmented, understanding patterns of seed dispersal and the potential for long-distance colonization takes on new importance. We studied the dispersal and establishment of the northern pitcher plant Sarracenia purpurea, which grows commonly in isolated bogs throughout Canada and eastern North America. Median dispersal distance of S. purpurea is only 5 cm, which is insufficient to explain its occurrence throughout formerly glaciated regions of North America. Establishment probability of seeds in the field is approximately 5%, and juveniles are normally found clustered around adult plants. The large-scale population genetic structure of this species can be accounted for by rare long-distance dispersal events, but its predictable occurrence in isolated habitats requires additional explanation. Reid's paradox remains an open question, and predicting long-range colonization into fragmented habitats by species with limited dispersal ability is a novel challenge.

Dispersal and survival of juvenile feral ferrets Mustela furo in New Zealand

Summary Introduced feral ferrets Mustela furo are a significant pest of both conservation and economic importance in New Zealand. Ferrets prey on indigenous wildlife and they also carry bovine tuberculosis (TB), a disease which may threaten New Zealand's international beef, dairy and venison markets. Very little is known about the role of dispersal and survival of juvenile ferrets in the recovery of ferret populations after control operations, and how these parameters might affect ferret impacts on native wildlife and their role in the spread of TB. Fifty‐two juvenile ferrets were radio‐collared at emergence from their natal dens on six study sites during 2 years, 1997–98 and 1998–99. On three sites, most introduced mammalian predators (including ferrets) had been removed by kill‐trapping from October to January to protect critical bird nesting areas. Three sites were left as non‐removal sites. Dispersal and survival of juvenile ferrets was therefore measured at low and normal ferret densities. Survival of juvenile ferrets showed a strong density‐dependent response to predator removal. In year 1, survival of juvenile ferrets from emergence to the end of the dispersal period (about 4 months) was 86% (95% confidence limits 63–100%) on removal sites, whereas survival on non‐removal sites was 19% (0–38%). In year 2, survival was 100% on removal sites compared with 71% (45–97%) on non‐removal sites. Dispersal of juvenile ferrets was not related to population density at source. A frequency distribution of dispersal distances of juvenile ferrets showed a typical negative‐exponential pattern. The median dispersal distance of juvenile ferrets was 5·0 km (range 0·5–45·0 km). There was no apparent sex bias in survival, dispersal distances or timing of dispersal in either season. Density‐dependent survival of juvenile ferrets should be a key parameter of interest in determining frequency and seasonal timing of ferret control. Ferret control to conserve native wildlife and minimize TB spread should be carried out in late autumn after dispersal, to provide a longer lag time before juveniles reinvade an area. This study fills a critical gap in the understanding of introduced mustelids in island systems such as New Zealand. It has direct implications for management of this important introduced predator, and it augments literature on dispersal and survival of mammalian carnivores.

Factors affecting breeding dispersal in the facultatively colonial lesser kestrel: individual experience vs. conspecific cues

Summary The role of individual experience vs. the use of conspecific cues on breeding dispersal decisions have seldom been determined in colonial birds. We studied causes of breeding dispersal in the lesser kestrel (Falco naumanni), a species that breeds in colonies of variable size as well as solitarily. During a 6‐year study in Spain, we gathered information on 486 subsequent breeding attempts and on 26 explanatory variables which evaluated individual experience, conspecific cues in terms of breeding performance and colony size, and different ecological and populational characteristics. Two decisions were separately analysed: whether or not to disperse, and how far to move. Generalized Linear Mixed Models (GLMMs) allowed us to identify the relative contribution of each explanatory variable while controlling for the non‐independence of individual dispersal decisions across years. Females seemed to disperse more often than males (34% vs. 19%), and both sexes apparently dispersed less with age. However, a GLMM showed that experience (i.e. the number of years a bird bred in a particular colony) was the only factor influencing breeding dispersal. Birds showed higher site fidelity the greater their experience in a colony, which could be related to benefits derived of increased local familiarity. A second GLMM showed that, before birds acquired experience in a particular colony, individual nest failure due to predation and proximity to other colonies increased the probability of dispersal, dispersal being also higher in colonies with poor conspecific breeding success. Furthermore, solitary nesting birds were more prone to disperse and dispersal probability decreased the larger the colony of origin, according to fitness expectations associated with colony size. A GLMM explaining dispersal distances retained two variables – birds dispersed farther the lower the breeding density in the surroundings, and the larger the distance to the nearest colony. Dispersing birds tended to settle within their previous foraging areas (median dispersal distance = 1·6 km), being constrained by the availability of nearby colonies. Lesser kestrels mainly cue on their own breeding performance and experience in a particular colony at the time of taking a dispersal decision. However, inexperienced birds also partially cue on the size and breeding success of their own colonies (but not on the size or breeding performance of other colonies), and birds moved larger distances when dispersing from areas of low populational density. These results support some degree of conspecific attraction.

Latitudinal Variation in Female Local Return Rate in the Philopatric Pied Flycatcher (Ficedula hypoleuca)

AbstractPhilopatry and dispersal distances of female Pied Flycatcher (Ficedula hypoleuca) are presented for European populations using data from 25 breeding areas from 40 to 70°N. Female annual survival probabilities according to capture–recapture models were similar in two study areas in central Spain (45 and 52%). The present study shows that survival is underestimated by using annual local return rate in one of the two breeding populations under study in central Spain. In southern and central Europe, females were found to return equally regularly to their breeding areas, whereas in northern Europe (latitude >60°N) females returned at lower rates. I did not find that median dispersal distance varied among sites, nor was breeding distance related to locate survival rate. Therefore, the present study suggests that the decline in between-year local return rate of female Pied Flycatchers with increasing latitude over Europe may be more probably caused by differences in mortality than by geographical differences in site fidelity.

The pattern, rate, and range of within‐patch movement of a stem‐galling fly

Summary1. Information on the movement of insects is critical to understanding the spatial spread, dynamics, and genetic structure of their populations, as well as their interactions with other species. With this in mind, the movement behaviour of the stem‐galling fly Eurosta solidaginis Fitch (Diptera: Tephritidae) was investigated.2. Fluorescent‐marked adults were released at a single location within pure patches of the host plant, tall goldenrod Solidago altissima, and their distributions censused repeatedly throughout the day.3. Following their release, male and female flies redistributed themselves in a manner that was well described by a simple‐diffusion model. The diffusion rate was independent of fly density and time since flies were released.4. Female flies dispersed at a significantly faster rate, and therefore farther on average, than males. Based on the diffusion model, it was estimated that at 2.5–3.0 h post release, males and females had a median dispersal distance of only 2.0 and 2.5 m respectively. Furthermore, 95% of the males were estimated to have dispersed no more than 5.9 m, and females no more than 6.4 m.5. Post‐release censuses suggested that flies were most active during mid morning, disappeared from the site at a rate of 10–15% per hour (most likely due to mortality), and survived for less than 2 days. Based on the rate of spread, diel activity, and liberal estimates of longevity in the field, 50% of the ovipositing females were predicted to have had a maximum lifetime range of movement within a patch of hosts of ≤ 51 m (95% were expected to have been limited to ≤ 130 m).6. These data are used to assess whether the absence of a positive correlation between host‐plant preference and offspring performance in this system could be due to the limited scale of dispersal of this species relative to the spatial scale at which its oviposition behaviour has been studied.

Field experiments on seed dispersal by wind in ten umbelliferous species (Apiaceae)

This report presents data from experiments on seed dispersal by wind for ten species of the family Apiaceae. Seed shadows were obtained in the field under natural conditions, using wind speeds between four and ten m/s. The flight of individual seeds was followed by eye, and seed shadows were acquired, with median distances varying from 0.7 to 3.1 m between species. Multiple regression models of wind speed and seed weight on dispersal distance were significant for six out of ten species; wind speed had significant effects in seven cases, but seed weight only once. A good correlation between mean terminal falling velocity of the seeds of a species and median dispersal distance, indicates the promising explanatory power that individual terminal velocity data might have on dispersal distance, together with wind speed and turbulence. The theory that seeds that seem to be adapted to wind dispersal travel much longer distances than seeds that have no adaptation was tested. Flattened and winged seeds were indeed found to be transported further by wind, but not much further. Moreover, the species with wind-adapted seeds were also taller, being an alternative explanation since their seeds experienced higher wind speeds at these greater heights. Furthermore, flattened and winged seeds were disseminated from ripe umbels at lower wind speeds in the laboratory. This means that the observed difference in dispersal distance would have been smaller when species specific thresholds for wind speed were incorporated in the field experiments. We argue therefore, that seed morphology is not always the best predictor in classifying species in groups with distinctly different dispersal ability.

Latitudinal Variation in Female Local Return Rate in the Philopatric Pied Flycatcher (Ficedula hypoleuca)

Abstract Philopatry and dispersal distances of female Pied Flycatcher (Ficedula hypoleuca) are presented for European populations using data from 25 breeding areas from 40 to 70°N. Female annual survival probabilities according to capture–recapture models were similar in two study areas in central Spain (45 and 52%). The present study shows that survival is underestimated by using annual local return rate in one of the two breeding populations under study in central Spain. In southern and central Europe, females were found to return equally regularly to their breeding areas, whereas in northern Europe (latitude >60°N) females returned at lower rates. I did not find that median dispersal distance varied among sites, nor was breeding distance related to locate survival rate. Therefore, the present study suggests that the decline in between-year local return rate of female Pied Flycatchers with increasing latitude over Europe may be more probably caused by differences in mortality than by geographical differ...

Effects of Quadrat Size and Shape, Initial Epidemic Conditions, and Spore Dispersal Gradient on Spatial Statistics of Plant Disease Epidemics

ABSTRACT The spatiotemporal spread of plant diseases was simulated using a stochastic model to study the effects of initial conditions (number of plants initially infected and their spatial pattern), spore dispersal gradient, and size and shape of sampling quadrats on statistics describing the spatiotemporal dynamics of epidemics. The spatial spread of disease was simulated using a half-Cauchy distribution with median dispersal distance mu (units of distance). A total of 54 different quadrat types, including 23 distinct sizes ranging from 4 to 144 plants, were used to sample the simulated epidemics. A symmetric form of the binary power law with two parameters (alpha, beta) was fitted to the sampled epidemic data using each of the 54 quadrats for each replicate simulation run. The alpha and beta estimates were highly correlated positively with each other, and their estimates were comparable to those estimated from observed epidemics. Intraclass correlation (kappa) was calculated for each quadrat type; kappa decreased exponentially with increasing quadrat size. An asymmetric form of the binary power law with three parameters (alpha (1), beta(1), beta(2)) was used to relate kappa to the disease incidence (p); beta1 was highly correlated to beta: beta1 approximately beta - 1. In general, initial conditions and quadrat size affected alpha, beta, alpha(1), beta(1), and beta(2) greatly. The parameter estimates increased as quadrat size increased, and the relationships were described well by a linear regression model on the logarithm of quadrat size with the slope or intercept parameters dependent on initial conditions and mu. Compared with initial conditions and quadrat size, the overall effects of mu and quadrat shape were generally small, although within each quadrat size and initial condition they could be substantial. Quadrat shape had the greatest effect when the quadrat was long and thin. The relationship of the index of dispersion (D) to p and quadrat size was determined from the alpha and beta estimates. D was greatest when p was 0.5 and decreased when p approached 0 or 1. It increased with quadrat size and the rate of the increase was maximum when p was 0.5 and decreased when p approached 0 or 1.

Patterns of movement and seed dispersal of a tropical frugivore.

Movement is a fundamental feature of vertebrate behavior and can modify processes within populations and communities. Because tropical avian frugivores disperse seeds of many plant species, the temporal and spatial patterning of their movement will influence seed distribution within a habitat. To date, little is known about movement patterns of these birds. Here we consider the movement of an understory frugivore, Mionectes oleagineus. Movements of 16 non-breeding females were monitored using continuous radio-telemetry to provide a general description of movement patterns and to examine the fractal geometry of the spatial component of movement. Most movements were of short distance and duration, with the frequency distributions of both measures strongly skewed to the left. Over the range of measurement scales considered, the fractal dimension of M. oleagineus's movement increased with increasing measurement scale up to ca.100 m, whereafter it appeared to flatten out. We combined movement data with M. oleagineus gut-passage rates for seeds of six plant species to predict seed shadows. Estimated seed shadows were leptokurtic for four of the six plant species, with median dispersal distances for all species from 42 to 56 m. Dispersal distances were of the order of reported pollen dispersal distances, suggesting that even small seed dispersers like M. oleagineus can provide significant dispersal for plant genotypes. Gut-passage rate appears to determine the shape of the seed shadow, while movement determines dispersal scale.

An Evaluation of a Mammalian Predator, Martes americana, as a Disperser of Seeds

Effective animal dispersers of fruit seeds have been primarily considered to be herbivorous birds and bats. To evaluate the seed-dispersal efficacy of a mammalian predator, we studied the American marten (Martes americana) as a disperser of two blueberries (Vaccinium alaskaense, V. ovalifolium) and salmonberry (Rubus spectabilis) on Chichagof Island, Alaska, by combining data on movement speed and gut-passage speed. Martens traveled a median distance of 133 m in 0-1 h and 501 m in 4-5 h. Median gut-passage time for V. alaskaense was 4.1 h and for R. spectabilis was 4.6 h. Median dispersal distances for both berry species were estimated to be about 500 m. In old-growth stands, V. ovalifolium seeds cleansed of pulp did not germinate differently than seeds cleansed of feces, suggesting that the marten gut does not harm germination. Vaccinium spp. seeds germinated better if cleansed of feces than if inside intact berries, indicating that passage through the gut of a marten improves germination over fruit fall alone. For V. alaskaense in old growth, seeds cleansed of feces germinated less than seeds cleansed of pulp, but still had high (41%) germination rates. We conclude that American martens move ingested seeds long distances, some of the highest reported for a mammal. This efficacy is in part due to the predatory adaptations of martens. Predators may have heretofore unrecognized value in shaping landscape patterns of berry-producing plants.

Effects of Initial Epidemic Conditions, Sporulation Rate, and Spore Dispersal Gradient on the Spatio-Temporal Dynamics of Plant Disease Epidemics

ABSTRACT A stochastic model that simulates the spread of disease over space and time was developed to study the effects of initial epidemic conditions (number of initial inocula and their spatial pattern), sporulation rate, and spore dispersal gradient on the spatio-temporal dynamics of plant disease epidemics. The spatial spread of disease was simulated using a half-Cauchy distribution with median dispersal distance mu (units of distance). The rate of temporal increase in disease incidence (beta(I), per day) was influenced jointly by mu and by the sporulation rate lambda (spores per lesion per day). The relationship between beta(I) and mu was nonlinear: the increase in beta(I) with increasing mu was greatest when mu was small (i.e., when the dispersal gradient was steep). The rate of temporal increase in disease severity of diseased plants (beta(S)) was affected mainly by lambda: beta(S) increased directly with increasing lambda. Intraclass correlation (kappa(t)), the correlation of disease status of plants within quadrats, increased initially with disease incidence, reached a peak, and then declined as disease incidence approached 1.0. This relationship was well described by a power-law model that is consistent with the binary form of the variance power law. The amplitude of the model relating kappa(t) to disease incidence was affected mainly by mu: kappa(t) decreased with increasing mu. The shape of the curve was affected mainly by initial conditions, especially the spatial pattern of the initial inocula. Generally, the relationship of spatial autocorrelation (rho(t,k)), the correlation of disease status of plants at various distances apart, to disease incidence and distance was well described by a four-parameter power-law model. rho(t,k) increased with disease incidence to a maximum and then declined at higher values of disease incidence, in agreement with a power-law relationship. The amplitude of rho(t,k) was determined mainly by initial conditions and by mu: rho(t,k) decreased with increasing mu and was lower for regular patterns of initial inocula. The shape of the rho(t,k) curve was affected mainly by initial conditions, especially the spatial pattern of the initial inocula. At any level of disease incidence, autocorrelation declined exponentially with spatial lag; the degree of this decline was determined mainly by mu: it was steeper with decreasing mu.

PHILOPATRY, NATAL DISPERSAL, AND INBREEDING AVOIDANCE IN AN ISLAND POPULATION OF SAVANNAH SPARROWS

Over a 9-yr period, we studied dispersal of young banded Savannah Sparrows (Passerculus sandwichensis) from their natal nest to the site where they first bred 1 yr later in a population on an isolated archipelago in the Bay of Fundy, Canada. On a broad geographic scale, young birds were highly philopatric, returning from wintering grounds several thousand kilometers away to nest on the same island and often in the same field where they had hatched the year before. In some cases, birds chose nest sites within a few meters of their natal nest. The median dispersal distance between a bird’s natal nest and its first nest as an adult was 228 m, a distance equivalent to about six times the diameter of an average territory. Nearly three-quarters of the breeding birds in the study population had been banded as nestlings or fledglings within a 10-ha area on one island, which suggests that most birds in the population originated within the 127-ha archipelago. Within the archipelago, males and females dispersed similar distances from their natal site. There was no correlation between natal dispersal distances of parents and their offspring, nor was there a correlation between natal dispersal distances of siblings, which indicated that natal dispersal has low heritability. Sex, hatching date, fledging mass, and population densities during the previous and current years were all poor predictors of natal dispersal distance. Males that were strongly philopatric recruited significantly more offspring during their lifetime than males raised outside the study area, although there were no differences between philopatric and dispersing females. Birds tended to shift to distinct parts of the island to breed if their parents of the opposite sex still occupied the territory where they had hatched. Dispersal was not affected by return of same-sex parents. Distances between nests of siblings raised the same year were farther apart than expected by chance, based on Monte Carlo simulations. Although many birds in the population had the opportunity to pair with kin, some mechanism, yet to be determined, enabled birds to avoid inbreeding: in 1073 nesting attempts involving birds of known parentage, no individuals were known to have paired with close relatives (coefficient of kinship >0.125). Because complete inbreeding avoidance occurred in <1% of Monte Carlo simulations, the absence of inbreeding among Kent Island Savannah Sparrows is unlikely to be due to chance. Understanding natal dispersal in birds requires a combination of models: ecogenetic models at broad geographic scales (e.g., adaptation to local environments), ecological constraints and neutral models at smaller spatial scales (e.g., unavailability of territories within particular habitats), and genetic models (e.g., inbreeding avoidance).

Dispersal of Young in Red Squirrels (Tamiasciurus hudsonicus)

-To examine how young red squirrels (Tamiasciurus hudsonicus) disperse, I used focal animal observations and radio-telemetry to monitor the activities and movements of six young squirrels from the time they emerged from the nest until they dispersed. Before dispersal, young squirrels spent most of their time within their mothers' territories, which served as natal ranges during the predispersal period. During the dispersing period, young squirrels explored far away from their natal ranges during the day but returned at night until they had established territories of their own. Some young squirrels never displayed this exploratory behavior and eventually claimed territories within their natal ranges. Regardless of whether young squirrels dispersed away from or claimed territories in their natal ranges, they emitted territorial calls significantly more often during the postdispersal period than in the predispersal period. In addition to the six young squirrels that were followed, two other marked young squirrels were sighted in the spring after their birth, and their dispersal distances were calculated. The median dispersal distance for eight young was 100 m, with four remaining in their natal ranges and four dispersing away.

Causes of Natal Dispersal in the Lesser Kestrel: Inbreeding Avoidance or Resource Competition?

1. Natal dispersal of seven cohorts (1988-94) of the colonial lesser kestrel (Falco naumanni) was studied in southern Spain. We ringed 1852 nestlings and resighted 321 (17.3%) which survived at least the first migration to Africa. Of the returning individuals, 57% settled in their natal colonies and the remaining 43% dispersed to other colonies. Resights fell off markedly with increasing distance from the natal colony; more than 90% of resights were within 30 km of the birthplace. 2. Contrary to most avian studies, dispersal from the natal site was not sex-biased in our population. The proportion of dispersing males (41%, n = 171) did not differ significantly from that of females (45%, n = 150). Excluding philopatric individuals, the median dispersal distance for males (19 km) did not differ from that of females (18.5 km). 3. Siblings (72 twos) dispersed or were philopatric independently of each other. This result provides no support for either a genetic basis of dispersal (which predicts the same strategy for both siblings) or the inbreeding avoidance hypothesis (which predicts greater dispersal in one sex than the other). This latter hypothesis also conflicts with our observation that the presence of the parent of the opposite sex in the natal colony has no effect on whether returning individuals disperse. 4. Generalized linear models (GLMs) showed that the probability of dispersal for an individual increased when the absolute growth of its natal colony decreased and also from beginning to end of the study, coincidental with a general population increase. These results point to a link between colony saturation and dispersal. GLMs also showed that dispersal distances tended to increase with size of the natal colony and higher breeding success. These results are also consistent with intraspecific competition for resources. 5. Some individuals were observed in two different colonies in their year of first return. These observations, and the lack of active inbreeding avoidance, suggest a scenario for the dispersal process: individuals would prefer to settle in their natal colonies. If they cannot secure a nest site or a mate, they will have to decide whether to stay as nonbreeders or to disperse in search of another breeding chance elsewhere. The actual decision to disperse could be triggered by the level of aggression the unmated individuals encounter in their natal colonies.

COMPETITION AND DISPERSAL FROM MULTIPLE NESTS

Models of competition among dispersers from multiple natal sites are developed. Analytical solutions demonstrate that competition among identical dispersers may cause large variation in dispersal distances, and that increased competition may increase or decrease dispersal distances depending on the nature and magnitude of competition. This pattern is more complex than are results from previous models that ignored competition among dispersers. The analytical results are in accordance with results of a spatially explicit individual-based simulation model. When individuals disperse simultaneously from multiple nests and the number of dispersers exceeds the number of vacant territories, individuals that are close to vacancies are more likely to be the successful dispersers, and dispersal distances will decline with increased competition. Including competition among dispersers in models of natal dispersal improved predictions of the median dispersal distance of banner-tailed kangaroo rats and Song Sparrows.

Competition and Dispersal from Multiple Nests

Models of competition among dispersers from multiple natal sites are developed. Analytical solutions demonstrate that competition among identical dispersers may cause large variation in dispersal distances, and that increased competition may increase or decrease dispersal distances depending on the nature and magnitude of competition. This pattern is more complex than are results from previous models that ignored competition among dispersers. The analytical results are in accordance with results of a spatially explicit individual-based simulation model. When individuals disperse simultaneously from multiple nests and the number of dispersers exceeds the number of vacant territories, individuals that are close to vacancies are more likely to be the successful dispersers, and dispersal distances will decline with increased competition. Including competition among dispersers in models of natal dispersal improved predictions of the median dispersal distance of banner-tailed kangaroo rats and Song Sparrows.

Predicted Pollen Dispersal by Honey-Bees and Three Species of Bumble-Bees Foraging on Oil-Seed Rape: A Comparison of Three Models

Comparisons among animal pollinators of the spatial distributions of pollen that they produce have typically been made among morphologically disparate pairs of species. In contrast, we investigated the potential extent of pollen dispersal by honey-bees ( Apis mellifera ) and bumble-bees ( Bombus lapidarius, B. pascuorum and B. terrestris ) foraging in rows of oil-seed rape ( Brassica napus cv Westar). We estimated the pollen carryover attributable to individual bees by using particulate fluorescent dye as a pollen analogue. Most of the dye was deposited at the first few flowers probed and smaller proportions were deposited up to the 20th successively probed flower. We found no significant interspecific differences in dye carryover mediated by individuals of A. mellifera, B. lapidarius and B. terrestris with respect to either the amount deposited or the rate of decline in deposition across successively probed flowers. We present evidence that the dye produced a reasonably good analogue of pollen transfer. Bees typically flew from one plant to another nearby in the same row and were strongly directional in their movements. Bee species differed significantly in their movement patterns, with B. terrestris having the greatest mean move length and directionality. We used three kinds of model (a numerical simulation and two different sets of diffusion-advection equations) to attempt to emulate bee movements. The predictions from all models were reasonably consistent with the observed bee movements, although the numerical simulation invariably made the most accurate predictions, particularly over the first few moves. Predicted bee movements were combined with least-squares models of dye deposition to estimate the spatial dispersal of pollen by each bee species. All models ranked the bees in the same order of decreasing effectiveness in dye dispersal: B. terrestris, A. mellifera, B. lapidarius, B. pascuorum , although, except for long-distance dispersal, there was only minor variation among the bee species in the predicted extents of dye dispersal (e.g. the models predicted that the median dispersal distance would be approximately two intervening plants irrespective of the species of bee). Overall, the consensus of the models' predictions is that most of the pollen from a source plant is deposited on immediate neighbours, but that long-distance pollen dispersal in this system extends over approximately 20-40 intervening plants from the originating plant, depending on the identity of the pollinator.

Dispersal of Male Sweetpotato Weevils (Coleoptera: Curculionidae) in Fields with or without Sweet Potato Plants

Dispersal distance of the male sweetpotato weevil, Cylas formicarius elegantulus (Summers), was estimated in the field. Two experiments were conducted. In experiment 1, release and capture were replicated 3 times from September to October in the same locality consisting mainly of sweet potato fields. Approximately 8,000 marked males were released and ≈30% of released males were captured by pheromone traps each time. Median dispersal distances per 2 d were 325.7 m in September, 119.8 m in early October, and <50 m in late October. The maximum dispersal distance in September was 500 m, whereas it was 200 m in October. In experiment 2, the dispersal distances were compared in the fields with and without sweet potato plants. The dispersal distance in the field without sweet potato plants was longer than that obtained in the sweet potato cultivated area. The maximum dispersal distance in the fields with sweet potatoes was 500 m, whereas it was 1,000 m in the fields without sweet potatoes.

Neighbourhood size and the importance of barriers to gene flow in an intertidal snail

The littorine gastropod Bembicium vittatum has direct development from benthic egg masses, and shows high levels of genetic subdivision in the Houtman Abrolhos Islands, Western Australia. Parallel studies of dispersal and the scale of variation of allozyme frequencies were used to estimate the extent of the neighbourhood, the area of complete genetic mixing. Estimates based on the variance of dispersal distances were 58 to 69 m, very similar to the estimate of 62 m from the median dispersal distance. The allozymes indicated mixing over greater distances, up to 150 to 300 m. Combined with estimates of population density at two sites, these values suggest neighbourhood sizes of 990 to 37 200 adults. However, estimation of effective population size based on a model of isolation by distance gave very low values of 22 to 38, raising doubts about the applicability of this method. Nevertheless, there was an overall pattern of isolation by distance along about 11 km of nearly continuous suitable habitat, with smooth clinal variation in allelic frequencies over distances of 2 to 6 km at individual loci. In contrast, there was no pattern of isolation by distance, and allelic frequencies showed less smooth patterns, along a discontinuous series of islands over the same distance. Within this discontinuous series, genetic subdivision, as measured by GST, was twice as large at distances of 1 km as for populations on continuous shores, demonstrating the importance of even relatively short water gaps as barriers to gene flow in this directly developing intertidal species.

Growth and dispersal of capybaras (Hydrochaeris* hydrochaeris) in the Llanos of Venezuela

Growth rates and dispersal patterns of capybaras, Hydrochaeris* hydrochaeris, were studied in a ranch in the Venezuelan seasonally flooded savannas by first marking juvenile animals at specific locations in March–April 1989 and then, on two occasions the following year (February and May 1990), searching a wide area for recaptures. Animals grew a mean 2.8 kg per month in the 10 months between the first and second captures and apparently lost weight over the next three months, possibly due to water scarcity. From 22 recaptures, median dispersal distance was 3366 m, about six times the average home range width. Maximum dispersal distance was 5600 m and minimum was 0. There were no differences between males and females in dispersal distances. A group of three males and three females apparently dispersed together. Competition for resources—water and grass—under a social system involving group‐living and territoriality is proposed as the cause of the patterns observed. No correlation was found between distance dispersed and initial and final weight or weight gain. Capybaras appeared to disperse predominantly north or south, possibly in order to reduce the chances of meeting established groups and to increase the probability of finding a stream, since all streams in the region run approximately west‐east.