Long-distance dispersal by a parasitoid (Anagrus delicatus,Mymaridae) and its host

Summary We demonstrate that an egg parasitoid, Anagrus delicatus (Mymaridae, Hymenoptera) and its host, Prokelesia marginata (Delphacidae, Homoptera) regularly disperse 1 km or more in a north Florida saltmarsh. Anagrus delicatus were caught on yellow sticky traps on offshore islets and oyster bars throughout the spring, summer, and fall, whereas P. marginata were caught during one pulse in the spring. Parasitism rates were higher on offshore islets than at mainland sites, even though egg densities were higher at the mainland sites. The majority of parasitoids caught offshore were females. Long-distance dispersal by A. delicatus may be a cause of inverse density-dependent or density-independent spatial patterns of parasitism and may represent a risk-spreading strategy.     

Spatial autocorrelation within a korean population ofRaphanus sativus var.hortensis f.raphanstroides

Spatial autocorrelation was applied to microgeographic variations in Korean wild radish,Raphanus sativus var.hortensis Baker f.raphanstroides. Separate counts of each type of join (combination of genotypes at a single locus) for each allele, and for each distance class of separation, were tested for significant deviation from random expectations by calculating the Standard Normal Deviation. Moran’s / was significantly different from the expected value in 16 of 112 cases (14.3%). Eleven of these values (9.8%) were negative, indicating genetic dissimilarity among pairs of individuals in the eight distance classes. Populations of wild radish are small in Korea and are distributed with tractors used for fishing. Occasional cutting of seed-bearing stems as fodder for animals also may bring a high level of gene flow. Thus, disturbance by many anglers, as well as by farmers, may contribute to the fact that the wild radish population of Onsan is unusual in lacking spatial genetic structure.     

Size-biased dispersal prior to breeding in a damselfly

Summary Dispersal is notoriously difficult to measure, so its potential population consequences are often unknown. If dispersal is density-dependent, it can act in population regulation. Adult damselflies Enallagma boreale (Zygoptera: Coenagrionidae) raised as larvae under a range of competitive regimes were individually measured and marked. Individuals that survived to reproductive maturity were either recovered at the natal pond or had dispersed to nearby water bodies. Dispersing individuals were heavier at emergence than those returning to the natal pond to breed. Therefore, an increased probability of dispersal does not appear to be a response to poor conditions in this species.     

Aerial dispersal of European red mites (Acari: Tetranychidae) in commercial apple orchards

Aerial dispersal of European red mite, Panonychus ulmi (Koch), in commercial apple orchards was estimated by trapping windborne mites. Studies were conducted at four orchards in eastern New York during 1989 and 1990 and at three orchards in western New York during 1989. In each orchard mites were trapped in three locations; the interior of the orchard, at the border of the orchard and in a field or woodlot beyond the orchard. Large numbers of mites were captured, even when the numbers of mites on apple foliage were well below levels where mite injury to leaves was visible (less than five per leaf). The log numbers of mites trapped were linearly related to the log density of mites on leaves and this relationship was consistent for each year and region the study was conducted. The trap captures among the three locations in and outside an orchard were highly correlated. The implications these findings may have on metapopulation dynamics and resistance to acaricide dynamics are discussed.     

Size-biased dispersal prior to breeding in a damselfly: conflicting evidence from a natural population

Summary The sizes of adult damselflies (Coenagrion puella) that were marked at emergence and recaptured as mature adults at the natal pond and at the next nearest pond were compared. In addition the sizes of a much larger sample of animals that were never recaptured were compared with those that returned to breed at the natal pond. There was no statistically significant difference in mass or wing length between adults in either comparison. This result contrasts with a previous study on a coenagrionid species in which, from an equally small sample, some evidence of size-biased dispersal was detected (dispersing adults were larger than those returning).     

Restricted dispersal in a flying beetle assessed by telemetry

Many insects living in ancient trees are assumed to be threatened as a result of habitat loss and fragmentation. It is generally expected that species in habitats with low temporal variability in carrying capacity have lower degree of dispersal in comparison to those in more ephemeral habitats. As hollow trees are long-lived, species in that habitat are expected to be sensitive to habitat fragmentation, due to a low capacity to establish new populations far from present ones. Using radio telemetry, we studied the dispersal for a beetle, Osmoderma eremita, living in hollow trees. O. eremita exhibited philopatry and only dispersed over short ranges. About 82–88% of the adults remained in the tree where they were caught. All observed dispersal movements ended up in nearby hollow trees and 62% in the neighbouring hollow tree. These results corroborate the suggestion that habitat fragmentation may be detrimental to insects living in temporally stable but spatially variable habitats. In order to preserve such species, we propose that conservation efforts should be focused on maintaining or increasing the number of suitable trees in and near presently occupied stands.     

Habitat niche specialization in an understory species in a warm temperate forest

Relationships between microhabitat variables; understory light conditions in summer and winter, altitude, slope inclination and topographic categories (valley, ridge, and slope) and the distribution of Aucuba japonica Thunb. (Cornaceae), a common understory shrub species in Japan were examined using non-contagious 66, 20 × 20 m² quadrats. The Morishita’s I _δ suggested that A. japonica distributions were strongly heterogeneous among the quadrats. Therefore positive spatial autocorrelation of A. japonica at a within-quadrat level (≤20 m) was obvious. Moran’s I statistics showed a significant positive spatial autocorrelation in A. japonica abundance within the distance shorter than 60 m. But the partial Mantel tests suggested that the mass effect from neighboring quadrats would little explain A. japonica abundance in a quadrat. The partial Mantel tests also clearly showed that A. japonica distributions were strongly structured by topography and understory light conditions. Using Monte Carlo randomization tests, we found that A. japonica was aggregately distributed in quadrats in valley which were covered by deciduous canopies. Better understory light conditions in winter together with valley edaphic conditions may increase the abundance of A. japonica there. It is concluded that habitat niche specialization is important in structuring distribution of A. japonica in this forest community.     

Spatial autocorrelation and dispersal limitation in freshwater organisms

Dispersal can limit the ranges of species and the diversity of communities. Despite its importance, little is known about its role in freshwater habitats and its relation to habitat type (lentic vs. lotic), especially for organisms with cryptic dispersal methods such as plankton. Poor dispersers are expected to show more clumped distributions or greater spatial autocorrelation (SA) in community composition than good dispersers. We examined patterns of SA across freshwater taxa with different dispersal modes (active vs. passive) and their association with habitat type (lake vs. stream) using 18 spatially explicit community composition data sets. We found significant relationships between SA and body size among taxa in lake habitats, but not in streams. However, the increase in SA with body size in lakes was driven entirely by fishes—organisms ranging in size from diatoms to macro-invertebrates showed equivalent levels of SA. These results support the idea that large organisms are less effective dispersers in aquatic environments, resulting in greater SA in community structure over broad scales. Streams may be effectively more connected than lakes as patterns of SA and body size were weaker in lotic habitats. Our data suggest that the critical threshold where greater body size increases dispersal limitation seems to come at the juncture between invertebrates and vertebrates rather than that between unicellular and multicellular organisms as has been previously suggested. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Localized migration and dispersal by the sweet potato whitefly,Bemisia tabaci

Laboratory populations of the sweet potato whitefly, Bemisia tabaci, have been shown to consist of both migratory and trivial flying morphs. The behavior of these forms as part of the process of short-range migration was examined under field conditions. Insects were marked in a field of melons using fluorescent dust during two consecutive growing seasons. During the first growing season, passive traps used to collect living whiteflies, were placed along 16 equally spaced transects radiating from the field to a distance of up to 1.0 km. Wind out of the north-east consistently carried migrating whiteflies to traps placed along transects in the south-western quadrant because cold air drainages dictate wind direction during early morning hours in the desert South-west. For this reason, during the second season traps were laid out over fallow ground in a rectangular grid extending 2.7 km to the south-west of the marked field. If dispersal was entirely passive, patterns could be described using a diffusion model. Statistical examination of the data, howèver, demonstrated that the distribution on all days was patchy. Geostatistical techniques were used to describe the observed patchiness. Traps in the immediate vicinity of the marked field caught more whiteflies than the daily median. Large numbers were also collected from near the periphery of the grid. White-flies were far less prevalent in the grid's center. As a result, the distribution of captured whiteflies can be described as bimodal. These patterns confirm behavior observed in the laboratory, i.e., a portion of the population are trivial fliers that do not engage in migration and are consequently captured in traps near the field, and a portion initially respond to cues associated with skylight, ignoring cues provided by the ground, and fly for a period of time before landing in distant traps. During both years movement out of the field had an exaggerated directional component on 13 of 14 days.     

Acorn dispersal estimated by radio-tracking

Bird-dispersed seeds are difficult to track, especially in the case of long-distance dispersal events. To estimate the oak dispersal distance and the seed shadow generated by the European jay (Garrulus glandarius), we inserted radio-transmitters in 239 acorns, placed them in bird-feeders and then located them by radio-tracking. Using this methodology we located the exact caching site of 94 Quercus ilex and 54 Q. suber acorns and determined the caching habitat characteristics (vegetation type, distance, spatial distribution). The results show that: (1) there is no differences in the dispersal distance distribution between the different acorn species or sizes, (2) dispersal distances range from approximately 3 m up to approximately 550 m (mean = 68.6 m; median = 49.2 m), (3) recently abandoned fields and forest tracks were the sites preferred by jays to cache acorns, whereas fields and shrublands were avoided and (4) seed shadows showed acorn aggregation zones (i.e. clusters of caches) close to the feeder as well as isolated caches at longer distances. The results also suggest that radio-transmitters are a cheap and reliable way to determine seed shadows and quantify both seed dispersal and post-dispersal seed predation for medium to large seeds.     

Fruit dispersal and trypanocarpy in brazilian cerrado grasses

28 grass species from one hectare of cerrado vegetation in Botucatu, State of São Paulo, were studied with regard to the morphology and dispersal of their fruits. 13 species have dispersal units with smooth surfaces which are predominantly dispersed by big herbivorous mammals or birds which swallow them together with their food, e.g., during grazing (accidental endozoochory). On the other hand, smaller animals, including birds, also feed directly on the grains; some of them may escape trituration and digestion or may be even stored (synzoochory). 11 species are principally epizoochorous: 8 of these are ± exclusively adapted to epizoochory, adhering to the animals by scabrid awns or sharply pointed calli, while the rest behave as epizoochores and anemochores. 4 other species are predominantly anemochorous with their dispersal units bearing soft hairs, sometimes together with very small scabrid awns; in a wet state they also can adhere to animals (occasional epizoochory). Beside this, most grass fruits can behave as passive ballists, as passing animals, wind and rain may cause their dispersal units to fall to the ground (passive autochory). — Among the 28 species, 12 have dispersal units which are awned. In 7 of them the awns are spirally twisted; the base of the dispersal unit runs out into a sharply pointed callus and has harpoone-like hairs. These awns show hygroscopical movements which may lead to a drilling of the basal part of the dispersal unit into the sandy cerrado soil. This trypanocarpy is important for establishment but without function for dispersal.     

Incorporating patterns of disperser behaviour into models of seed dispersal and its effects on estimated dispersal curves

The processes determining where seeds fall relative to their parent plant influence the spatial structure and dynamics of plant populations and communities. For animal dispersed species the factors influencing seed shadows are poorly understood. In this paper we test the hypothesis that the daily temporal distribution of disperser behaviours, for example, foraging and movement, influences dispersal outcomes, in particular the shape and scale of dispersal curves. To do this, we describe frugivory and the dispersal curves produced by the southern cassowary, Casuarius casuarius, the only large-bodied disperser in Australia’s rainforests. We found C. casuarius consumed fruits of 238 species and of all fleshy-fruit types. In feeding trials, seeds of 11 species were retained on average for 309 min (±256 SD). Sampling radio-telemetry data randomly, that is, assuming foraging occurs at random times during the day, gives an estimated average dispersal distance of 239 m (±207 SD) for seeds consumed by C. casuarius. Approximately 4% of seeds were dispersed further than 1,000 m. However, observation of wild birds indicated that foraging and movement occur more frequently early and late in the day. Seeds consumed early in the day were estimated to receive dispersal distances 1.4 times the ‘random’ average estimate, while afternoon consumed seeds received estimated mean dispersal distances of 0.46 times the ‘random’ estimate. Sampling movement data according to the daily distribution of C. casuarius foraging gives an estimated mean dispersal distance of 337 m (±194 SD). Most animals’ behaviour has a non-random temporal distribution. Consequently such effects should be common and need to be incorporated into seed shadow estimation. Our results point to dispersal curves being an emergent property of the plant–disperser interaction rather than being a property of a plant or species.     

Migratory moths as dispersal vectors of an introduced plant-pathogenic fungus in Japan

The plant-pathogenic fungus Claviceps paspali infects florets of the dallisgrass Paspalum dilatatum and induces the plant to produce honeydew (containing massive amounts of conidia), which in turn attracts insects for dispersal of the fungal spores. In Japan, the association between C. paspali and its host plant is common, although both P. dilatatum and C. paspali are introduced species. To determine the dispersal agents of the introduced C. paspali, we examined which insects sapped the P. dilatatum honeydew produced by C. paspali. The study was conducted from September to October 2003 in central Japan. Adults of 48 moth species and two lacewing species visited infected spikelets of P. dilatatum and sapped the honeydew at night. The dominant moths, which carried C. paspali conidia on their proboscises, migrate long distances every year. Therefore, migratory moths may transmit C. paspali spores from diseased to healthy host plants and may have spread C. paspali to other areas in Japan where C. paspali did not previously exist.     

Small-scale spatial structure within patterns of seed dispersal

A large proportion of dispersing propagules land near their maternal plant, even in species that have evolved structures which enhance dispersal. For these propagules, their post-dispersal spatial pattern is likely to reflect the overall shape and scale of the parental plant canopy and, especially in poorly dispersing species, aggregation of propagules on the plant prior to dispersal. Localised patterns within seed shadows are also likely to be affected by secondary movement after dispersal, leading to either more or less small-scale aggregation, depending on the mechanism. Our general aim was to study the small-scale spatial structure within patterns of seed dispersal of Raphanus raphanistrum L. to generate hypotheses about the sequence of processes and events leading to the spatial pattern of dispersal in this species. More specifically, we determined the sizes of small-scale structures within the seed shadows on the ground after dispersal and the extent to which these match the sizes of pre-dispersal aggregations within the parental canopy. Variation in plant size and shape was provided by four levels of inter-specific competition resulting from differing wheat crop densities. Positions of propagules were determined using a three-dimensional digitizer, and the data for each plant were analysed using point pattern analysis. Not surprisingly, larger plants, growing at lower plant density, had larger seed shadows, showing an overall influence of maternal plant size. The pattern of propagules exhibited significant small-scale aggregates, with similar sizes on the plant and on the ground. There was no evidence that aggregation size was greater on the ground or increased with time, but the strength of the aggregation increased with length of time on the ground.     

Temporal and spatial variation in dispersal in the greater flamingo (Phoenicopterus ruber roseus)

We studied movements of individually marked greater flamingos (Phoenicopterus ruber roseus) born in the Camargue, southern France, between their two most important breeding colonies in the western Mediterranean (Camargue and Fuente de Piedra, Spain) from 1986 to 1992. The two sites differ in the frequency with which they offer suitable conditions for breeding. Flamingos have bred each year in the Camargue since 1974, but in only 12 of the past 22 years at Fuente de Piedra. Higher colony fidelity is thus expected in the less variable environment (Camargue), but if dispersal occurs competition might be an important factor causing this dispersal. Following years during which breeding birds in the Camrgue were disturbed (1988 and 1990) a higher proportion of adults changed colonies between breeding attempts (= breeding dispersal, 12.4%), while only 0.4% of flamingos breeding in the Camargue dispersed in the other years. As expected, flamingos breeding at Fuente de Piedra showed a higher rate of breeding dispersal (8.14%). No differences were observed between males and females. The importance of breeding failure as a factor causing breeding dispersal in flamingos was also confirmed by the movements of individual birds. The proportion of young flamingos that moved from their natal colony to start breeding at Fuente de Piedra (= natal dispersal) was independent of sex and age, but increased when breeding access to the Camargue colony was more difficult. However, natal dispersal was also higher in 1988 and 1990 (40.5%) than in the remaining years (1.2%), as was breeding dispersal. We discuss possible ways in which the increased natal dispersal among inexperienced birds could be linked with the increased breeding dispersal of adults in the same year.     

Biochemical and morphological evidence for host race evolution in desert mistletoe,Phoradendron californicum (Viscaceae)

Allozymes and morphological characters were used to test whether host race evolution—the genetic divergence of parasitic populations caused by adaptation to different host species—has occurred in desert mistletoe,Phoradendron californicum. Populations ofPhoradendron californicum from two hosts,Acacia greggii andProsopis glandulosa, were surveyed from the Mojave and Colorado deserts. Electrophoretic data indicated genetic differentiation of mistletoes occurring on these hosts. Three of four morphological characters (internode length, main shoot lateral shoot diameter ratio and berry color) also showed significant host-specific differentiation. These data support the hypothesis that host race formation has occurred or is occurring in this parasitic angiosperm.     

Short-distance dispersal behavior and establishment of the parasitoid Gonatocerus ashmeadi (Hymenoptera: Mymaridae) in Tahiti: Implications for its use as a biological control agent against Homalodisca vitripennis (Hemiptera: Cicadellidae)

The egg parasitoid Gonatocerus ashmeadi Girault (Hymenoptera: Mymaridae), was introduced into French Polynesia as a biological control agent to control the invasive plant feeding pest Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae). The short-distance dispersal of G. ashmeadi was monitored as part of the biological control program. G. ashmeadi showed exponential dispersal capacity with 47 m/day being a minimum estimate of its natural rate of spread at high host densities (>150 nymphs per minute of sweep net sampling) in urbanized areas at sea level, which were characterized by a high diversity of exotic ornamental plants. This rate of spread contrasted starkly with almost nonexistent establishment and dispersal where host densities were very low (<2 nymphs per minute of sweep net sampling) at high elevation (800 m) with relatively undisturbed native vegetation. Survey results across different altitudes revealed an effect of vegetative diversity and host density on the measurable mobility and establishment of G. ashmeadi. In contrast, no significant influence of wind direction was found on G. ashmeadi dispersal rate or direction. Survey results for G. ashmeadi from French Polynesia suggest that the best release establishment strategies for classical biological control of H. vitripennis are: (1) many small releases where host density is high, or (2) larger and fewer releases where host densities are low.     

Biological control of an alien tree, Acacia cyclops, in South Africa: impact and dispersal of a seed-feeding weevil, Melanterius servulus

Several Australian Acacia species have become highly invasive in a number of ecologically sensitive areas of South Africa. Many have useful attributes that, to an extent, have hampered biocontrol efforts by restricting potential agents to those reducing seed production whilst not affecting vegetative growth. The outcome of the introduction of a seed-feeding weevil, Melanterius servulus, against Acacia cyclops in 1994, was assessed by investigating the plant’s reproductive phenology, as well as levels of damage caused by the beetles. The study provided essential information on host/agent interactions and, on the build up and spread of populations of the agent. Seed damage is commonly as high as 90% (exceptionally it reaches 100%), and dispersal rates of the beetles on average is almost 2 km per year. The consistently high levels of seed damage achieved hold promise for the future of the program, however, its ultimate success will be further enhanced through the integration of biological control with other manual clearing operations that are currently employed on a large scale in South Africa.