Low propensity for aerial dispersal in specialist spiders from fragmented landscapes

Aerial dispersal by ballooning is a passive flight, by which wind drag generates an upward lift on a silk thread. It is likely to reflect an aerial lottery, in which the absence of flight direction control is a serious cost for long-distance dispersal in a fragmented landscape. For species occurring in one patchily distributed habitat type, dispersal should evolve in a different way from morphological traits, directly linked to active dispersal. Therefore, we expect that if the risk of landing in an unsuitable habitat is lower than the probability of reaching a suitable habitat, selection should benefit a well-developed ballooning behaviour. We investigated interspecific variation in the ballooning-initiating tiptoe behaviour as it is linked to spider dispersal performance. Our results indeed indicate that ballooning performance is negatively related to habitat specialization in spiders from patchy grey dunes, so habitat specialists are characterized by poorly developed dispersal behaviour. These findings are concordant with recent insights that dispersal is selected as risk spreading in generalists, while it is selected against in specialist species.     

Repeatability of dispersal behaviour in a common dwarf spider: evidence for different mechanisms behind short‐ and long‐distance dispersal

Abstract 1. The response of dispersal towards evolution largely depends on its heritability for which upper limits are determined by the trait’s repeatability. 2. In the Linyphiid spider E. atra, we were able to separate long‐ and short‐distance dispersal behaviours (respectively ballooning and rappelling) under laboratory conditions. By performing repeated behavioural trials for females, we show that average dispersal trait values decrease with increasing testing days. By comparing mated and unmated individuals during two periods (before and after mating for the mated group, and the same two periods for the unmated group), we show that mating has no effect on the mean displayed dispersal behaviour or its within‐individual variation. Repeatabilities were high and consistent for ballooning motivation, but not for rappelling. 3. Ballooning motivation can be regarded as highly individual‐specific behaviour, while general pre‐dispersal and rappelling behaviours showed more individual variation. Such difference in repeatability between long‐ and short‐distance dispersal suggests that short‐ and long‐distance dispersal events are triggered by different ecological and evolutionary mechanisms.     

Inbreeding depresses short and long distance dispersal in three congeneric spiders

Dispersal is one of the most important precopulatory inbreeding avoidance mechanisms and subject to landscape related selection pressures. In small populations, inbreeding within and between populations may strongly affect population dynamics if it reduces fitness and gene‐flow. While inbreeding avoidance is generally considered to be a key evolutionary driver of dispersal, potential effects of inbreeding on the dispersal process, are poorly known. Here, I document how inbreeding within a population, so by mating among relatives, affects the survivorship and the dispersal behaviour of three congeneric spider Erigone species (Araneae: Linyphiidae) that differ in habitat preference and regional rarity. The three species were chosen as a model because they allow the assessment of both long and short distance dispersal motivation (respectively ballooning and rappelling) under laboratory conditions. Inbreeding reduced both long and short distance dispersal modes in the three congeneric species. Because survival was depressed after inbreeding, with a tendency of reduced survival loss in the rare and highly stenotopic species, energetic constraints are likely to be the underlying mechanism. Inbreeding consequently depresses silk‐related dispersal in three related spiders. This may induce an inbreeding depression vortex with important consequences for range expansion and metapopulation dynamics of aerially dispersing species from highly fragmented landscapes.     

Ballooning dispersal using silk: world fauna, phylogenies, genetics and models

Aerial dispersal using silk ('ballooning') has evolved in spiders (Araneae), spider mites (Acari) and in the larvae of moths (Lepidoptera). Since the 17th century, over 500 observations of ballooning behaviours have been published, yet there is an absence of any evolutionary synthesis of these data. In this paper the literature is reviewed, extensively documenting the known world fauna that balloon and the principal behaviours involved. This knowledge is then incorporated into the current evolutionary phylogenies to examine how ballooning might have arisen. Whilst it is possible that ballooning co-evolved with silk and emerged as early as the Devonian (410-355 mya), it is arguably more likely that ballooning evolved in parallel with deciduous trees, herbaceous annuals and grasses in the Cretaceous (135-65 mya). During this period, temporal (e.g. bud burst, chlorophyll thresholds) and spatial (e.g. herbivory, trampling) heterogeneities in habitat structuring predominated and intensified into the Cenozoic (65 mya to the present). It is hypothesized that from the ancestral launch mechanism known as 'suspended ballooning', widely used by individuals in plant canopies, 'tip-toe' and 'rearing' take-off behaviours were strongly selected for as habitats changed. It is contended that ballooning behaviour in all three orders can be described as a mixed Evolutionary Stable Strategy. This comprises individual bet-hedging due to habitat unpredictability, giving an underlying randomness to individual ballooning, with adjustments to the individual ballooning probability being conferred by more predictable habitat changes or colonization strategies. Finally, current methods used to study ballooning, including modelling and genetic research, are illustrated and an indication of future prospects given.     

Ballooning propensity of canopy and understorey spiders in a mature temperate hardwood forest

1. Spiders frequently disperse and colonise habitats through ballooning, a passive aerial dispersal process. Ballooning is pre‐eminent in open habitat spider communities and its propensity can be modulated by habitat conditions and availability, and by life‐history traits such as body size, degree of specialisation, and feeding behaviour. 2. Using spiders from the canopy and understorey of a north‐temperate hardwood forest as a model system, our main objectives were to detect if foliage spiders of a mature forest disperse through ballooning, and identify life‐history traits that influence ballooning propensity. 3. Our results demonstrate that foliage spiders living in the canopy and understorey of a mature forest do balloon, and in some cases have very high ballooning propensities similarly to open field spiders. Species level models showed that small body size had a strong positive effect on ballooning for juveniles of species with large‐bodied adults, while individuals of small‐bodied species initiated ballooning regardless of size, habitat or development stage. A generalised linear mixed model indicated that small size web‐building spiders from the Retro Tibial Apophysis (RTA) and Orbicularia clades had the highest propensity for foliage spiders of this north‐temperate hardwood forest. 4. In conclusion, we provide the first demonstration that forest spiders can have high ballooning propensities and that ballooning propensity is negatively affected by body size and positively affected by the prominent use of silk to catch prey. However, spiders originating from the canopy and understorey of a north‐temperate hardwood forest did not differ in their ballooning propensities.     

Levels of gene flow among populations of a wolf spider in a recently fragmented habitat: current versus historical rates

Gene flow among populations is important for countering the deleterious effects of random genetic drift and inbreeding, as well as spreading beneficial mutations. Wind-driven aerial dispersal is known to occur in numerous plants and invertebrates. Its evolution suggests that historically, suitable habitat patches were dense enough to make such undirected dispersal evolutionarily advantageous. Using microsatellite markers we assessed the population genetic structure of seven populations of a wolf spider (Rabidosa rabida) capable of ballooning. Historically, each spider population received a mean of 1.5 migrants per generation from the other six populations. Over the past several generations the number of migrants reaching a population is only 0.2. This statistically significant reduction in gene flow coincides with high levels of habitat fragmentation and suggests that undirected aerial dispersal is ineffective in this fragmented landscape. Further, individuals within populations showed signficantly elevated levels of homozygosity relative to Hardy–Weinberg expectations, suggesting that cursorial dispersal may be very limited and genetic structure within populations exists. Inbreeding coefficients averaged 0.18 over all seven populations with very little variation among populations (s = 0.02). Fitness was lower in smaller populations relative to larger ones. Altered landscapes pose evolutionary dilemmas for many metapopulations and species that depend on undirected movement for dispersal may be particularly vulnerable to habitat fragmentation.     

Environmental and genetic background of tiptoe-initiating behaviour in the dwarfspider Erigone atra

Ballooning is a widespread behavioural trait in invertebrates, which enhances passive aerial dispersal. We investigated the influence of common lineage (family effect) and postnatal environmental conditions on latency to initiate preballooning tiptoe behaviour (ballooning latency) in the dwarfspider Erigone atra (Blackwall, 1841). The ballooning latency of individual spiders was tested in a specially designed test chamber. In a 3-week experiment, in which the spiders were fed only during the first day of each week, ballooning latency had low repeatability at weekly intervals. Ballooning latency declined with increasing food deprivation during the first week but not during the second and the third weeks. At intervals of less than 1 h, however, ballooning latency showed high repeatability. We also investigated whether maternal and postnatal environmental conditions (i.e. during juvenile development) influence phenotypic variation in ballooning latency. We reared offspring of several families under two feeding and two temperature conditions. Environmental conditions explained more variation in ballooning latency than family. Ballooning latency was lower in spiders reared at 20°C than in those reared at 15°C. In addition, spiderlings fed four prey per 3 days were faster ballooners than those fed only four prey per week. An interaction between factors was present, indicating the existence of different reaction norms between the two environmental conditions. The expression of ballooning latency behaviour thus strongly depends on current nutrition, feeding history and the feeding and temperature conditions during juvenile development. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Natal dispersal and parental escorting predict relatedness between mates in a passerine bird

Although relatedness between mates is of considerable evolutionary and ecological significance, the way in which the level of relatedness is determined by different behavioural processes remains largely unknown. We investigated the role of behaviour in predicting mate relatedness in great tits using genotypic markers and detailed observations. We studied how mate relatedness is influenced by natal dispersal, inbreeding/outbreeding avoidance after natal dispersal and a behaviour not previously considered that influences membership to social aggregations, namely family escorting behaviour by parents. Among locally born individuals, the level of mate relatedness decreased with natal dispersal distance for females, but not for males. In contrast, mate relatedness was negatively related to the extent of family movements for males, but not for females. However, family movements did not predict dispersal distance for either sex. Local recruits were more related to their mates than immigrants, but this was only significant for females. No evidence was found for inbreeding/outbreeding avoidance after dispersal. Our results suggest that, in highly mobile species, mating options are spatially and/or socially limited, and that parents influence mating options of their offspring before dispersal.     

Ballooning dispersal in arthropod taxa: conditions at take-off

We have solved a long-standing and seemingly paradoxical set of questions that relate to the conditions which govern spider ballooning. We show that observations of spider ballooning excursions are best explained by meteorological conditions which maximize dispersal. Dispersal is predicted to be most effective in terms of distance when the stability of the atmosphere is non-ideally convective and is less effective during purely convective or neutrally stable conditions. Ballooners are most likely to travel a few hundred metres, but dispersal distances of several hundred kilometres are possible.     

Predictors of long-distance dispersal in the Siberian flying squirrel

During dispersal the distances moved differ between individuals. The evolutionary causes of dispersal rate are much studied, for example, it is observed that dispersal is often a condition- and phenotype-dependent strategy. However, more empirical information is needed on factors affecting the dispersal distance. We study factors behind dispersal distance in the juvenile Siberian flying squirrel. The longer dispersing individuals abandoned natal site earlier in the season and were larger, perhaps being born earlier, than shorter dispersing individuals. These patterns did not hold between same-sex siblings, indicating that the early long-distance dispersal was more a between than a within-litter related phenomenon. Our results indicate differences between litters that are related to dispersal strategies of individuals. In flying squirrels, long-distance dispersal is not merely a secondary effect of short-distance dispersal. Instead, the distribution of dispersal distance is affected by factors enhancing long-distance dispersal.     

Short-term response of ground-dwelling arthropods to storm-related disturbances is mediated by topography and dispersal

Wind disturbances and consequent salvage logging lead to drastic changes in forest soil conditions, vegetation and microclimate, potentially affecting arthropod communities. In mountain regions, topography is expected to be particularly important to modulate the effect of canopy removal and soil disturbance potentially amplifying the ecological contrast between forest and disturbed areas. Here, we studied the short-term response of ground beetles (Carabidae), spiders (Araneae), and harvestmen (Opiliones) in wind-damaged spruce forests along statistically orthogonal gradients in elevation, slope, and aspect. We addressed three main ecological questions: (i) Does the effect of wind disturbance on diversity depend on topography? (ii) Are there specific taxon-related responses to disturbances?, and (iii) What is the role of dispersal in shaping species assembly dynamics? We generally observed that increasing slope and elevation amplified the differences between undisturbed forest and windfall areas. On the one hand, the diversity of ground beetles and harvestmen seemed to be negatively affected by wind disturbance, causing a loss of specialized forest species with a low rate of colonization of species typical of open habitats. On the other hand, several novel spider species were able to rapidly colonize windfalls and community composition strongly shifted from forest to disturbed areas. Species with long-range dispersal strategies (e.g. flying and ballooning) were those more likely to colonize windfalls. Our findings suggest that disturbance effects on ground-dwelling organisms were modulated by underlying environmental gradients and that short-term response of different taxa was dependent on their dispersal ability.     

寡居种悦目金蛛若蛛群居机制初步研究

对寡居种悦目金蛛Argiope amoena若蛛群居生活对其结网的影响、温度和种群大小对若蛛存活率的影响、若蛛对限制性空间的利用及其扩散方式进行了观察和研究.结果表明,若蛛从卵袋出蛰后不经历群居扩散即具备结网能力,可结完整网,也具备在扩散前不进食水存活的能力;在变温条件下若蛛存活率要远远高于室温条件下的存活率,而在室温条件下若蛛开始死亡和半数死亡的时间比其聚居期长得多;若蛛主要通过群体空中扩散的方式进行扩散,可以减小能耗与敌害的威胁;不同种群大小的若蛛在限制性空间的分布均从第5d左右开始发生较大变化的,这与在空间不受限的条件下若蛛在第5d开始扩散的结果是一致的.据此我们推断:个体扩散代价最小化(包括御敌和能耗)和维持种群较高的生存率可能是寡居种悦目金蛛若蛛群居生活的主要原因.     

Big and aerial invaders: dominance of exotic spiders in burned New Zealand tussock grasslands

As post-disturbance community response depends on the characteristics of the ecosystem and the species composition, so does the invasion of exotic species rely on their suitability to the new environment. Here, we test two hypotheses: exotic spider species dominate the community after burning; and two traits are prevalent for their colonisation ability: ballooning and body size, the latter being correlated with their dispersal ability. We established spring burn, summer burn and unburned experimental plots in a New Zealand tussock grassland area and collected annual samples 3 and 4 years before and after the burning, respectively. Exotic spider abundance increased in the two burn treatments, driven by an increase in Linyphiidae. Indicator analysis showed that exotic and native species characterised burned and unburned plots, respectively. Generalised linear mixed-effects models indicated that ballooning had a positive effect on the post-burning establishment (density) of spiders in summer burn plots but not in spring plots. Body size had a positive effect on colonisation and establishment. The ability to balloon may partly explain the dominance of exotic Linyphiidae species. Larger spiders are better at moving into and colonising burned sites probably because of their ability to travel longer distances over land. Native species showed a low resilience to burning, and although confirmation requires longer-term data, our findings suggest that frequent fires could cause long lasting damage to the native spider fauna of tussock grasslands, and we propose limiting the use of fire to essential situations.     

Dispersal and adaptive radiation of Bidens (Compositae) across the remote archipelagoes of Polynesia

The genus Bidens (Compositae) comprises c. 230 species distributed across five continents, with the 41 Polynesian species displaying the greatest ecomorphological variation in the group. However, the genus has had a long and complicated taxonomic history, and its phylogenetic and biogeographic history are poorly understood. To resolve the evolutionary history of the Polynesian Bidens, 152 individuals representing 91 species were included in this study, including 39 of the 41 described species from Polynesia. Four chloroplast and two nuclear DNA markers were utilized to estimate phylogenetic relationships, divergence times, and biogeographic history. Bidens was found to be polyphyletic within Coreopsis, consistent with previous assessments. The Polynesian radiation was resolved as monophyletic, with the initial dispersal into the Pacific possibly from South America to either the Hawaiian or Marquesas Islands. From the Marquesas, Bidens dispersed to the Society Islands, and ultimately to the Austral Islands. The initial diversification of the crown group in the Pacific is estimated to have occurred ~1.63 mya (0.74–2.72, 95% HPD), making Polynesian Bidens among the youngest and most rapid plant diversification events documented in the Pacific. Our findings suggest that relatively rare long‐distance dispersal and founder‐event speciation, coupled with subsequent loss of dispersal potential and within‐island speciation, can explain the repeated and explosive adaptive radiation of Bidens throughout the archipelagoes of Polynesia.     

Between-population variation in homeward orientation behaviour in two riparian wolf spiders

Animals make decisions based on various sources of information that differ in spatial and temporal scale of validity and/or applicability. This decision-making is expected to be shaped by evolutionary processes and is especially relevant in stressful situations. The importance of inherited sources of information or experience involved in orientation behaviour remains to date unclear. By means of a field-experiment, we evaluated variation in zonal recovery of two sympatric riparian Pardosa wolf spiders after releasing individuals offshore from a non-familiar river bank. After acclimatisation under controlled laboratory conditions, both species showed strong directional movements towards the natal river bank shore. Additionally, the more stenotopic riparian wolf spider showed considerable between-individual variation in orientation behaviour. In conclusion, information with respect to an individual's origin acts as an important cue for wolf spider orientation during movement. Our findings provide insights into decision-making processes in stressful situations and point to between-population variation in orientation behaviour, which relates to inherited factors and/or early-life (learned) experience.     

Effects of aerial dispersal, habitat specialisation, and landscape structure on spider distribution across fragmented grey dunes

Species distribution patterns have been explained by Hutchinson's niche theory, metapopulation theory and source-sink theory. Empirical verification of this framework, however, remains surprisingly scant. In this paper, we test the hypothesis that landscape characteristics (patch size and connectivity), aerial dispersal ability and niche breadth interact in explaining distribution patterns of 29 spider species inhabiting fragmented grey dunes. Distribution patterns only depended on aerial dispersal potential, and the interaction between patch connectivity and area. Niche breadth, measured as the degree of habitat specialisation in the total coastal dune system, did not contribute to the observed distribution patterns. Additional variation in patch occupancy frequency was strongly species-dependent and was determined by different responses to the degree of patch connectivity for ballooning dispersal. Results from this study suggest that dispersal ability largely affects our perception of a species "fundamental niche", and that source-sink and metapopulation dynamics may have a major impact on the distribution of species. From a conservation point of view, specialised (and hence intrinsically rare) species can be predicted to become rarer if fragmentation increases and connectivity decreases. This study is, to our knowledge, one of the few linking species distribution (and not patch occupancy, species diversity or richness) to landscape ecological (patch connectivity and area) and auto-ecological (niche breadth, dispersal potential) features.     

Effects of extreme climatic events on small-scale spatial patterns: a 20-year study of the distribution of a desert spider

Individuals of most animal species are non-randomly distributed in space. Extreme climatic events are often ignored as potential drivers of distribution patterns, and the role of such events is difficult to assess. Seothyra henscheli (Araneae, Eresidae) is a sedentary spider found in the Namib dunes in Namibia. The spider constructs a sticky-edged silk web on the sand surface, connected to a vertical, silk-lined burrow. Above-ground web structures can be damaged by strong winds or heavy rainfall, and during dispersal spiders are susceptible to environmental extremes. Locations of burrows were mapped in three field sites in 16 out of 20?years from 1987 to 2007, and these grid-based data were used to identify the relationship between spatial patterns, climatic extremes and sampling year. According to Morisita??s index, individuals had an aggregated distribution in most years and field sites, and Geary??s C suggests clustering up to scales of 2?m. Individuals were more aggregated in years with high maximum wind speed and low annual precipitation. Our results suggest that clustering is a temporally stable property of populations that holds even under fluctuating burrow densities. Climatic extremes, however, affect the intensity of clustering behaviour: individuals seem to be better protected in field sites with many conspecific neighbours. We suggest that burrow-site selection is driven at least partly by conspecific cuing, and this behaviour may protect populations from collapse during extreme climatic events.     

Role of breeding and natal movements in lifetime dispersal of a forest‐dwelling rodent

The lifetime movements of an individual determine the gene flow and invasion potential of the species. However, sex dependence of dispersal and selective pressures driving dispersal have gained much more attention than dispersal at different life and age stages. Natal dispersal is more common than dispersal between breeding attempts, but breeding dispersal may be promoted by resource availability and competition. Here, we utilize mark–recapture data on the nest‐box population of Siberian flying squirrels to analyze lifetime dispersal patterns. Natal dispersal means the distance between the natal nest and the nest used the following year, whereas breeding movements refer to the nest site changes between breeding attempts. The movement distances observed here were comparable to distances reported earlier from radio‐telemetry studies. Breeding movements did not contribute to lifetime dispersal distance and were not related to variation in food abundance or habitat patch size. Breeding movements of males were negatively, albeit not strongly, related to male population size. In females, breeding movement activity was low and was not related to previous breeding success or to competition between females for territories. Natal philopatry was linked to apparent death of a mother; that is, we did not find evidence for mothers bequeathing territories for offspring, like observed in some other rodent species. Our results give an example of a species in which breeding movements are not driven by environmental variability or nest site quality. Different evolutionary forces often operate in natal and breeding movements, and our study supports the view that juveniles are responsible for redistributing individuals within and between populations. This emphasizes the importance of knowledge on natal dispersal, if we want to understand consequences of movement ecology of the species at the population level.     

Increased propensity for aerial dispersal in disturbed habitats due to intraspecific variation and species turnover

Animal dispersal depends on multiple factors, such as habitat features and life‐history traits of the species. We studied the propensity for ballooning dispersal in spiders under standardized laboratory conditions. The 1269 tested individuals belonged to 124 species and originated from 16 sites with wide variation in habitat type. Spiders from disturbed habitats ballooned 5.5 times more than spiders from stable habitats. In Meioneta rurestris , for which we had enough data for a single‐species analysis, individuals were most dispersive if they originated from highly disturbed habitats. While the data for the other species were not sufficient for single‐species analyses, a hierarchical model that included the data simultaneously on all species suggested that dispersal propensity generally increases within species with the level of habitat disturbance. Dispersal probability showed a trend to increase with niche width, but the higher commonness of species with wide niches provides an alternative explanation for this pattern. As the prevalence of especially dispersive species was highest in disturbed habitats, variation in dispersal propensity was influenced by both inter‐ and intraspecific factors. We conclude that the positive correlation between niche width and dispersal propensity enables generalist species to utilize highly disturbed habitats, whereas the persistence of specialist species with restricted dispersal ability requires the conservation of stable habitats.     

Couch potatoes do better: Delayed dispersal and territory size affect the duration of territory occupancy in a monogamous mammal

In territorial, socially monogamous species, the establishment and defense of a territory are an important strategy to maximize individual fitness, but the factors responsible for the duration of territory occupancy are rarely studied, especially in long‐lived mammals. A long‐term monitoring program in southeast Norway spanning over 18 years allowed us to follow the individual life histories of Eurasian beavers (Castor fiber) from adolescence in their natal family group to dispersal and territory establishment until the end of territory occupancy. We investigated whether territory size, resource availability, population density, and dispersal age could explain the duration of territory occupancy, which ranged from 1 to 11 years. The duration of territory occupancy was positively related to dispersal age, suggesting that individuals that delayed dispersal had a competitive advantage due to a larger body mass. This is in support with the maturation hypothesis, which states that an animal should await its physical and behavioral maturation before the acquisition of a territory. Further, we found that individuals that established in medium‐sized territories occupied them longer as compared to individuals in small or large territories. This suggests that large territories are more costly to defend due to an increased patrolling effort, and small territories might not have sufficient resources. The lifetime reproductive success ranged from zero to six kits and generally increased with an increasing duration of territory occupancy. Our findings show the importance of holding a territory and demonstrate that dispersal decisions and territory selection have important consequences for the fitness of an individual.