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

1.  Most predictions of species distribution and abundance changes in response to global warming relate the individual requirements of a single isolated species to climate variables through some form of climate mapping. This method fails to account for the effects of species dispersal and species interactions, both of which may strongly affect distribution and abundance.



2.  We therefore examined the effects of dispersal and species interactions on the distribution and abundance of three Drosophila species in a laboratory system that mimicked a latitudinal cline of 15 °C. We then investigated how species distribution and abundance in this system responded to simulated global warming.



3.  Dispersal allowed populations to persist at non-optimum temperatures, overriding physiologically imposed range limits.



4.  Temperature determined the outcome of competition. In pairwise interactions, Drosophila subobscura eliminated D. melanogaster or D. simulans at low temperatures but was itself eliminated at high temperatures.



5.  Competitive interactions changed abundance and range sizes thus shifting the position of species optima. These changes depended on both the number and the identity of the competing species.



6.  Enemy–victim interactions altered range and abundance. Adding the parasitoid Leptopilina boulardi affected the host assemblage directly at high temperatures where the parasitoid was present, and indirectly (mediated by dispersal) at low temperatures where it was scarce or absent. Host species coexisted for longer at low temperatures in clines when parasitoids were present than when they were absent.



7.  Simulated global warming produced complex, counter-intuitive effects on distribution and abundance, including the reversal of species’ relative abundance at some temperatures.



8.  Because dispersal and species interactions strongly influenced both range and abundance (sometimes in unexpected ways) current species distributions are no guide to what they might be under global climate change. Furthermore, since both these factors are missing from climate envelope models of range and abundance change, their predictions are, at best, incomplete.

Individualistic species responses invalidate simple physiological models of community dynamics under global environmental change

Patterns of distribution and processes of differentiation have often been contrasted between terrestrial and marine biotas. The islands of Oceania offer an excellent setting to explore this contrast, because the geographic setting for terrestrial and shallow-water, benthic, marine organisms are the same: the myriad islands strewn across the vast Pacific. The size of species ranges and the geographic distribution of endemism are two biogeographic attributes that are thought to differ markedly between terrestrial and marine biotas in the Pacific. While terrestrial species are frequently confined to single islands or archipelagoes throughout Oceania, marine species tend to have wide to very wide distributions, and are rarely restricted to single island groups except for the most isolated archipelagoes. We explore the conditions under which species can reach an island by dispersal and differentiate. Genetic differentiation can occur either through founder speciation or vicariance; these processes are requisite ends of a continuum. We show that founder speciation is most likely when few propagules enter the dispersal medium and survive well while they travel far. We argue that conditions favorable to founder speciation are common in marine as well as terrestrial systems, and that terrestrial-type, archipelagic-level endemism is likely common in marine taxa. We give examples of marine groups that show archipelagic level endemism on most Pacific island groups as well as of terrestrial species that are widespread. Thus both the patterns and processes of insular diversification are variable, and overlap more between land and sea than previously considered.

Diversification in the Tropical Pacific: Comparisons Between Marine and Terrestrial Systems and the Importance of Founder Speciation

nMissouri, USA, and one week in New South Wales, Australia, using sticky traps in Missouri and to w nets in Australia . More than 2,000 spiders in Missouri and more than 800 spiders in Australia wer e identified to family and estimates made of their live masses . Both aeronaut faunas are dominated b y the family Linyphiidae, with the remaining families making up different proportions at the two sites . The vast majority of aeronauts weighed between 0 .2 and 1 .0 mg, with the mass-frequency distribution s at both sites tailing off rapidly beyond 2 .0 mg . The most massive Missouri aeronaut weighed 25 .5 mg , and the most massive Australian aeronaut 19 .1 mg . These are the first published extensive data on th e live masses of ballooning spiders from the field and this is the first taxonomic analysis of a Souther n Hemisphere aeronaut fauna .

Ballooning spiders in missouri, usa , and new south wales, australia : family and mass distribution s

The mechanisms of colonization, competition and extinction have fascinated biologists since they began to appreciate that ecological patterns, however stable they might appear, are the outcome of complex dynamic interactions The pioneer naturalists who recorded the similarities and differences between floras and faunas in different regions were often confronted by the question of whether todays patterns were the relict consequence of processes that operated in the past but no longer did no, or could be explained by the dispersal capacity of species in a world with todays geography Darwin and Hooker corresponded over the cause of the floristic and faunistic similarities between widely separated temperate lands in the southern hemisphere, and Darwin once wrote to Hooker about one widely dispersed island tree ‘I believe you are afraid to send me a ripe Edwardsia pod lest I should float it from New Zealand to Chile’ The study of the disjunct and distinctive biotas of oceanic islands has always involved assumptions (if not calculations) about the isolating influence of ocean barriers and about the rates of colonization as a source of new genes diluting the pool already present and moderating the trend towards endemism

Summary and conclusions: characteristics and consequences of biological invasions

Six cruises were made aboard ships in the Pacific during 1967 and 1968 to continue studies of trans-oceanic dispersal of insects. Specimens obtained together with new and modified equipment and techniques used are discussed. Suggestions are included for further changes to improve the program.

http://hbs.bishopmuseum.org/pi/pdf/11(2)-455.pdf

E. P. Holzapfel

Papers

Trapping of air-borne insects on ships in the pacific, part 8