No abstract

Body size is one of the most significant features of animals. Not only is it correlated with many life history and ecological traits, but it also may influence the abundance of species within, and their membership of, assemblages. Understanding of the latter processes is frequently based on a comparison of model outcomes with the frequency of species of different body mass within natural assemblages. Consequently, the form of these frequency distributions has been much debated. Empirical data usually concern taxonomically delineated groups, such as classes or orders, whereas the processes ultimately apply to whole assemblages. Here, we report the most complete animal species-body size distribution to date for those free-living species breeding on sub-Antarctic Marion Island and using the terrestrial environment. Extending over 15 orders of magnitude of variation in body mass, this distribution is bimodal, with separate peaks for invertebrates and vertebrates. Under logarithmic transformation, the distribution for vertebrates is not significantly skewed, whereas that for invertebrates is right-skewed. Contrary to expectation based on a fractal or pseudofractal environmental structure, the decline in the richness of species at the smallest body sizes is a real effect and not a consequence of unrecorded species or of species introductions to the island. The scarcity of small species might well be a consequence of their large geographic ranges.

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Body size was measured for 67 of the approximately 120 invertebrate species on Marion Island. These include more than 60% of the 29 acarine families, and more than 80% of the remaining terrestrial invertebrate species. Thus the data are regarded as representative of the entire invertebrate fauna of sub-Antarctic, Marion Island. Length–mass and fresh–dry mass relationships were calculated for orders, families and species to provide a means of estimating body size parameters for species in collections and those which are known from only a few specimens. A comparison of the regression slopes for the different taxonomic ranks indicates that it is better to use regressions from the lowest possible taxonomic level for prediction of body mass. Differences between length–mass relationships for Marion Island insects and continental assemblages raises the question as to the applicability of continental regressions to sub-Antarctic species. This study provides a useful means for estimating body size parameters for Antarctic and sub-Antarctic invertebrates and provides baseline data on an important species trait that seems to be changing with local and global environmental changes.

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It has been suggested previously that the presence and abundance of indigenous species have a marked influence on the likelihood of invasion of a community. It has also been suggested that such biotic resistance has a negligible influence on the outcome of an invasion, but that the abiotic characteristics of the environment being invaded are more important. The latter has been claimed to be especially important on the islands of the Southern Ocean. In order to test these competing hypotheses we examined the distribution and abundance of indigenous and introduced springtails across 13 habitats, which differ considerably in the properties of their soils, and soil temperature, on the eastern quarter of sub‐Antarctic Marion Island. There was no evidence of negative abundance covariation or species associations within habitats, nor were there significant relationships between species richness or abundance of the indigenous as opposed to the introduced collembolans across habitats. Interspecific interactions thus seem to have played no readily identifiable role in the outcome of invasions by Collembola on Marion island. In contrast, the indigenous and introduced species responded very differently to abiotic variables. The indigenous Collembola prefer drier, more mineral soils with a low organic carbon content, and species richness tends to be highest in cold, fellfield areas. On the other hand, the introduced springtails prefer moist, warm sites, with organically enriched soils, introduced species richness was negligible in cold, fellfield areas. Disturbance also appeared to influence positively the species richness and abundance of introduced species at a site. These results provide independent support for the idea that abiotic factors, especially temperature, significantly influence the likelihood of biological invasions on Southern Ocean islands. They also suggest that predicting the outcome of climate change on community structure in this region is likely to be problematic, especially in the case of the Collembola.

No abstract

SUMMARY We have examined the gas-exchange characteristics of five southern African centipede species from three orders. Two scolopendromorph species exhibit discontinuous gas-exchange cycles (DGCs) identical to those recorded for several insect and chelicerate species. Another scolopendromorph and a lithobiomorph species exhibit weak periodic patterns, and a scutigermorph species shows continuous gas exchange. A crucial component for DGCs in tracheated arthropods is the presence of occludible spiracles. However, on the basis of studies of temperate centipedes, most recent invertebrate biology texts hold the view that centipedes, as a group, cannot close their spiracles. Using flow-through normoxic and normoxic—anoxic—normoxic respirometry and electron microscopy, we conclusively demonstrate that at least one of the scolopendromorph species, Cormocephalus morsitansL., can close its spiracles fully, thus accounting for its DGCs. Homologies in spiracular structure and DGCs suggest that several other tracheated arthropod taxa probably have this ability too and that DGCs have evolved convergently at least four times in the Arthropoda. Spiracular closure and discontinuous gas-exchange cycles are probably more widespread in arthropods than has previously been suspected.