Patterns in the abundance and distribution of littoral and supralittoral arthropods on Marion Island

Abstract

This study provides the first quantitative analysis of the littoral and supralittoral arthropod assemblages of sub-Antarctic Marion Island. Seventeen mite species (126 203 individuals) from 11 families were found on the shore at Macaroni Bay. Three families dominated the assemblages in both abundance and diversity: the Hyadesiidae, Ameronothridae and Halacaridae. Six insect species from three orders were found on the shore. Species richness increased from one in the littoral, to four and two species in the Mastodia and Caloplaca zones, respectively. The littoral, chironomid midge, Telmatogeton amphibius was the most abundant insect species, constituting 80% of all insects counted. Arthropod assemblages corresponded closely to the cryptogram dominated zonation patterns previously identified for the Marion Island shore. This clear association between arthropod and cryptogam zonation patterns provided a clear indication of habitat specificity in many of the species, and a quantitative analysis of habitat specificity on a species by species basis supported this tenet. The specificity of most species to the shore, which forms part of the epilithic biotope, is most likely a consequence of the considerable age of this biotope compared to the younger, post-glacial vascular vegetation. Tourist species, i.e. species transient to an assemblage, inflated species richness in zones and the distribution ranges of species across zones. It is suggested that previous, qualitative analyses of shoreline arthropod communities may have overestimated species ranges and richnesses because of the inclusion of tourist species. It is suggested that if a sound understanding of patterns in and processes underlying Antarctic arthropod assemblages is to be achieved, quantitative analyses must be expanded in the region. In this study the spatial patterns of littoral and supralittoral arthropods from sub-Antarctic Marion Island are examined. Primary consumers were by far the most diverse group on the shore, with nine algivore and ten fungivore species from a total of 23 species. Positive species associations were found in the Mastodia and Caloplaca zones and positive abundance covariation in the Verrucaria, Mastodia and Caloplaca zones. There were no negative associations between any taxa, indicating that interspecific interactions on the shore are either minimal or absent. Significant interactions were related to the diversity of the respective habitats, with higher diversity resulting in higher levels of positive associations and abundance covariation. High levels of aggregation clearly demonstrated that species were not randomly distributed within habitats. Intraspecific aggregation was generally higher than interspecific aggregation in the five habitats and if competition was to occur it would most likely be among conspecifics. The absence of suitable biological information for species precluded further analyses of competition. However, if competitive interactions were found to occur between heterospecifics then coexistence would best be explained using the aggregated nature of superior competitors, allowing weaker competitors to coexist in zones. Positive associations between species were attributed to favourable environmental conditions, the availability of limiting resources (e.g. shelter) and the structure of the dominant cryptogram species. Body sizes, spanning five orders of magnitude (0.5 J.1g - 26 mg), were measured for 59 of the approximately 120 invertebrate species on Marion Island. Mass-length and fresh-dry mass relationships were calculated for orders, families and species (for those with sufficient data). A comparison of their slopes indicates that for prediction of body mass it may be useful to use regressions from the lowest taxonomic rank possible. Differences between the mass-length relationship for Marion Island insects (log mass = -4.294 + 3.151 log length) and other relationships on continental assemblages raises the questions as to the applicability of these results. This study should prove useful for estimating body sizes for other, similar taxa in the Antarctic and provide baseline information on an important species trait that seems to be changing with local and global environmental changes.