Arthropod predation in an Antarctic terrestrial community
(1) Field predation by the mite Gamasellus racovitzai (GA) on three other species of micro-arthropods was determined using polyacrylamide gel electrophoresis (PAGE). Attack rates were derived and prey selection examined in maritime Antarctic communities at Signy Island. (2) During summer, c. 86% of the predator population had detectable prey traces. The species composition of these traces in Gamasellus from three major habitat types reflected the proportions of prey observed in the field community, although there were differences between the life-stages of Gamasellus. The collembolan Cryptopygus antarcticus (CR) formed c. 80% of all prey traces. (3) The non-feeding component of the predator population was stable at c. 14% despite the 10-fold range in Cryptopygus densities between moss-turf, fellfield and ornithogenic sites. (4) Data from field and laboratory experiments showed no detectable functional response over a 300-fold range in prey density. Attack rates upon Cryptopygus calculated by quantitative electrophoresis analysis of field samples confirm this. (5) In laboratory experiments, Gamasellus presented with different size-classes of prey showed maximum attack rates of c. 4 Cryptopygus per predator per day (CR/GA day-1), decreasing with prey size to as little as 0.1 CR/GA day-1 or less for juveniles and c. 0.2 CR/GA day-1 for adults. Mean field attack rates calculated by electrophoresis analysis were more consistent at 0.2 CR/GA day-1 for protonymphs and deutonymphs and 0.4 CR/GA day-1 for adults. These attack rates would easily be sustainable at the lowest field densities of Cryptopygus (c. 1 cm-2) ever likely to be encountered by Gamasellus at Signy Island. (6) Experiments using mixtures of two prey (Cryptopygus and a second collembolan, Parisotoma octooculata) showed that Gamasellus prefers the former species, with feeding being maximal at the ratio of 4 Cryptopygus: 1 Parisotoma. Live weight gain of the predator is also greatest at this ratio. (7) The data validate the method of Lister, Usher & Block (1987) for the quantification of field attack rates by predatory micro-arthropods. They are discussed in relation to the strategies adopted by an invertebrate predator to maintain its population in extreme habitats containing few prey species.