COMPARATIVE NUTRITIONAL ECOLOGY OF GRASS-FEEDING IN A SUB-ANTARCTIC BEETLE - THE IMPACT OF INTRODUCED SPECIES ON HYDROMEDION-SPARSUTUM FROMSOUTH GEORGIA

South Georgia has many introduced plant and animal species, a conseque nce of its long history of human habitation. Introduced reindeer have a strong effect on the vegetation of the Stromness Bay area by causing the replacement of indigenous species by grazing-tolerant grasses suc h as the exotic Poa annua, and in certain circumstances, the indigenou s Festuca contracta. Recently it has been argued that an introduced pr edatory carabid has contributed to declines in the abundance and an in crease in the body size of adults of the indigenous perimylopid beetle Hydromedion sparsutum. However, it also appears that body size of the se beetles is smaller in areas where exotic grasses predominate compar ed to undisturbed areas. Here we test the hypothesis that by causing t he spread of poorer quality grasses, especially the exotic Poa annua, reindeer may be having an indirect effect on N. sparsutum. To do this eve examined the nutritional ecology of H. sparsutum larvae on four gr ass species which form a major part of its diet, viz. the indigenous P arodiochloa flabellata, Phleum alpinum and Festuca contracta, and the exotic Poa annua. Larvae showed the highest growth rate on Parodiochlo a flabellata: followed by Phleum alpinum, F. contracta and Poa annua. These differences are due to poorer absorption of the exotic grass, an d poorer utilization of the absorbed material in the case of F. contra cta. Poor growth of larvae on F, contracta appears to be due to its lo w water and nitrogen contents, whereas in the case of P. annua a combi nation elf low water content and high nitrogen content may be responsi ble for low growth rates. Low growth rates associated with poor qualit y food may lead either to a prolongation of the life cycle or of the l ength of feeding bouts of an insect. Neither option appears to be feas ible for H. sparsutum, and this means that the outcome of feeding on p oorer-quality foods would be a reduction in final adult size. This has fitness consequences for the beetle. Hence it appears that by causing the spread of grasses that are unsuitable for growth of H. sparsutum, reindeer may be hating an indirect effect on this beetle species.