Patterns of energy flow in populations of the dominant insect consumers on Marion Island

Abstract

Five insect species were identified as important primary consumers or decomposers in the terrestrial ecosystem of sub-Antarctic Marion Island. Patterns of energy flow in individuals ("life strategies") and populations (population energetics) of the five species were studied against the evolutionary templet of their respective microhabitats. Energy flow through populations of the five species was used for a quantitative description of energy flow through the primary consumer component of the Marion Island terrestrial ecosystem, while the life history and ecological attributes of the five species were compared against the background of past and present geological, climatological, evolutional and ecological processes in the sub-Antarctic. The insects are Embryonopsis halticel/a Eaton (Lepidoptera: Yponomeutidae); Ectemnorhinus similis (C. 0. Waterhouse) and E. marioni Jeannel (Coleoptera: Curculionidae); Pringleophaga marioni Viette (Lepidoptera: Tineidae) and Paractora dreuxi mirabilis Seguy (Diptera: Helcomyzidae). E. halticella is a host-specific folivore of the tussock grass Poa cookii, and dominates the herbivore guild on the island, while E. similis and E. marioni are polyphagous feeders of both angiosperms and bryophytes. P. marioni is a polyphagous detritivore and an important decomposer of plant litter. The kelp fly P. dreuxi mirabilis is the dominant decomposer of stranded kelp in the island's littoral zone. Herbivore populations (of E. halticella larvae and E. similis adults) assimilate less than 5 % of the annual net primary production at Marion Island. P. dreuxi mirabilis larval populations may assimilate up to 10 % of stranded kelp and play an important role in enhancing the microbial decay of wrack. The predominantly moss-feeding (E. marioni adults) and litter-feeding (P. marioni larvae) terrestrial insects do not contribute significantly to energy flow, but the latter species is the primary agent of nutrient mineralization and recycling in the terrestrial ecosystem. Each of the five species exhibits a combination of life history and ecological traits which is not satisfactorily explained by the r-K-A selection continuum, chiefly because of the intensity of resource restraints. Physiological economy is at a premium, and each species has evolved a distinct suite of adaptive characteristics which include slow growth, low metabolic rates, long life cycles, and deferred maturity. The lack of interspecific competition, and the general paucity of biotic restraints inherent in the sub-Antarctic Island situation, may be important factors in the evolution of the energy flow patterns exhibited by individuals and populations of sub-Antarctic insects.