This article is a sequel to a word-model description of the ecosystem of sub-Antarctic Marion Island, published in this journal 30 years ago (Smith 1977). It expands on the qualitative considerations of patterns of energy flow and nutrient cycling presented in that paper, by providing quantitative information from subsequent research at the island. Primary production of the island's lowland plant communities is high on an annual basis, because the vegetation has a long growing season due to the lack of severely cold winters or drought. Daily productivity is actually low due to low radiation levels and a cool growing season. The vegetation is particularly efficient regarding its use of nutrients for its growth, but still requires substantial amounts of nutrients to support the high annual production. Seabirds and seals import large quantities of nutrients from the ocean when they breed and moult on the island. They markedly enhance soil and plant nutrient status in the areas in which they occur, and also in adjacent areas. However, by far the greater part of the island's inland vegetation is not directly influenced by birds or seals and most of the nutrients required for plant growth are provided by decomposition of plant litter and peat. Soil invertebrates are crucial facilitators of decomposition processes, which are otherwise restricted by low soil temperatures and high soil moisture contents. Introduced house mice have invaded almost all parts of the island and predate heavily on the invertebrates, thus affecting nutrient mineralisation. This threatens not only the functioning (lowered nutrient availability leading to slower plant growth and the production of a lower quality, more decomposition-recalcitrant plant litter), but also the structure (an altered balance between production and decomposition leads to a change in the relation between peat formation and degradation, which is an important determinant of vegetation succession) of the island's ecosystem. It is suggested that mice may also affect the island's ecology by predating on seabird chicks.