The role of the soil seed store in the survival of an invasive population of Poa annua at Point Thomas Oasis, King George Island, maritime Antarctica

Abstract

Poa annua, an invasive species in the Antarctic, had established a relatively large population at Point Thomas Oasis (Maritime Antarctic) recorded from 1985/1986 austral summer. We present a detailed account of the expansion of P. annua across its 30 years of establishment (1985/1986, 1986/1987, 1987/1988, 1989/1990 and 1991/1992 – historic data, 2014/2015 and 2015/2016 – our own observations) and how this relates to the historical human activity at this site. We also evaluate the first stages of the control effort, i.e. tussock removal by hand. For a subset of the removed tussocks we also removed soil in order to assess the importance of the soil seed bank in the course of this invasion. The population showed a spatio-temporal dynamic increase in area from 1985/1986 to 1987/1988 growing season followed by a decrease in area noted in 1989/1990 and subsequent increase in area and diversity of occupied habitats. Analysis of distribution dynamics did not show a significant difference in the number of tussocks recorded in the 2014/2015 austral summer between cartogram squares occupied in 1991/1992 or earlier, and those occupied just in 2014/2015. There was, however, a high correlation in the number of tussocks per square between the 2014/2015 and 2015/2016 census. Field observations conducted in 2015/2016 within the whole Point Thomas Oasis indicated that on 6 out of 51 sites cleared during the 2014/2015 austral summer, P. annua had re-established, mostly on sites where no soil was collected. Of the soil samples removed from beneath tussocks, seed germinated in 19% of the samples (160 seedlings in total). These results indicate that the strategy of soil seed bank formation has been very effective in extreme Antarctic conditions, providing a ‘temporal rescue effect’ which has enabled the survival of the population, and hinders the effective eradication of this invasive species. Keywords: Seed germination, Temporal rescue effect, Cartogram, Eradication