Smith V R, Steenkamp Marianna
Department of Botany, University of the Orange Free State, 9301, Bloemfontein, South Africa.
Oecologia. 1990 Nov;85(1):14-24. doi: 10.1007/BF00317338.
Marion Island (47°S, 38°E) has one of the most oceanic climates on earth, with consistently low air temperatures, high precipitation, constantly high humidity, and low incident radiation. Since 1968 mean surface air temperature has increased significantly, by 0.025° C year. This was strongly associated with corresponding changes in sea surface temperature but only weakly, or not at all, with variations in radiation and precipitation. We suggest that changing sealevel (atmospheric and oceanic) circulation patterns in the region underlie all of these changes. Sub-Antarctic terrestrial ecosystems are characterized by being species-poor and having a simple trophic structure. Marion Island is no exception and a scenario is presented of the implications of climatic change for the structure and functioning of its ecosystem. Primary production on the island is high and consequently the vegetation has a large annual requirement for nutrients. There are no macroherbivores and even the insects play only a small role as herbivores, so most of the energy and nutrients incorporated in primary production go through a detritus, rather than grazing, cycle. Ameliorating temperatures and increasing CO levels are expected to increase productivity and nutrient demand even further. However, most of the plant communities occur on soils which have especially low available levels of nutrients and nutrient mineralization from organic reserves is the main bottleneck in nutrient cycling and primary production. Increasing temperatures will not significantly enhance microbially-mediated mineralization rates since soil microbiological processes on the island are strongly limited by waterlogging, rather than by temperature. The island supports large numbers of soil macro-arthropods, which are responsible for most of the nutrient release from peat and litter. The activities of these animals are strongly temperature dependent and increasing temperature will result in enhanced nutrient availability, allowing the potential for increased primary production due to elevated temperature and CO levels to be realized. However, housemice occur on the island and have an important influence on the ecosystem, mainly by feeding on soil invertebrates. The mouse population is strongly temperature-limited and appears to be increasing, possibly as a result of ameliorating temperatures. We suggest that an increasing mouse population, through enhanced predation pressure on soil invertebrates, will decrease overall rates of nutrient cycling and cause imbalances between primary production and decomposition. This, along with more direct effects of mice (e.g. granivory) has important implications for vegetation succession and ecosystem structure and functioning on the island. Some of these are already apparent from comparisons with nearby Prince Edward Island where mice do not occur. Other implications of climatic change for the island are presented which emphasize the very marked influences that invasive organisms have on ecosystem structure and functioning. We suggest that changing sealevel circulation patterns, by allowing opportunities for colonization by new biota, may have an even more important influence on terrestrial sub-Antarctic ecosystems than is suggested merely on the basis of associated changes in temperature or precipitation.
马里恩岛（南纬47°，东经38°）拥有地球上最海洋性的气候之一，气温持续偏低，降水量大，湿度始终很高，且入射辐射量低。自1968年以来，地表平均气温显著上升，每年上升0.025摄氏度。这与海面温度的相应变化密切相关，但与辐射和降水的变化关系微弱或毫无关联。我们认为，该地区海平面（大气和海洋）环流模式的变化是所有这些变化的根本原因。亚南极陆地生态系统的特点是物种匮乏且营养结构简单。马里恩岛也不例外，本文呈现了气候变化对其生态系统结构和功能的影响。该岛的初级生产力很高，因此植被每年对养分的需求量很大。岛上没有大型食草动物，甚至昆虫作为食草动物所起的作用也很小，所以初级生产中纳入的大部分能量和养分通过碎屑循环而非放牧循环。气温升高和二氧化碳水平增加预计将进一步提高生产力和养分需求。然而，大多数植物群落生长在养分有效含量特别低的土壤上，有机储备中的养分矿化是养分循环和初级生产的主要瓶颈。气温升高不会显著提高微生物介导的矿化速率，因为该岛的土壤微生物过程受涝渍的强烈限制，而非温度。该岛有大量土壤大型节肢动物，它们负责泥炭和枯枝落叶中大部分养分的释放。这些动物的活动强烈依赖温度，温度升高将导致养分有效性提高，从而使因温度升高和二氧化碳水平增加而增加初级生产的潜力得以实现。然而，岛上有小家鼠，它们对生态系统有重要影响，主要是通过捕食土壤无脊椎动物。小家鼠种群受温度强烈限制，且似乎在增加，可能是温度升高的结果。我们认为，小家鼠数量增加，通过增强对土壤无脊椎动物的捕食压力，将降低养分循环的总体速率，并导致初级生产与分解之间的失衡。这一点，连同小家鼠更直接的影响（如种子捕食），对该岛的植被演替以及生态系统结构和功能具有重要意义。其中一些影响已通过与附近没有小家鼠的爱德华王子岛的比较而显现出来。本文还介绍了气候变化对该岛的其他影响，强调了入侵生物对生态系统结构和功能的显著影响。我们认为，海平面环流模式的变化，通过为新生物群的定殖提供机会，可能对亚南极陆地生态系统产生比仅基于温度或降水的相关变化所表明的更为重要的影响。
