Impacts of the feral house mouse (Mus musculus) on spiders on subantarctic Macquarie Island

Macquarie Island is a Nature Reserve under the Tasmanian National Parks and Wildlife Act 1970 and also a World Heritage Area but it has been modified significantly as the result of the introduction and establishment of exotic species including the house mouse, Mus musculus (Brothers and Copson 1988). Current attitudes favour the reversal of changes caused by such introductions, however, to date, efforts on the island have concentrated on the control of rabbits (Oryctolagus cuniculus) and cats (Felis catus). Although cats were extirpated some few years ago, this was followed by a considerable increase in the rabbit population. Control of both rabbits and rodents is currently being addressed (Anon 2007).Invertebrates are rarely considered in conservation decisions even though invertebrate interactions have been established as playing an integral role in maintaining ecosystem function emphasizing their ecological importance (Majer 1987; Wilson 1987; Kremen et al. 1993). Examples of their various roles are their importance in soil aeration and drainage, litter decomposition and nutrient cycling, pollination, seed distribution and survival and herbivory (Majer 1987). Comparative studies of secondary production by insects and vertebrates invariably show that insects are greater producers of biomass and conduits of energy through communities than vertebrates (Price 1984). In the subantarctic environment, where many of these processes occur at a low rate much of the time (Hnatiuk 1993), altering the composition of invertebrate communities could have a significant impact on ecosystem processes (Hanel and Chown 1998). Moreover, macroinvertebrates have been shown to be responsible for most litter decomposition on subantarctic Marion Island (Chown and Smith 1993; Smith 1993; Hanel and Chown, 1998). Introduced rodents have the potential to indirectly alter ecosystems of subantarctic islands through their impact on the invertebrate fauna (Crafford 1990). On Macquarie Island, Copson (1986) found that spiders made up a significant proportion of the diet of the house mouse. Of 108 stomach contents examined, spiders were recorded in 84% of stomachs and were common or abundant in 49% of those. The three spider species that occur on Macquarie Island (Greenslade 2006) are probably the major predators of small invertebrates. It is possible therefore that alteration of spider density has significant flow-on effects in both the invertebrate community and the systems of which they are a part. It is not clear however if mouse predation is important in the regulation of spider densities.The aim of this study was to test the hypothesis that predation by M. musculus affects the densities of the three spider species, Myro kerguelensis O. P. Cambridge, Parafroneta marrineri (Hogg) and Haplinis mundenia (Urquhart) present on Macquarie Island. An exclusion experimental design was used. This work was completed as part of ASAC project 104 (ASAC_104).

麦夸里岛（Macquarie Island）是依据《1970年塔斯马尼亚国家公园与野生动物法》设立的自然保护区，同时也是世界遗产地，但因外来物种（exotic species）的引入与定殖，其中涵盖小家鼠（Mus musculus）（Brothers与Copson，1988），其生态系统已发生显著改变。当前的保护理念倾向于逆转此类外来物种引入所引发的生态变化，但迄今为止，岛上的防控工作仅聚焦于穴兔（Oryctolagus cuniculus）与家猫（Felis catus）的种群管控。尽管家猫已于数年前被彻底根除，但随后兔种群数量出现了大幅回升。目前，针对穴兔与啮齿类（rodents）的综合防控工作正有序推进（匿名，2007）。
尽管无脊椎动物（invertebrates）的种间相互作用已被证实是维持生态系统功能（ecosystem function）的核心环节，凸显了其极高的生态重要性（Majer，1987；Wilson，1987；Kremen等，1993），但在保护决策制定过程中，无脊椎动物却极少被纳入考量范畴。无脊椎动物的生态功能涵盖土壤通气与排水、枯落物分解（litter decomposition）、养分循环（nutrient cycling）、传粉（pollination）、种子传播与存活以及植食作用（herbivory）等多个关键环节（Majer，1987）。针对昆虫与脊椎动物次级生产（secondary production）的对比研究普遍表明，昆虫的生物量（biomass）产出以及作为群落间能量流动通道的作用均远超脊椎动物（Price，1984）。在亚南极环境（subantarctic environment）中，多数生态过程长期处于低速率运行状态（Hnatiuk，1993），因此无脊椎动物群落组成的改变可能会对生态系统过程产生显著影响（Hanel与Chown，1998）。此外，研究证实，亚南极马里恩岛的枯落物分解主要由大型无脊椎动物（macroinvertebrates）主导（Chown与Smith，1993；Smith，1993；Hanel与Chown，1998）。外来啮齿类可通过影响无脊椎动物区系（invertebrate fauna），间接改变亚南极岛屿的生态系统结构（Crafford，1990）。在麦夸里岛，Copson（1986）的研究发现，蜘蛛构成了小家鼠食谱的重要组成部分。在检测的108份胃容物（stomach contents）样本中，84%的样本内均检出蜘蛛，其中49%的样本中蜘蛛为常见或优势类群。麦夸里岛现存的三种蜘蛛（spider species）（Greenslade，2006）大概率是小型无脊椎动物的主要捕食者。因此，蜘蛛种群密度的变化可能会对无脊椎动物群落及其所在的生态系统产生显著的连锁效应（flow-on effects）。但目前尚不清楚鼠类捕食是否对蜘蛛种群密度的调控具有关键作用。
本研究旨在验证以下假说：小家鼠（M. musculus）的捕食作用会影响麦夸里岛现存的三种蜘蛛——Myro kerguelensis O. P. Cambridge、Parafroneta marrineri (Hogg)以及Haplinis mundenia (Urquhart)的种群密度。本研究采用排除法实验设计（exclusion experimental design），相关工作作为ASAC项目104（ASAC_104）的一部分完成。