Transoceanic dispersal of terrestrial species by debris rafting

Rare, long-distance dispersal events are a key process in generating and maintaining patterns in biological diversity and species distributions across space and time. The 9.0 magnitude earthquake that struck the eastern coast of Japan in 2011, and the subsequent 38 m high tsunami washed large amounts of shoreline debris into the Pacific Ocean that led to a large-scale biological rafting event carrying nearly 300 marine species to the western shores of North America. Whether oceanic, trans-Pacific dispersal via rafting generates long distance dispersal events for small, flightless, terrestrial species is unknown. By sampling beach debris associated with known hot-spots of tsunami debris along the north and east shores of Graham Island, Haida Gwaii, Canada, I document significantly dissimilar invertebrate communities associated with tide-line beach debris and the occurrence of several putative Japanese species of soil-dwelling mites (Acari: Oribatida). Previous explanations of Haida Gwaii’s unique flora and fauna have been attributed to a proximity to the Beringian land bridge and the accumulated evidence of near-offshore glacial refugia during the last glacial period. However, my research also suggests that stochastic, trans-Pacific rafting events contribute to the biodiversity and biogeography of soil communities on the west coast of North America.
Methods
Species abundance data were generated at locations with previously documented Japan Tsunami Marine Debris (JTMD) along the north and east shorelines (North Beach and East Beach) of Graham Island, Haida Gwaii, British Columbia, Canada. Samples were collected from beach, tidal edge, and forest locations in Summer 2017 over a one-week period following the morning high tide.  Beach samples consisted of approx. 20 cm2 of debris from multiple items collected along the shoreline representing the daily tide line.  Shoreline vegetation and leaf litter at the tidal edge zone were sampled to represent the first possible, but likely non-optimal habitat that would allow for colonization of new terrestrial migrants.  Forest floor samples were also collected for comparison with long-term resident species.  Where possible beach, edge and forest samples were collected at the same location.  All samples were extracted on portable Berlese faunal extractors over 72 hrs into 75% ethanol, and specimens observed were subsequently identified to species-level using keys from the primary literature, and compared against >3000 curated specimens in the author’s taxonomic collection.

稀有长距离扩散事件是塑造并维持生物多样性格局以及物种时空分布模式的核心过程。2011年袭击日本东海岸的9.0级大地震及其引发的38米高海啸将大量海岸碎屑冲入太平洋，由此触发了大规模生物浮筏事件，近300种海洋物种借此抵达北美西海岸。目前尚不清楚通过浮筏实现的跨太平洋海洋扩散，能否为小型无飞行能力的陆生生物带来长距离扩散事件。本研究通过在加拿大海达瓜依（Haida Gwaii）格雷厄姆岛（Graham Island）南北两岸已知的海啸碎屑热点区域采集海滩碎屑样本，记录到与潮间带海滩碎屑相关的无脊椎动物群落存在显著差异，并发现了数种疑似源自日本的土壤栖息螨类（Acari: Oribatida）。此前针对海达瓜依独特动植物区系的解释，多归因于其邻近白令陆桥，以及末次冰期近岸冰川避难所的相关累积证据。但本研究同时表明，随机发生的跨太平洋浮筏事件，对北美西海岸土壤群落的生物多样性及生物地理格局具有塑造作用。

研究方法
物种丰度数据采集自加拿大不列颠哥伦比亚省海达瓜依格雷厄姆岛南北两岸（北滩与东滩）已记录的日本海啸海洋碎屑（Japan Tsunami Marine Debris, JTMD）采样点。样本采集工作于2017年夏季开展，在早潮过后的一周内，分别从海滩、潮间带边缘与林地区域取样。海滩样本采集自沿岸每日潮线区域的多件碎屑，样本量约为20平方厘米。潮间带区域的海岸植被与枯落物样本，用于模拟陆生新移殖物种可能抵达的首个（但通常并非最优）栖息生境。同时采集林地地表样本，用于与长期定居的本土物种进行对照。在条件允许的情况下，会在同一地点同步采集海滩、潮间带边缘与林地样本。所有样本均使用便携式贝莱斯氏动物分离器（Berlese faunal extractors）在75%乙醇中抽提72小时；随后依据经典文献中的分类检索表，将观测到的标本鉴定至物种水平，并与作者分类馆藏中超过3000份已整理标本进行比对。