船運帶來商機人潮, 也帶外來種

人為活動促使許多生物跨越自然的地理障礙
分布至自然情況下不會出現的地理區域
這樣的生物稱做外來種(alien species)
入侵種(invasive species)則是指會影響當地原生種的外來種

入侵種是造成生物多樣性衰減的主因之ㄧ
入侵種除了直接與原生種競爭食物與空間
也可能改變整個生態系!

找出外來種引入的途徑是減緩或解決外來種問題的根本
世界各地的海洋生物隨著航運進行生物相大交換
早期的船隻是利用石頭或其他重物來穩定船艙
後來則改成抽水入船艙(叫作壓艙水ballast water)
因此許多水中浮游生物就隨著壓艙水被帶至另外一地
(關於壓艙水裡的生物相, 相關研究非常多!)

那這些外來種從哪裡來?
除了以船運航線和頻度來推測之外
也能透過DNA的分析追尋其可能的來源地!

由於外來種引入的數量和次數是影響其是否能建立族群的關鍵
但通常此因子也不易準確評估
Brawley等人便透過歷史的記載的船運紀錄
代表外來種可能來源與輸入次數
以及兩種外來種--褐藻和玉黍螺--的DNA分析
推判出海洋外來種的引入和船運有明確的關連

此研究刊登於5月4日的美國國家科學院院刊(PNAS)搶鮮版(early edition)
Brawley et al. 2009. Historical invasions of the intertidal zone of Atlantic North America associated with distinctive patterns of trade and emigration. doi: 10.1073/pnas.0812300106
全文pdf檔可免費下載


搶鮮版介紹如下

Origins of invasive marine organisms

Trade and emigration from Europe to North America in the 19th century resulted in thousands of ships crossing the Atlantic Ocean. Shipping records from 1773-1861 indicate that nearly all ships arriving in Pictou Harbour, Nova Scotia originated in Scotland, England, or Ireland. Some of these ships brought marine organisms on the rocks used as ship ballast. Two invasive species, the rockweed Fucus serratus and the periwinkle snail Littorina littorea, have become common on Canada's North Atlantic coast, and the snail quickly became abundant on the Atlantic coast of the United States. By using genetic fingerprints and historical shipping records, Susan Brawley et al. determined that F. serratus and L. littorea found in Pictou Harbour likely originated in Great Britain and Ireland. Microsatellite analysis of F. serratus indicated 2 separate introductions to Nova Scotia: one originating in Galway, Ireland, and one in Greenock, Scotland. L. littorea showed a similar pattern, with 8 of the 9 cytochrome b haplotypes matching those from snails captured in Ireland and Scotland. These findings suggest that shipping records may provide a proxy for recognizing the introduction of invasive marine species, and the authors suggest that these species form the tip of the "invasive iceberg."