樹木小苗的長距離播遷可使其逃離專食性鱗翅目幼蟲的摧殘

文獻來源：JM Norghauer, J Grogan, JR Malcolm, JM FelWli. 2010. Long-distance dispersal helps germinating mahogany seedlings escape defoliation by a specialist caterpillar. Oecologia 162:405–412. DOI 10.1007/s00442-009-1476-9

具有嚴格專一性的植食者或病原菌造成幼苗死亡與密度或距離變因有關，且可能會促進熱帶雨林樹種的高生物多樣性，然而目前證據不足。長久以來有許多假說試圖解 釋熱帶雨林具有高生物多樣性的原因，其中以天敵假說（Enemies hypothesis or Janzen-Connell hypothesis）最廣受討論。天敵假說認為愈接近母樹的區域，種子密度愈高，因此容易吸引大量天敵取食或攻擊，導致該處種子或幼苗死亡率高，此現象 稱為密度-距離依變效應（Density-distance dependent effect）。雖然鱗翅目幼蟲是森林中最豐富且對寄主植物最具專一性的類群，但是他們對幼苗造成的干擾仍未明瞭。即便指針對一個樹種，進行一個支持天敵 假說的可靠實驗也是困難的，因為這不只是處理種子、幼苗和記錄其結果而已。試驗測試還需要（1）一種已被鑑定的且具高度專一食性的植食性昆蟲，(2)該昆 蟲所取食且能在原處進行測量的食草樹種，與(3)對該多株該樹種的樹木進行多年的評估及確認。作者在此研究中呈現了新熱帶樹種-大葉桃花心木 (Swietenia macrophylla, Meliaceae)的新生小苗與一種夜蛾科幼蟲(Steniscadia poliophaea)之間在空間上的詳盡互動關係，該幼蟲專門取食大葉桃花心木的初生苗。試驗樣區位於巴西的熱帶森林的下層，結果顯示，由該種夜蛾科幼 蟲造成之幼苗被攻擊的比例、葉損失面積和幼苗死亡率之最高峰，均落在最接近母樹的樣點，且隨著距離母樹愈遠有顯著的下降，此結果與天敵假說吻合。作者的結 論是，大於50公尺的長距離傳播事件提供大葉桃花心木幼苗在早期的生存優勢，且鱗翅目幼蟲在天敵假說中扮演的媒介角色在熱帶森林中被低估了。

Abstract
Herbivores and pathogens with acute host specificity may promote high tree diversity in tropical forests by causing distance- and density-dependent mortality of seedlings, but evidence is scarce. Although Lepidoptera larvae are the most abundant and host-specific guild of herbivores in these forests, their impact upon seedling distributions remains largely unknown. A firm test of the mechanism underpinning the Janzen–Connell hypothesis is difficult, even for a single tree species, because it requires more than just manipulating seeds and seedlings and recording their fates. Experimental tests require: (1) an insect herbivore that is identified and highly specialised, (2) linkage to an in situ measure (or prevention) of herbivory, and (3) evaluation and confirmation among many conspecific adult trees across years. Here we present experimental evidence for a spatially explicit interaction between newly germinating seedlings of a Neotropical emergent tree, big leaf mahogany (Swietenia macrophylla, Meliaceae), and caterpillars of a noctuid moth (Steniscadia poliophaea). In the understory of a southeastern Amazon forest, the proportion of attacks, leaf area lost, and seedling mortality due to this specialised herbivore peaked near Swietenia trees, but declined significantly with increasing distance from mature fruiting trees, as predicted by the Janzen–Connell hypothesis. We conclude that long-distance dispersal events (>50 m) provided an early survival advantage for Swietenia seedlings, and propose that the role of larval Lepidoptera as Janzen– Connell vectors may be underappreciated in tropical forests.