正榕與授粉小蜂的物候及族群互動的季節變異

Abstract

榕果-榕小蜂的共生關係中，榕樹需仰賴其專一性的授粉蜂傳遞花粉，而授粉蜂也依賴榕果繁衍後代，如何將小蜂族群衍續，對榕樹而言是重要的課題。本研究以台北市的正榕 (Ficus microcarpa) 與其授粉蜂之間的物候模式來探討其如何適應季節變化明顯的環境。研究內容分為兩部份，第一部份調查榕果物候及授粉蜂的族群動態以探討環境因子－榕果產量－授粉蜂族群三者的關聯（第二章）。本研究選定國立台灣大學校園周圍的29棵正榕植株，自2008年4月至2009年7月進行約每週一次的野外調查。每次調查記錄榕果豐度並採集榕果，利用雌花期榕果內的小蜂估算授粉蜂族群量，最後探討氣象因子與各花期榕果量的關聯。結果發現正榕產果高蜂主要在春季和夏季，冬季榕果產量十分稀少。正榕的開花物候與氣溫、全天空輻射及雨量皆顯著相關。授粉蜂的族群量在夏季達最高峰，秋季漸減。冬季榕果大多無法授粉，顯示冬季對授粉蜂會造成瓶頸效應，待來年春天時授粉蜂族群再次擴張。第二部份檢視花期不同步性的季節變異和授粉率的關係，並探討此現象在適應上扮演的角色（第三章）。利用第二章的榕果豐度資料，分別將株內不同步性、株間不同步性、族群不同步性量化，比較三者的季節變異；另外分析榕果自花授粉機會的指標──「株內雌雄花期重疊 (intra-tree sexual-phase overlap)」的頻率──在季節間的變異，以及表現此現象的植株在授粉蜂進駐率上是否與其他植株有差異。結果顯示，株內不同步性在春、夏季較高；株間不同步性在冬天較高；而族群不同步性無季節差異。株內雌雄花期重疊在夏季較常發生，授粉蜂進駐率在發生此現象的樹上並未顯著高於其他樹，顯示株內雌雄花期重疊，亦即使小蜂於株內傳播，並非正榕小蜂的渡冬方式。在冬天，正榕小蜂以蟲癭的方式渡過環境較差的日子，並在少數環境較好的日子仍可於株間傳播。此外正榕會連續性的雌花期榕果生產，在春季較常出現，延長雌花期的現象被認為可增加個體和族群的授粉率，本研究提供了此一假說的實際觀察。

In the mutualistic relationship between fig and fig wasps, the Ficus trees are pollinated exclusively by their species-specific pollinating fig wasps. How to sustain the pollinator population is a crucial issue for Ficus. This study focused on the phenological patterns between these mutualistic species and explored the adaptive reproduction strategy of a Ficus microcarpa population in Taipei, where weather varies seasonally. The study was divided into two major parts: the first section focused on fig phenology and pollinator population dynamics (Chapter two). Field surveys were conducted on 29 F. microcarpa trees in one week intervals from April 2008 to July 2009. The number of syconia in each developmental phase was recorded. The number of foundresses inside receptive syconia were counted and used as an index of pollinator population size. The results showed that temperature, solar radiation and rainfall are positively correlated with syconia abundance. Syconia abundance was highest in spring and summer, and decreased to a very low level in winter. The pollinator population exhibited a similar trend but with greater fluctuation. It reached the greatest density in summer and dropped in fall. Receptive figs are poorly pollinated in winter, and thus pollinator population should suffer a bottleneck effect during this season. The pollinator population then recovered the following spring. The second part of this study explored the pattern of flowering asynchrony and its relationship with pollination rates, and then discussed the potential adaptive reproduction strategy of F microcarpa (Chapter 3). I used the syconia abundance data recorded in chapter one to further quantify asynchrony at the intra-tree, inter-tree and population levels. The index for chance of self-pollination (frequency of “intra-tree sexual-phase overlap”) was also counted. In addition, the difference in pollinator colonization rate was tested between trees with sexual-phase overlap and trees without. The results showed that both intra-tree asynchrony and the frequency of intra-tree sexual phase overlap were higher in spring and summer. The pollinator colonization rate was slightly (but not significantly) higher for the trees presenting sexual-phase overlap. The phenomenon of successively (>3 weeks) producing female-phase syconia was more common in spring. Finally, the strategy for F. microcarpa to overcome harsh winters and the role of asynchrony in reproductive success were discussed.