水生食物網網絡中的寄生關係

Abstract

寄生蟲是生態群聚中重要的一部分；但是直到最近，寄生蟲才較常被納入食物網的研究中。利用最近發表的文獻資料與相關的網絡分析工具，我的研究嘗試釐清寄生蟲在水生生態系的食物網網路中所扮演的角色。首先，本研究闡明寄生蟲在食物網中並非隨機分布的，而是與其宿主在食物網的位置有關。宿主在食物網的位置如果擁有較多的獵物種類、較靠近所有的獵物、或是能從食物網的底層往上累積較多的資源，其寄生蟲之種類多樣性便較高。相對的，若是宿主在食物網的位置有較多的掠食者、較靠近所有的掠食者、或是位在許多食物鏈路徑的必經之處，則此宿主的寄生蟲往上傳遞的比例越高。再者，本研究利用生態模式模擬食物網加入寄生蟲後，其網絡強固性之變化。首次將寄生蟲所造成之網絡強固性減少，分成網路的結構複雜性改變與寄生蟲特性兩個成因來探討；研究結果顯示造成食物網強固性減少的主因在於寄生蟲對宿主資源利用的可替代性較低。本研究建議利用寄生蟲對次級滅絕特別敏感的特性，可將寄生蟲當成食物網功能是否完整之領先指標。最後，本研究以過去食物網中物種的拓樸重要性指標為基礎，延伸加入食性營養關係中能量流動方向性的正負符號概念，發展出一套新的具正負符號的拓樸重要性指標。此一指標涵括食物網中的上行與下行效應、直接與間接作用，可用來定量物種間彼此的交互作用。應用此一新指標在包含寄生蟲的食物網資料上，可幫助我們進一步釐清寄生蟲在食物網中的角色與和其他功能群間的交互關係。

Parasites are ubiquitous in ecological communities but they haven’t been routinely included in food web studies until very recent. Using recently published data and the tool of network analysis, I elucidated features associated with the pattern of parasitism in food web networks. First, I showed that parasites are not only occurring in non-random fashion in food webs but also positively associated with the prominent network positions occupied by their hosts. Meanwhile, a host species with high parasite diversity tends to have a wide diet range, occupy a network position close to many prey species, or occupy a network position that can better accumulate resources from species at lower trophic levels, whereas a host species with higher vulnerability to predators, being at a network position close to many predatory species, or being involved in many different food chains, tends to serve as a good intermediate host in parasite transmission. Second, by conducting simulation experiments with different food web models and extinction scenarios, I demonstrated that the reduction in food web robustness after considering parasites is mainly contributed by the life cycle constrain of parasites. The finding further demonstrates that parasites are prone to secondary extinctions and their extinctions occur earlier than those involving non-parasite species. The evident vulnerability nature of parasite to species loss designates parasite a proper leading indicators of food web integrity. Lastly, with the extension of a previously developed methodology, a new approach is presented and used to quantify the interaction structure of a food web and consequently the topological importance of species when the food web is viewed as a signed digraph. This method is also capable to quantify the strength of inter-specific interaction as well as in what way species interact with each other after counting both direct and indirect cascades in both top-down and bottom-up directions. As it has the potential to quantify a wide range of ecological interactions, its further application on revealing the interaction structures between parasites and other functional groups in food web with parasites is evidently achievable.