生態系統之流量網路分析─外來物種與指標物種之互動

Abstract

以往生態系統的研究，多半集中在物種與物種間的兩兩交互作用特性上。然而，一旦生態系統的規模擴大，物種間的交互作用就越顯複雜，其複雜性可能到達難以估算的程度。隨著小世界網路、無尺度網路等複雜網路研究的興起，可將物種與物種間的交互作用簡化為物種與物種間的關係，並用統計的方式去觀察生態網路整體的性質。但這種方式卻接連遭遇到生態網路既不是小世界網路，也不是無尺度網路的定位問題。 本研究站在前人複雜網路的研究基礎上，從流量網路的角度出發，重新納入複雜系統中物種與物種間的交互作用，發現生態網路在連結數分布上與流量分布上並不一致，而流量分布則具有無尺度網路的冪次律特性，可將生態網路重新給與無尺度網路的定位。 進一步，本研究以流量網路分析來探討外來物種的影響。以往研究外來物種對生態系統的影響多集中在外來物種本身的特性上，一個與原生物種習性、食性相似，卻在生殖與掠食能力上具有優勢者，可預期會對生態系統造成影響，但每個物種在生態系統中的角色不盡相同，其受外來物種影響的程度與影響擴散的範圍亦不盡相同。 因此，本研究從生態保育的角度出發，鎖定觀察外來物種對生態系統中指標物種之影響，並評估指標物種受影響之程度與影響擴散的範圍。進而從其影響程度與範圍得出指標物種間的相關性及保育上的優先性，期望透過保育指標物種的方式，減低外來物種對生態系的威脅。

In the past, research about the ecosystem was concentrated on the cause-effect interaction between species. However, when the ecosystem becomes more and more complex, the interaction between species becomes more and more difficult to analyze. Fortunately, with the development of complex network models, the interaction between species has been simplified as relations in a network. Therefore, we can apply statistical methods to analyze the collective characteristics of ecological networks. But this method encountered a series of challenges about how to type the ecological networks, because relation-based ecological networks looked like neither small-world networks nor scale-free networks. Our research is based on complex network but taking the interaction of species into consideration and transforming an ecosystem into a flow network. We discovered that flow distribution does not consist with degree distribution but obey the power-law existing in scale-free network. Accordingly, we can hereafter classify ecological network as scale-free network. Based on this theoretical finding, we studied the influence of alien species on an ecosystem. Previous research about the impact of alien species on ecosystems was focused on the ability of alien species. If an alien species not only has advantages on predation and reproduction but also has similar feeding habits with a native species, we could expect that it would make influence on the ecosystem. But each species plays different roles on an ecosystem, one species would not suffer the same impact as others and the expanding range of impact would not be identical, either. Therefore, from the viewpoint of ecological conservation, we evaluated the impact suffered by indicator species and the expanding range of the impact when the indicator species was influenced by alien species. As a consequence, we can identify the relation between indicator species and priority of ecological conservation. Hence, we can alleviate the threat of alien species by protecting certain indicator species.