合歡山高山植物與傳粉者交互作用網絡

Abstract

植物與傳粉者的互利共生網絡多樣性越高，越能抵抗擾動並維持生態穩定。探究高山地區此類交互作用網絡，有助解析物種關聯結構與制定保育策略。國外研究大多顯示，隨海拔升高，網絡專一性與模組性下降，巢狀性上升。然而亞洲地區對於這方面的研究相對缺乏，臺灣因為位處東亞地區，有北回歸線經過中南部，為具有潛力研究高山植物與傳粉者交互作用的地區。先前在臺灣合歡山進行研究，結果顯示在臺灣大多數的高山植物仰賴熊蜂、食蚜蠅、非食蚜蠅拜訪，這三個分類群的生態棲位重疊指數低；亦有研究指出探討植物與傳粉者交互作用必須同時考慮拜訪率以及昆蟲分上攜帶的花粉。本研究的研究問題是探討使用純粹拜訪網絡以及花粉傳遞網絡是否呈現一致，目的是希望初探臺灣高山植物與傳粉者的交互作用網絡結構並且補足亞洲地區高山植物與傳粉者互動網絡上的研究空缺。我們的假說是在臺灣合歡山地區的植物與傳粉者交互作用網絡應呈現低專一性、高度模組化、高巢狀性的結構特徵，高度拜訪率的傳粉昆蟲會貢獻最多的花粉於整個植物與傳粉者的交互作用網絡。 為了瞭解合歡山植物與傳粉者之間的交互作用網絡，我們選擇合歡山樣區（24°08’48.6”N 121°16’56.7”E）做為採樣地點，於2023年6月24日至2023年8月24日進行昆蟲捕捉，以每兩星期為一個單位將採樣時間分成五個時期。我們請教專家學者們的意見，參考最新發表之文獻進行樣區的傳粉昆蟲鑑定，釐清傳粉昆蟲的組成。為了比較拜訪網絡以及花粉傳遞網絡的結構數值差別並且找出關鍵物種，我們使用稀疏曲線（rarefaction）分析，決定取樣數值並且清洗昆蟲身上攜帶的花粉進行辨識，再進行拜訪網絡以及花粉傳遞網絡重建與整體結構數值專一性（H_2）、巢狀性（NODF、weighted NODF）、模組化（Madularity Q）比較，進行中心性分析betweenness、closeness、normalized degree找出關鍵物種，並且繪製滅絕曲線找出每個時期影響網絡的優勢物種。 結果顯示合歡山樣區的植物與傳粉者交互作用網絡主要由雙翅目與膜翅目組成，且膜翅目在第三期至第五期開始有大量的數量出現，出現的時間會與花期重疊。在拜訪網絡方面會發現在第一期及、第四期第五期舞虻貢獻極高的拜訪率，而信義熊蜂則從第二期開始出現，到第四期達到高峰，並且與非食蚜蠅類共同主導整個植物交互作用網絡。花粉傳遞網絡部分則是由楚南熊蜂貢獻最多的花粉量，並且傾向專一性拜訪玉山懸鉤子。而雙翅目在第四期及第五期則攜帶最多的花粉數目。結構數值方面，不論是拜訪網絡還是花粉傳遞網絡，皆呈現低度專一性、低度模組性、以及高度巢狀性結構。根據滅絕曲線分析，可以發現在拜訪網絡中的優勢物種為信義熊蜂及舞虻。而在花粉傳遞網絡當中，楚南熊蜂、食蚜蠅Dasysyrphus sp.、食蚜蠅 Eristalinus paria為優勢物種，因為在整個網絡當中攜帶最多的花粉。 雖然不論是拜訪網絡還是花粉傳遞網絡皆符合我們的假說，在合歡山地區呈現低專一性、低模組化、高巢狀性的結構，屬於廣泛性拜訪的網絡，但是我們認為合歡山的植物與昆蟲交互作用網絡，相對脆弱，因為使用花粉傳遞網絡進行結構數值分析會更接近平均值。因此，在評估植物與傳粉者交互作用網絡時應該同時考慮拜訪網絡以及花粉傳遞網絡，才能有效的評估整個網絡應該優先保護的物種。昆蟲取食的過程中能量的消耗以及高山上植物花色的演化，可能是造成整個網絡結構呈現低度專一性的原因。為了維持整個網絡的穩定授粉，避免在整個網絡的過程中因為某些物種的消失而導致失去傳粉昆蟲，因此演化出高度巢狀性的結構。在合歡山樣區，因為具備低度的模組化，整體環境可能較無法抵抗外來的干擾；在我們的中心性分析當中，信義熊蜂為整個合歡山網絡的關鍵物種，而外來種植物貓兒菊已經取代原生種植物變成關鍵物種，有可能會造成臺灣原生種植物的結子率降低，國家公園對於外來種植物的管理應該要更留意管理外來種植物，以避免本土植物的滅絕。本研究提供了對於副熱帶高山地區植物與傳粉者互動網絡的初步理解，補足過去東亞地區在此領域研究的空缺，並且希望有更多臺灣在地的研究人員共同攜手合作，一起為臺灣的高山生態環境盡一份心力。

The higher diversity of the mutualistic network between plants and pollinators, the higher ability to against the perturbation and maintain the stability of ecosystem. Most of the researches revealed that the specialization and the modularity decreased and the nestedness increased along the altitude gradient in abroad. However, this kind of researches are deficient in Asia. Taiwan, a small island located in subtropic Asia within a lot of high mountains and the Tropic of Cancer cross the south, is a potential area to investigate the alpine plant-pollinator interaction. The previous study revealed the plants depend on the bumble bees, syrphid flies and non-syrphid flies to visit and the niche overlap is respectively low（0.3）between these three groups of pollinators. There is also another research suggest that we should consider both of the visiting numbers and pollen carriage from the pollinators to explore the plant-pollinator interaction. Our research question is to check if the plant-pollinator interaction networks are the same by visiting-based and pollen carriage. We aim to explore the structure of alpine plant-pollinator interaction network in Taiwan and make the vacancy up in Asia. Our hypothesis is the plant-pollinator interaction network is low specialization, high modularity and high nestedness in Hehuanshan in Taiwan. In order to understand the plant-pollinator interaction in Hehuanshan, we chose the study site in Hehuanshan located in Taroko National Park（24°08’48.6”N 121°16’56.7”E）. Insects were collected from June 24 to August 24, 2023, we separated the sampling time into 5 terms and each term contain 2 weeks. In order to understand the pollinator’s composition, we consulted experts and used the latest published studies to identify them in the sampling areas. In order to compare the differences between visiting-based and pollen carriage interaction, we got the threshold by using the rarefaction to decide the sample size, washed pollinators to identify the pollens. And then, we compare the structure properties, specialization（H_2）, modularity, nestedness between visiting network and pollen carriage network. Using betweenness, closeness and normalized degree to find the keystone species. Drawing the extinction curve to find the dominant species. The plant–pollinator interaction network in the Hehuanshan study site was primarily composed of Diptera and Hymenoptera. Hymenoptera appeared in large numbers from the term3 to the term5 and their occurrence overlapping with the flowering season. In the visiting network, Empdidae exhibited exceptionally high visitation rates in term1, term4 and term5. Bombus formosellus appeared from term2 and peaked in term4, dominating the network with non-syrphid flies in term4. In the pollen carriage network, Bombus sonani contributed the largest amount of pollen and exhibited a tendency for specialized visitation on Rubus rolfei. Diptera carried the highest amount of pollen loads during term4 and term5. In all the terms, both of the visitating and pollen carriage networks exhibited low specialization, low modularity, and high nestedness. The extinction curve analysis reveal the dominant species in the visiting network were Bombus formosellus. In the pollen carriage network, Bombus sonani, Dasysyrphus sp. and Eristalinus paria are the dominant species because they carried the most amount of pollen. Although both the visitation and pollen carriage networks fit our hypothesis that exhibited low specialization, low modularity, and high nestedness, the result reveal that the plant-pollinators network in Hehuanshan is a generalized network and the plant-pollinator interaction network in Hehuanshan appears relatively vulnerable to against the perturbation because of lower z-score in structure properties during pollen carriage networks. The reason of low specialization observed in the network may because of energy consuming during foraging and the evolution of floral coloration in alpine plants. High nested structure may likely evolve to prevent the loss of pollinators and maintain the pollination. The low modularity of the network suggests that the network has the low ability to against the perturbation in Hehuanshan. According to our centrality analysis, Bombus formosellus functions as a keystone species. Moreover, when assessing plant–pollinator interaction networks, both visitation and pollen carriage networks should be considered to effectively identify the conservation priority. Furthermore, the exotic plant, Hypochaeris radicata, has become the keystone species, this situation may reduce the seed set of native plants, national park management should pay attention in controlling the exotic plant to prevent the extinct of native plant. Our study provided a preliminary understanding of plant–pollinator interaction networks in subtropical alpine regions and complement the study gap of East Asia. We hope more researchers to collaborate in the conservation of alpine ecosystem in Taiwan.